Class 9 Science NCERT Notes – Chapter 8: Force and Laws of Motion (PDF, MindMap, Q&A, Quizzes)

Chapter 8 (Physics): Force and Laws of Motion – Class 9 NCERT Science Detailed Study Notes.

1. The Concept of Force

1.1 Historical Perspective and Introduction

For centuries, scientists and philosophers believed that rest was the “natural state” of an object, based on observations like a ball coming to rest after being hit. This view was challenged and ultimately replaced by the work of Galileo Galilei and Isaac Newton, who developed a new understanding of motion and its causes.

The central question this topic addresses is what causes motion and changes in motion. The fundamental cause is identified as force.

1.2 Defining and Understanding Force

Force itself has not been directly seen, tasted, or felt. Its existence and nature are understood by observing its effects on objects.

• Definition: The concept of force is based on actions like a push, hit, or pull. These are all ways to bring objects into motion by making a force act on them.
• Effects of Force: Applying a force to an object can result in several changes:
  • Change the state of motion: Put a stationary object into motion or stop a moving object.
  • Change the magnitude of velocity: Make an object move faster or slower.
  • Change the direction of motion: Alter the path of a moving object.
  • Change the shape and size: Deform an object, such as expanding a spring or making a rubber ball oblong.

2. Balanced and Unbalanced Forces

The effect of a force depends on whether it is balanced by other forces.

• Balanced Forces: When two or more forces of equal magnitude act on an object from opposite directions, they are called balanced forces. These forces do not change the state of rest or of motion of an object. For example, if a wooden block is pulled from opposite sides with equal forces, it will not move.
• Unbalanced Forces: An unbalanced force exists when forces of different magnitudes act on an object, or when a single force is unopposed. An unbalanced force is required to:
  • Bring a stationary object into motion.
  • Accelerate the motion of an object (change its speed or direction).
• The Role of Friction: Friction is a force that arises between two surfaces in contact and acts in the direction opposite to motion. When pushing a heavy box, it may not move initially because the force of friction balances the pushing force. To move the box, the pushing force must be greater than the friction force, creating an unbalanced force.
• Sustaining Motion: Contrary to early belief, an object does not need a continuous unbalanced force to maintain its motion. An object moves with a uniform velocity when the forces acting on it (e.g., pushing force and frictional force) are balanced, resulting in no net external force.

3. Newton’s First Law of Motion: The Law of Inertia

3.1 Galileo’s Foundation

Galileo Galilei’s experiments with objects on inclined planes were crucial. He deduced that objects move with a constant speed when no force acts on them.

• Observations:
  • A marble rolling down an inclined plane increases its velocity.
  • A marble rolling up an inclined plane decreases its velocity.
  • On a frictionless double-inclined plane, a marble released from a certain height on one side will roll up to the same height on the opposite side.
• Conclusion: If the second plane were made horizontal and frictionless, the marble would continue to travel forever, trying to reach its original height. This suggests that no net force is needed to sustain uniform motion, and an unbalanced force is only required to change the motion.

3.2 Newton’s Formulation

Building on Galileo’s work, Isaac Newton formulated his first law of motion:

An object remains in a state of rest or of uniform motion in a straight line unless compelled to change that state by an applied force.

3.3 Inertia and Mass

The first law introduces the concept of inertia.

• Definition: Inertia is the natural tendency of an object to resist a change in its state of motion or of rest. All objects possess inertia.
• Inertia and Mass: The mass of an object is a quantitative measure of its inertia.
  • Heavier or more massive objects offer larger inertia.
  • For example, a train has much more inertia than a bicycle, and a stone has more inertia than a rubber ball of the same size.
  • The SI unit of mass is the kilogram (kg).

3.4 Everyday Examples of Inertia

• In a Vehicle:
  • Braking: When a moving bus brakes suddenly, passengers fall forward. The lower body stops with the bus, but the upper body tends to continue moving forward due to inertia.
  • Accelerating: When a bus starts suddenly from rest, passengers fall backward. The lower body moves with the bus, but the upper body tends to remain at rest due to inertia.
  • Turning: In a sharp turn, passengers are thrown to the side because their bodies tend to continue moving in a straight line due to inertia.
• Activities:
  • Carom Coins: Hitting the bottom coin of a stack sharply causes it to move out, while the rest of the stack falls vertically due to their inertia (tendency to stay at rest).
  • Coin and Card: Flicking a card from under a coin on a tumbler causes the card to shoot away, while the coin drops into the tumbler due to its inertia.
  • Leaves on a Tree: Vigorously shaking a branch causes the branch to move, but the leaves tend to remain at rest due to their inertia, causing them to detach.

4. Newton’s Second Law of Motion

4.1 Momentum

The first law is qualitative. The second law provides a quantitative measure of force. It introduces the concept of momentum (p).

• Definition: Momentum is the product of an object’s mass (m) and velocity (v).
• Formula: p = mv
• Properties: Momentum has both magnitude and direction. Its direction is the same as the velocity.
• SI Unit: kilogram-metre per second (kg m s⁻¹).
• Impact: The impact produced by an object depends on both its mass and velocity (its momentum). A small bullet at high velocity can have a large impact, as can a slow-moving truck with large mass.

4.2 Statement of the Second Law

The rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of the force.

This means the force required to change an object’s momentum depends on both the magnitude of the momentum change and the time over which the change occurs.

4.3 Mathematical Formulation

• Consider an object of mass m moving with initial velocity u, accelerated to a final velocity v in time t by a constant force F.
• Initial momentum, p₁ = mu.
• Final momentum, p₂ = mv.
• Change in momentum = p₂ - p₁ = m(v - u).
• Rate of change of momentum = m(v - u) / t.
• According to the law, F ∝ m(v - u) / t.
• Since acceleration a = (v - u) / t, we have F ∝ ma.
• This becomes an equation: F = kma, where k is a constant of proportionality.
• The unit of force, the newton (N), is defined such that k=1. One newton is the force that produces an acceleration of 1 m s⁻² in an object of 1 kg mass.
• Final Formula: F = ma
• Unit of Force: The SI unit is kg m s⁻², which is called a newton (N).

4.4 Applications of the Second Law

The principle Ft = mv - mu (Force × time = change in momentum) explains many phenomena:

• Catching a Cricket Ball: A fielder pulls his hands back while catching a ball. This increases the time (t) over which the ball’s momentum is reduced to zero. Increasing the time decreases the rate of change of momentum, thus reducing the force (F) exerted on the fielder’s hands.
• High Jump: Athletes land on a cushioned or sand bed. This increases the time it takes for them to stop, reducing the force of impact on their bodies.

5. Newton’s Third Law of Motion

5.1 Statement of the Third Law

When one object exerts a force on another object, the second object instantaneously exerts a force back on the first. These two forces are always equal in magnitude but opposite in direction.

An alternative statement is:

To every action, there is an equal and opposite reaction.

5.2 Key Characteristics

• Action-Reaction Pairs: Forces always occur in pairs.
• Act on Different Objects: The action force and the reaction force never act on the same object. Therefore, they cannot cancel each other out.
• Simultaneous: The action and reaction forces occur at the same instant.

5.3 Examples

• Walking: To walk forward, you push the ground backward with your feet (action). The ground exerts an equal and opposite force on your feet, pushing you forward (reaction).
• Gun Recoil: When a gun is fired, it exerts a forward force on the bullet (action). The bullet exerts an equal and opposite force on the gun, causing it to recoil (reaction). The gun’s acceleration is much less than the bullet’s because its mass is much greater.
• Sailor and Boat: When a sailor jumps forward out of a boat, their feet push the boat backward (action). The boat pushes the sailor forward (reaction), but also moves backward due to the sailor’s push.
• Spring Balances: Two connected spring balances pulled apart will show the same reading, demonstrating that the force exerted by one on the other is equal and opposite.

*****************************************************************************************

Q&A Section

Short-Answer Questions (25 Questions)

1. What was the common belief about the “natural state” of an object before the work of Galileo and Newton?
2. List three distinct effects that a force can have on an object.
3. Define balanced forces and explain what happens to an object when subjected only to balanced forces.
4. What is an unbalanced force, and what is its primary effect on an object?
5. Explain the role of friction when a person tries to push a heavy box that initially does not move.
6. Describe Galileo’s thought experiment with a marble on a frictionless horizontal plane and what it suggests about motion.
7. State Newton’s First Law of Motion in your own words.
8. What is inertia, and why is the first law also called the law of inertia?
9. How is the mass of an object related to its inertia? Provide an example.
10. Using the concept of inertia, explain why passengers fall forward when a moving bus stops suddenly.
11. Using the concept of inertia, explain why leaves may fall when you shake a tree branch vigorously.
12. Define momentum and state its SI unit.
13. Is momentum a scalar or a vector quantity? Explain your reasoning.
14. State Newton’s Second Law of Motion in terms of momentum.
15. What is the mathematical formula for Newton’s Second Law, and what do the variables represent?
16. Define one newton (N), the SI unit of force.
17. Explain why a cricket fielder pulls his hands back while catching a fast-moving ball.
18. How does landing on a cushioned bed help a high jump athlete avoid injury?
19. State Newton’s Third Law of Motion.
20. In an action-reaction pair, do the two forces act on the same object or different objects? Why is this important?
21. Explain why a gun recoils when it is fired.
22. Use Newton’s Third Law to describe the forces involved when a person walks on the ground.
23. If action and reaction forces are equal and opposite, why don’t they cancel each other out?
24. A sailor jumps off a boat onto a dock. Explain why the boat moves backward.
25. When an insect hits the windshield of a fast-moving car, which experiences a greater force: the insect or the car? Explain using Newton’s laws.

Multiple Choice Questions (20 Questions)

1. The property of an object to resist a change in its state of motion is called:
   • a) Force b) Momentum c) Inertia d) Acceleration
2. According to Galileo’s experiments, what is required to change the motion of an object?
   • a) A continuous force b) An unbalanced force c) Balanced forces d) A large mass
3. If a block is pulled from two opposite sides with equal forces of 10 N each, the net force on the block is:
   • a) 20 N b) 10 N c) 0 N d) Impossible to determine
4. Mass is a measure of an object’s:
   • a) Velocity b) Inertia c) Force d) Momentum
5. Newton’s First Law of Motion is also known as:
   • a) The law of momentum b) The law of action-reaction c) The law of uniform motion d) The law of inertia
6. An object moving with uniform velocity has:
   • a) An unbalanced force acting on it b) No net external force acting on it c) A changing momentum d) A constant acceleration
7. The SI unit of momentum is:
   • a) kg m s⁻² b) N m c) kg m s⁻¹ d) kg / m s⁻¹
8. Newton’s Second Law of Motion relates force to:
   • a) The change in position b) The rate of change of velocity c) The rate of change of momentum d) The product of mass and distance
9. The formula F = ma is a mathematical expression of:
   • a) The First Law of Motion b) The Second Law of Motion c) The Third Law of Motion d) The Law of Inertia
10. A force of 1 newton is the force that gives a mass of 1 kg an acceleration of:
    • a) 1 m s⁻¹ b) 9.8 m s⁻² c) 1 cm s⁻² d) 1 m s⁻²
11. When you vigorously shake a tree branch, leaves get detached because of:
    • a) The force of the shake b) The reaction force from the leaves c) The inertia of the leaves d) The lack of friction
12. The momentum of an object with mass m and velocity v is given by:
    • a) mv² b) ½ mv² c) ma d) mv
13. When catching a fast-moving cricket ball, a fielder increases the time of impact to:
    • a) Increase the ball’s final momentum b) Decrease the force exerted by the ball c) Increase the force exerted by the ball d) Maintain the ball’s velocity
14. According to Newton’s Third Law, for every action there is:
    • a) A smaller reaction b) An equal and opposite reaction c) A larger reaction d) No reaction
15. The forces in an action-reaction pair always:
    • a) Act on the same object b) Cancel each other out c) Act on two different objects d) Are perpendicular to each other
16. The recoil of a gun is an example of:
    • a) Newton’s First Law b) Newton’s Second Law c) Newton’s Third Law d) The concept of friction
17. Which of the following has the most inertia?
    • a) A five-rupees coin b) A rubber ball c) A bicycle d) A train
18. What force opposes the motion of a bicycle when a rider stops pedalling?
    • a) Momentum b) The rider’s inertia c) The force of friction d) Balanced forces
19. To accelerate an object, one needs to apply:
    • a) Inertia b) Momentum c) A balanced force d) An unbalanced force
20. A sailor jumps forward from a boat. The boat moves backward because:
    • a) The boat has less inertia than the sailor. b) The boat exerts a reaction force on the water. c) The sailor exerts a backward action force on the boat. d) The sailor’s momentum is transferred to the boat.

*****************************************************************************************

Answer Keys

Short-Answer Questions Answer Key

1. Before Galileo and Newton, the common belief was that rest is the “natural state” of an object. This was based on everyday observations where moving objects eventually come to a stop.
2. A force can change an object’s state of motion (start or stop it), change the magnitude of its velocity (speed it up or slow it down), or change its direction of motion. It can also change the shape and size of an object.
3. Balanced forces are equal forces acting in opposite directions on an object. When an object is subjected only to balanced forces, its state of rest or of uniform motion does not change.
4. An unbalanced force is a net force that is not zero, occurring when opposing forces are unequal or a force is unopposed. Its primary effect is to cause a change in the object’s motion, meaning it causes acceleration.
5. When a person pushes a heavy box, a friction force arises between the box and the floor, acting in the opposite direction. If the box does not move, it is because this friction force is equal to (and balances) the pushing force.
6. Galileo argued that if a marble rolls down one incline and up another, it tries to reach the same height. If the second plane were made horizontal and frictionless, the marble would travel forever, suggesting that no net force is needed to sustain uniform motion.
7. An object at rest will stay at rest, and an object in motion will stay in motion with the same speed and in the same direction, unless acted upon by an unbalanced force.
8. Inertia is the natural tendency of any object to resist a change in its state of motion or rest. The first law is called the law of inertia because it is a formal statement of this principle.
9. Mass is the quantitative measure of inertia; the more massive an object is, the greater its inertia. For example, a train has much more inertia than a bicycle because it has a much larger mass.
10. When the bus stops, the passengers’ feet stop with it. However, due to inertia, their upper bodies tend to continue moving forward at the same speed the bus was travelling, causing them to lurch forward.
11. When the branch is shaken, it is set into motion. The leaves, due to their inertia, tend to remain in their state of rest. This resistance to motion causes them to become detached from the moving branch.
12. Momentum is defined as the product of an object’s mass and its velocity (p = mv). Its SI unit is the kilogram-metre per second (kg m s⁻¹).
13. Momentum is a vector quantity. This is because it is the product of mass (a scalar) and velocity (a vector), so it has both magnitude and the same direction as the velocity.
14. The rate of change of momentum of an object is proportional to the applied unbalanced force and occurs in the direction of the force.
15. The formula is F = ma. In this equation, F represents the net unbalanced force acting on the object, m is the mass of the object, and a is the resulting acceleration.
16. One newton (N) is defined as the amount of force required to produce an acceleration of 1 m s⁻² in an object with a mass of 1 kg.
17. By pulling his hands back, the fielder increases the time over which the ball’s high velocity is reduced to zero. This decreases the rate of change of momentum, which in turn reduces the magnitude of the force on his hands.
18. The cushioned bed increases the time it takes for the athlete’s body to come to a stop after the jump. This longer time duration decreases the rate of change of momentum, thereby reducing the impact force on the athlete.
19. Newton’s Third Law states that for every action, there is an equal and opposite reaction. These forces are equal in magnitude and opposite in direction.
20. In an action-reaction pair, the forces act on two different objects. This is important because if they acted on the same object, they would cancel out, and no acceleration would be possible.
21. When the gun exerts a forward force on the bullet (the action), the bullet simultaneously exerts an equal and opposite force back on the gun (the reaction). This reaction force causes the gun to move backward, which is known as recoil.
22. A person pushes backward on the ground with their foot (action). In response, the ground exerts an equal and opposite force forward on the person’s foot (reaction), which propels the person forward.
23. Action and reaction forces do not cancel each other out because they act on two different objects. To cancel, forces must act on the same object.
24. As the sailor jumps forward, he pushes backward on the boat with his feet (action). The boat, in turn, pushes forward on the sailor (reaction). The backward push on the boat causes it to move in that direction.
25. According to Newton’s Third Law, the force the car exerts on the insect is exactly equal in magnitude and opposite in direction to the force the insect exerts on the car. Both experience the same force.

Multiple Choice Questions Answer Key

1. c) Inertia
2. b) An unbalanced force
3. c) 0 N
4. b) Inertia
5. d) The law of inertia
6. b) No net external force acting on it
7. c) kg m s⁻¹
8. c) The rate of change of momentum
9. b) The Second Law of Motion
10. d) 1 m s⁻²
11. c) The inertia of the leaves
12. d) mv
13. b) Decrease the force exerted by the ball
14. b) An equal and opposite reaction
15. c) Act on two different objects
16. c) Newton’s Third Law
17. d) A train
18. c) The force of friction
19. d) An unbalanced force
20. c) The sailor exerts a backward action force on the boat.

*****************************************************************************************

Essay Questions (10 Questions with Answers)

1. Explain the evolution of thought from the “natural state of rest” to Newton’s First Law of Motion, highlighting the key contributions of Galileo.
   • Answer: For centuries, philosophers believed that rest was the natural state for all objects, a conclusion drawn from observations where moving objects like a rolling ball eventually stop. This implied that a continuous force was needed to sustain motion. Galileo Galilei fundamentally challenged this view through his experiments with inclined planes. He observed that a marble’s velocity increases rolling down an incline and decreases rolling up one. From this, he deduced that on a perfectly horizontal, frictionless surface, an object in motion would continue to move with a constant velocity forever, as there would be no force to slow it down. This crucial insight—that an unbalanced force is required not to sustain motion, but to change it—laid the groundwork for Newton. Newton formalized this into his First Law of Motion, stating that an object remains in a state of rest or uniform motion in a straight line unless compelled to change that state by an applied force. The law also introduced the concept of inertia, the inherent resistance of an object to changes in its state of motion.
2. Using Newton’s first and second laws, explain the relationship between unbalanced force, mass, and acceleration. Why is it harder to move a train than a bicycle?
   • Answer: Newton’s First Law states that an unbalanced force is required to change an object’s state of motion (i.e., to cause acceleration). Newton’s Second Law quantifies this relationship with the formula F = ma, where F is the unbalanced force, m is mass, and a is acceleration. This equation shows that for a given mass, a larger force produces a larger acceleration. It also shows that for a given desired acceleration, a larger mass requires a larger force. The concept of inertia, defined in the first law and measured by mass, explains why this is the case. It is harder to move a train than a bicycle because the train has a much larger mass, and therefore, much greater inertia. Inertia is the resistance to a change in motion. To produce any change in the train’s state of rest (an acceleration), a significantly larger unbalanced force is needed to overcome its massive inertia compared to the small force needed to overcome the bicycle’s small inertia.
3. Describe Newton’s Third Law of Motion and use the examples of a recoiling gun and a person walking to illustrate its principles. Why don’t the action-reaction forces cancel each other out?
   • Answer: Newton’s Third Law states that for every action, there is an equal and opposite reaction. This means that forces always occur in pairs; when one object exerts a force on a second object, the second object simultaneously exerts a force of equal magnitude and opposite direction on the first.
   • In the case of a recoiling gun, the gun exerts a forward force on the bullet (action). The bullet, in turn, exerts an equal and opposite force on the gun (reaction), causing it to move backward or recoil.
   • When a person walks, their foot pushes backward on the ground (action). The ground exerts an equal and opposite forward force on their foot (reaction), which propels the person forward.
   • These action-reaction forces do not cancel each other out because they act on two different objects. The action force (gun on bullet) acts on the bullet, while the reaction force (bullet on gun) acts on the gun. Since they act on different bodies, they cannot be summed to zero and both objects experience a force.
4. Define momentum and explain its importance in understanding the effects of collisions, using the examples of a fast-moving cricket ball and a slow-moving truck.
   • Answer: Momentum is a physical quantity defined as the product of an object’s mass and its velocity (p = mv). It is a measure of the “quantity of motion” an object has. The importance of momentum is that it combines both mass and velocity to describe the impact an object can produce. A large impact can be caused by an object with a small mass moving at a very high velocity, or by an object with a very large mass moving at a slow velocity.
   • For example, a fast-moving cricket ball, despite its small mass, has significant momentum due to its high velocity and can hurt a spectator upon impact. Conversely, a slow-moving truck (e.g., at 5 m s⁻¹), despite its low velocity, has enormous momentum due to its large mass and can be lethal. These examples show that both mass and velocity are crucial for determining the effect of a collision, a concept captured by momentum.
5. Explain how the mathematical formulation of Newton’s Second Law (F = ma) can be derived from the statement about the rate of change of momentum.
   • Answer: Newton’s Second Law states that the applied unbalanced force (F) is proportional to the rate of change of momentum. Let’s consider an object of mass m, whose velocity changes from an initial value u to a final value v over a time t.
   • The initial momentum is p₁ = mu, and the final momentum is p₂ = mv.
   • The change in momentum is Δp = p₂ - p₁ = mv - mu = m(v - u).
   • The rate of change of momentum is the change in momentum divided by time: Δp / t = m(v - u) / t.
   • According to the law, F ∝ m(v - u) / t.
   • We know that acceleration a is the rate of change of velocity, so a = (v - u) / t. Substituting this into the proportionality gives F ∝ ma.
   • To turn this into an equation, we introduce a constant of proportionality, k, giving F = kma. The unit of force, the newton (N), is specifically defined to make k=1, which simplifies the final, widely used formula to F = ma.
6. Use Newton’s Second Law to explain the technique used by a fielder to catch a cricket ball and by organizers of a high jump event to protect athletes.
   • Answer: Newton’s Second Law can be expressed as F = (mv - mu) / t, which means the force (F) is equal to the rate of change of momentum. This can be rearranged to Ft = mv - mu. This relationship shows that for a given change in momentum (e.g., bringing a moving object to rest), the force experienced is inversely proportional to the time over which the change occurs.
   • When catching a cricket ball, the fielder’s goal is to reduce the ball’s momentum to zero. By pulling their hands backward, the fielder increases the time (t) of the impact. This increased time decreases the rate of change of momentum, and therefore significantly reduces the force (F) exerted by the ball on their hands, preventing injury.
   • Similarly, in a high jump event, athletes land on a cushioned bed or sand. These soft surfaces increase the time it takes for the athlete to come to a complete stop after landing. This longer duration of impact decreases the rate of change of the athlete’s momentum, thus reducing the force exerted on their body and preventing injury.
7. Compare and contrast balanced and unbalanced forces. Provide real-world examples for each, including the role of friction.
   • Answer: Balanced forces are two or more forces of equal magnitude acting on an object in opposite directions. Their net effect is zero, so they do not cause a change in the object’s state of motion. An object at rest remains at rest, and an object in uniform motion continues its uniform motion. An example is a book resting on a table, where the force of gravity pulling it down is perfectly balanced by the normal force from the table pushing it up.
   • Unbalanced forces occur when the net force on an object is not zero. An unbalanced force is required to change an object’s state of motion—that is, to cause acceleration (starting, stopping, speeding up, slowing down, or changing direction). An example is kicking a football; the force of the kick is an unbalanced force that changes the ball’s state from rest to motion.
   • Friction plays a key role. When children first push a heavy box, it may not move because the static friction force is equal and opposite to their push, creating a balanced force situation. To move it, they must push harder to create an unbalanced force where their push exceeds the friction. Once moving at a constant velocity, the pushing force may again be balanced by the kinetic friction force.
8. Discuss the statement: “Action and reaction forces are always equal in magnitude, but they may not produce accelerations of equal magnitudes.” Use the Earth-and-falling-apple system or a gun-and-bullet system as an example.
   • Answer: This statement is correct and highlights the interplay between Newton’s Third Law and Second Law. The Third Law states that the action force and reaction force are always equal in magnitude. However, the effect of that force—the acceleration—depends on the mass of the object it acts upon, as described by the Second Law (a = F/m).
   • Consider a gun firing a bullet. The gun exerts a large forward force on the bullet (action). The bullet exerts an equal and opposite backward force on the gun (reaction). The forces are identical in magnitude. However, the bullet has a very small mass (m_bullet), so it experiences a very large acceleration (a_bullet = F / m_bullet). The gun has a much larger mass (m_gun), so it experiences a much smaller backward acceleration, which is the recoil (a_gun = F / m_gun). Thus, equal forces produce vastly different accelerations due to the difference in mass.
9. An insect hits the windshield of a moving motorcar. A student argues that the motorcar exerted a larger force on the insect, causing it to die. Another student argues the insect and motorcar experienced the same force. Who is correct and why? Explain the difference in the change of momentum for both.
   • Answer: The student who argued that the insect and motorcar experienced the same force is correct. This is a direct application of Newton’s Third Law of Motion: the force exerted by the motorcar on the insect is exactly equal in magnitude and opposite in direction to the force exerted by the insect on the motorcar.
   • The reason for the vastly different outcomes is explained by Newton’s Second Law (F = ma or FΔt = Δp). Although the force (F) is the same for both, their masses are vastly different. The insect, with its tiny mass, undergoes an enormous and fatal deceleration. The motorcar, with its huge mass, experiences an infinitesimally small and unnoticeable deceleration from the same force.
   • According to the second law, the change in momentum (Δp = FΔt) is also the same for both the car and the insect, but in opposite directions. The insect’s velocity changes drastically (e.g., from its flying speed to the car’s speed, or to zero), resulting in a large change in velocity. The motorcar’s momentum also changes by the same amount, but because its mass is so large, the corresponding change in its velocity is negligible.
10. Explain how the everyday experience of riding in a bus or car (starting, stopping, and turning) can be used to illustrate Newton’s First Law of Motion (the Law of Inertia).
    • Answer: Newton’s First Law, the Law of Inertia, states that an object resists changes in its state of motion. This is clearly demonstrated when riding in a vehicle.
    • Stopping: When a moving bus brakes suddenly, passengers lurch forward. The vehicle stops, and the passengers’ feet, in contact with the floor, also stop. However, their upper bodies, due to inertia, tend to continue moving forward with the same velocity the bus had, causing the forward lurch. Safety belts are designed to apply a force to counter this inertia.
    • Starting: When a bus accelerates from rest, passengers are pushed backward. The bus and the passengers’ feet begin to move forward. Due to inertia, the upper bodies of the passengers tend to remain in their state of rest. As the lower body moves forward with the bus, the upper body’s resistance to this change in motion makes it feel as if it is being pushed back.
    • Turning: When a car makes a sharp turn, passengers tend to be thrown to the outside of the turn. This is because their bodies, due to inertia, want to continue moving in a straight line (the original direction of travel). As the car turns, it applies an unbalanced force to change its direction, but the passengers’ bodies resist this change and try to maintain their straight-line path, causing them to press against the side of the car.

*****************************************************************************************

Glossary of Key Terms

Term	Definition
Acceleration	The rate of change of velocity.
Action-Reaction Forces	The pair of opposing forces described by Newton’s Third Law. They are always equal in magnitude, opposite in direction, and act on two different objects.
Balanced Forces	Forces of equal magnitude acting from opposite directions on an object. They do not change the state of rest or of motion of an object.
Force	A push, hit, or pull that, when unbalanced, can change an object’s state of motion, magnitude of velocity, direction of motion, or its shape and size.
Friction	A force that arises between two surfaces in contact, acting in a direction opposite to the motion or intended motion.
Inertia	The natural tendency of an object to resist a change in its state of motion or of rest. It is the property described by Newton’s First Law.
Law of Inertia	Another name for Newton’s First Law of Motion.
Mass	A quantitative measure of the inertia of an object. The SI unit is the kilogram (kg).
Momentum (p)	The product of an object’s mass (m) and velocity (v), given by the formula p = mv. Its SI unit is kg m s⁻¹.
Newton (N)	The SI unit of force. One newton is the force required to produce an acceleration of 1 m s⁻² in an object of mass 1 kg.
Newton’s First Law	An object remains in a state of rest or of uniform motion in a straight line unless compelled to change that state by an applied force.
Newton’s Second Law	The rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of the force. Mathematically expressed as F = ma.
Newton’s Third Law	To every action, there is an equal and opposite reaction. These forces act on two different bodies.
Unbalanced Force	A net force acting on an object that is not zero. An unbalanced force is required to change an object’s state of motion (i.e., to cause acceleration).
Uniform Motion	Motion in a straight line at a constant velocity.
Velocity	The rate of change of an object’s position with respect to a frame of reference; it has both speed and direction.