Chapter 8: Force and Laws of Motion Long Questions | science Class 9 CBSE

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Chapter 8: Force and Laws of Motion Long Questions | science Class 9 CBSE | ByteNotes.in

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Question 1

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Q1. What is force? Explain how force can affect the motion and shape of an object with examples.

• Force is a push or pull acting upon an object due to interaction with another object.
• It can change the state of motion of an object (start, stop, accelerate, or decelerate).
• Force can also change the direction of motion of an object.
• It can alter the shape and size of objects (e.g., squeezing a rubber ball).
• Pushing a stationary box can move it; hitting a spring stretches it.
• Examples: Pulling a drawer, kicking a football, compressing a spring.

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Question 2

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Q2. Differentiate between balanced and unbalanced forces. Give suitable examples.

• Balanced forces are equal in magnitude and opposite in direction; they do not change the state of motion.
• Unbalanced forces are unequal and cause a change in motion or rest.
• Balanced force example: Two people pulling a block with equal force from opposite sides — the block doesn't move.
• Unbalanced force example: One person pulling harder causes the block to move in that direction.
• Balanced forces maintain the status quo; unbalanced forces result in acceleration or deceleration.
• Motion only changes when unbalanced force acts.

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Question 3

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Q3. State Newton’s First Law of Motion. Explain it with examples from daily life.

• First Law: An object remains in a state of rest or uniform motion unless acted upon by an unbalanced external force.
• Also known as the law of inertia.
• A body in motion resists change to its velocity due to inertia.
• Example: Passengers lurch forward when a moving bus suddenly stops.
• Example: Coin falling straight into a glass when a card is flicked away quickly.
• Inertia depends on the mass of the object.

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Question 4

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Q4. What is inertia? How is it related to mass? Give examples to support your answer.

• Inertia is the natural tendency of an object to resist changes in its state of motion or rest.
• Mass is a measure of inertia — higher the mass, greater the inertia.
• Example: A train resists motion more than a bicycle when equal force is applied.
• A coin needs more force to move compared to a similar-sized rubber ball.
• Inertia explains why objects at rest stay at rest unless pushed or pulled.
• Greater inertia implies greater force needed to change motion.

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Question 5

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Q5. Explain Galileo’s observations on motion and how they contributed to Newton’s First Law.

• Galileo observed that objects move with constant velocity when no external force acts.
• He used inclined planes to study motion and noticed that friction slowed down objects.
• On a frictionless surface, an object would continue moving indefinitely.
• This led to the idea that no force is needed to sustain uniform motion.
• Newton built upon this concept to frame his First Law of Motion.
• Galileo’s experiments challenged the earlier belief that rest is the natural state.

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Question 6

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Q6. State and mathematically express Newton’s Second Law of Motion. What does it imply?

• Second Law: The rate of change of momentum of an object is directly proportional to the applied unbalanced force.
• Mathematically: F = ma, where F is force, m is mass, and a is acceleration.
• Force causes acceleration depending on the mass of the object.
• It provides a way to measure force.
• Example: A larger force causes greater acceleration in lighter objects.
• SI unit of force is newton (N).

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Question 7

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Q7. What is momentum? Derive the relation between force and the rate of change of momentum.

• Momentum (p) = mass × velocity, i.e., p = mv.
• It has both magnitude and direction (vector quantity).
• Change in momentum = final momentum – initial momentum.
• Force = Rate of change of momentum = (mv – mu)/t.
• This derivation leads to F = ma, Newton's Second Law.
• Greater the rate of momentum change, higher the force.

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Question 8

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Q8. Describe an activity to demonstrate Newton’s First Law of Motion using carom coins.

• Stack a few carom coins vertically on a table.
• Strike the bottom coin sharply with a striker.
• The bottom coin moves out, and the rest fall vertically.
• This demonstrates that upper coins remain at rest due to inertia.
• Their state of rest is changed only when an unbalanced force acts.
• Supports the concept of inertia and Newton’s First Law.

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Question 9

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Q9. A cricket fielder pulls his hands back while catching a fast ball. Explain why using Newton’s Second Law.

• Pulling hands back increases the time of impact.
• According to Second Law, force = change in momentum/time.
• Increasing time reduces force experienced by the hands.
• This prevents injury and provides a smooth catch.
• Change in momentum is same; reduced acceleration lowers force.
• Common example of impulse control using time.

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Question 10

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Q10. State Newton’s Third Law of Motion and explain it with practical examples.

• Third Law: For every action, there is an equal and opposite reaction.
• Forces act in pairs on two different bodies.
• Example: When you push a wall, it pushes back with equal force.
• Jumping out of a boat causes the boat to move backward.
• Recoil of a gun is opposite to bullet's direction.
• Walking involves pushing the ground backward, which pushes you forward.

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Chapter 8: Force and Laws of Motion | Long Questions

Q1. What is force? Explain how force can affect the motion and shape of an object with examples.

Q2. Differentiate between balanced and unbalanced forces. Give suitable examples.

Q3. State Newton’s First Law of Motion. Explain it with examples from daily life.

Q4. What is inertia? How is it related to mass? Give examples to support your answer.

Q5. Explain Galileo’s observations on motion and how they contributed to Newton’s First Law.

Q6. State and mathematically express Newton’s Second Law of Motion. What does it imply?

Q7. What is momentum? Derive the relation between force and the rate of change of momentum.

Q8. Describe an activity to demonstrate Newton’s First Law of Motion using carom coins.

Q9. A cricket fielder pulls his hands back while catching a fast ball. Explain why using Newton’s Second Law.

Q10. State Newton’s Third Law of Motion and explain it with practical examples.

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