## FORCE AND LAWS OF MOTION

Class 9 - Physics

Very Short Answer Questions that you must know.

Q1: Give an example that force can make a stationary body move.

Answer: A football can be set to move by kicking it, i.e. by applying a force.

Q2: Name the scientist who experimented with moving balls on a ramp and proved that objects move with constant speed and when no force acts on them.

Q3: Force can change the shape and size of an object. Give an example.

Answer: By hammering, a block of metal can be turned into a thin sheet. By hammering, a stone can be broken into pieces.

Q4: Why does a bicycle slow down when the cyclist stop pedalling?

Answer: This is because of the frictional forces acting opposite to the direction of motion.

Q5: What are balanced forces?

Answer: If the resultant of applied forces is equal to zero, it is called balanced forces.

Q6: Which law of motion gives the measure of force?

Answer: Newton's second law of motion.

Q7: Do balanced forces cause any change of state of an object?

Q8: Write the SI and CGS unit of force.

CGS Unit: Dyne

Q9: Can balanced forces change the shape and size of an object?

Answer: Yes. e.g. when equal forces are applied from both sides over a balloon, the size and
shape of balloon is changed.

Q10: Can every force produce motion in every object?

Q11: What are unbalanced forces?

Answer: If the resultant of applied forces are greater than zero, the forces are called unbalanced forces.

Q12: When a force acts on a body, what are the two essential effects it can produce?

Answer: The effects are as follows:
It can change the state of motion of a body.
It can change the shape of the body.

Q13: Which Newton's law is also known as the Law of Inertia?

Answer: Newton’s First Law of Motion

Q14: A person standing in a moving bus falls forward if driver applies brakes suddenly. Name the inertia applying resistance here?

Q15: Define 1 Newton force.

Answer: 1 Newton is the magnitude of force which produces an acceleration of 1m/s² in a body of mass 1 kg.

Q16: What do you mean by an imapct force?

Answer: The force produced by the impact of a fast moving object on another is called impact force.

Q17: Define inertia. How it is measured?

Answer: The property of an object because of which it resists to get disturb its state is called inertia. Inertia of an object is measured by its mass.

Q18: Define force of friction.

Answer: The force acting between any two surfaces in contact and tending to oppose motion is called force of friction.

Q19: Why do we tend to fall outwards when a car takes a steep turn?

Answer: We tend to fall outwards when a car takes a steep turn due to interia of direction.

Q20: If the body is found to be accelerated, is the force acting on it balanced or unbalanced?

Q21: Is it possible that the train in which you are sitting appears to move while it is at rest?

Answer: Yes, it is possible, when two trains are at rest on the two parallel tracks. If one train is started suddenly then passengers sitting in the stationary train appear to move.

Q22: When a body moves on flat friction-less surface, will its speed change?

Q23: Which has more inertia, a table tennis ball or a rubber ball of the same size?

Answer: Rubber ball (as it is heavier than a table tennis ball)

Q24: Suppose you were pushing on a heavy bucket of stones with 100 N of force and it did not move. How many newtons of force would be balancing the force that you were applying?

Answer: 100N but in opposite direction to our pushing force.

Q25: What did Galileo conclude on the basis of his experiments on the motion of objects?

Answer: A body continues to move with the same velocity if no unbalanced force acts on it.

Q26: Is force a scalar or vector quantity?

Q27: What do you mean by a resultant force?

Answer: When two or more forces act on a body simultaneously, then the single force which produces the same effect as produced by vector sum of all the forces acting together is known as the resultant force.

Q28: State Newton's third law of motion.

Answer: To every action there is an equal an opposite reaction.

Q29: Do action and reaction act on the same body?

Answer: No, action and reaction act on different bodies.

Q30: Name the principle on which a rocket works.

Answer: Newton's third law of motion.

Q31: How is force related to acceleration produced in a body by its application?

Answer: Force is directly proportional to acceleration produced in a body by its application

Q32: What is the SI unit of impulse.

Q33: When you apply a force for a certain amount of time, it creates ________ (fill in the blank).

Q34: What is the total momentum of a bullet and a gun before firing?

Q35: Define momentum.

Answer: Momentum is a physical quantity defined as the product of mass multiplied by velocity.
i.e. p = m × v

Q36: A body is moving with uniform acceleration. Is its momentum constant?

Q37: What is the SI unit of momentum?

Q38: Name the physical quantity that corresponds to the rate of change of momentum.

Q39: What is the net momentum of gun and bullet system before firing?

Answer: The net momentum of gun and bullet system before firing is zero, as velocities of both (gun and bullet) are zero.

Q40: Which principle is involved in the working of a jet plane?

Answer:  Newton's third law of motion.

Q41: How is force related to momentum of a body?

Answer: Force is directly proportional to the rate of change of momentum.

Q42: Why mass is sometimes called coefficient of linear inertia?

Answer: It is easier to pull a lighter body than a heavier body. Therefore, more the mass more will be the inertia. That is why mass is sometimes termed as coefficient of linear inertia.

Q43: Why is the person hurt less when he falls on a spongy surface?

Answer: In case of spongy surface, the change in momentum takes longer duration of time. So force exerted on his body is less.

Q44: When a force acting on a body has equal and opposite reaction, then why should the body move at all?

Answer: The action and reaction act on different bodies. Therefore, the body moves under the action of the  acting force.

Q45: Does Newton's third law apply to a system where bodies do not actually touch each other?

Answer: Yes, whenever the bodies are in actual contact or even if there is an interaction between the bodies (e.g., attraction or repulsion between two magnets charges, etc.), Newton's third law is applicable.

Q46: What is formula of impulse?

Answer: Impulse = Force × time  