Class 9 Science Chapter 8 Question Answers - Force and Laws of Motion

Q1: Name two factors which determine the momentum of a body.
Ans: The two factors that determine the momentum of a body are its mass and velocity.

Q2: If a man jumps out of a boat, the boat moves backwards. Why?
Ans: When a man jumps forward from a boat, he exerts a force on the boat in the backwards direction. As a reaction, the boat moves backwards. This is the example and application of Newton's third law of motion.

Q3: What is the direction of momentum?
Ans: The direction of momentum is the same as that of velocity.

Q4: Why are road accidents at high speeds very much worse than accidents at low speeds?
Ans: At high speeds, the momentum is large and the time of impact is small, resulting in a greater force during collision. 

Q5: State Newton’s first law of motion.
Ans: An object remains in a state of rest or of uniform motion in a straight line unless acted upon by an external unbalanced force.

Q6: Which type of force brings an object into motion?
Ans: Unbalanced force

Q7: Name the scientist who introduced the property of momentum.
Ans:  Sir Isaac Newton introduced the concept of momentum as part of his laws of motion.

Q8: Why do athletes run some distance before jumping?
Ans: To gain momentum for a longer jump.

Q9: What decides the rate of change of momentum of an object?
Ans: he rate of change of momentum of an object depends on the unbalanced force applied to it. (Newton’s second law).

Q10: The fruits fall off the branches when a strong wind blows. Give a reason.
Ans: Fruits fall off branches during strong winds due to the principle of inertia. 

Q11: Define force.
Ans: Force is a push or pull that changes or tends to change the state of motion of an object.  Mathematically, according to Newton's second law of motion, F = ma.

Q12: If a ball is thrown up in a moving train, it comes back into the person’s hands. Why?
Ans: If a ball is thrown up in a moving train, it comes back into the person’s hands because it retains the forward motion of the train due to inertia, so it comes back to the thrower’s hand

Q13: State Newton’s third law of motion.
Ans: Newton's third law of motion states that, for every action, there is an equal and opposite reaction.

Q14: What is a frictional force?
Ans: The frictional force is the force that opposes the motion of an object. It acts between two surfaces in contact.

Q15: Define the SI unit of force. Or what is one Newton force?
Ans: A Newton is the force which acts on a body of mass 1 kg to produce an acceleration of 1 m/s² in it.

Q16: Name the physical quantity which is determined by the rate of change of linear momentum.
Ans: Force is the physical quantity that results from the rate of change of linear momentum.

Q17: It is easier to push an empty box than to push a box full of books. Give a reason.
Ans: It is easier to push an empty box than to push a box full of books because an empty box has less mass, it has less inertia, so it's easier to push.

Q18: Is force a scalar quantity or a vector quantity?
Ans: Force is a vector quantity. 

Q19: What is inertia?
Ans: The natural tendency of an object to resist a change in its state of rest or uniform motion is known as inertia. 

Q20: There are three solid balls, made up of aluminium, steel, and wood of the same shape and volume. Which of them would have the highest inertia? Why?
Ans: The ball made of steel has the highest inertia among the three materials because steel has the highest density, so it has the greatest mass and therefore the highest inertia.

Q21: No force is required to move an object with constant velocity. Why?
Ans:  According to Newton’s first law, no force is needed to keep an object in motion with constant velocity in the absence of external forces like friction.

Q22: Explain our walking in terms of Newton’s third law of motion.
Ans: When we walk on the ground, our foot pushes the ground backwards, and in return, the ground pushes our foot forward. The forward reaction exerted by the ground on our foot makes us walk forward.
Q23: Which will have more inertia, a body of mass 10 kg or a body of mass 20 kg?
Ans: A body with a mass of 20 kg will have more inertia than a body with a mass of 10 kg.

Q24: Which type of force does not change the state of rest or of motion of an object?
Ans: Balanced force

Q25: State Newton’s second law of motion.
Ans: The second law of motion states that the rate of change of momentum of an object is proportional to the unbalanced force applied. This change occurs in the direction of the applied force.

Q26: What is momentum?
Ans: Momentum is the product of mass and velocity. It is a vector quantity and is measured in kg·m/s.

Q27: Why do action and reaction not cancel each other?
Ans: Action and reaction do not cancel each other because they act on different objects.

Q28: What is a balanced force?
Ans: Balanced forces are forces that act on an object from opposite directions and do not change its state of rest or motion. 

Q29: What is the momentum of a body at rest?
Ans: Zero.