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

Q1: Name two factors which determine the momentum of a body.
Ans: Two factors on which momentum of a body depend are mass and velocity. Momentum is directly proportional to the mass and velocity of the body.

Q2: If a man jumps out from a boat, the boat moves backwards. Why?
Ans: When a man jumps out of a boat to the bank of the river, he supplies a force in the forward direction. Due to the reaction of this, the boat moves backwards.

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: The time of impact of vehicles is very small at high speed. So, they exert very large forces on each other. Hence, road accidents at high speeds are highly fatal.

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 in motion?
Ans: Unbalanced force.

Q7: Name the scientist who introduced the property of momentum.
Ans: Newton.

Q8: Why do athletes run some distance before jumping?
Ans: Athlete has the inertia of motion and thus continues to move past the line.

Q9: What decides the rate of change of momentum of an object?
Ans: The rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of force.

Q10: The fruits fall off the branches when a strong wind blows. Give reason.
Ans: Fruits tend to continue in the state of rest on account of inertia while branches suddenly come into motion.

Q11: Define force.
Ans: It is a push or pull on an object that produces acceleration in the body on which it acts.

Q12: If a ball is thrown up in a moving train, it comes back to the person’s hands. Why?
Ans: This is because no horizontal force acts on it. It moves with the same horizontal speed.

Q13: State Newton’s third law of motion.
Ans: To every action, there is an equal and opposite reaction and they act on two different bodies.

Q14: What is frictional force?
Ans: The force that always opposes the motion of an object is called the force of friction.

Q15: Define SI unit of force. or What is one Newton force?
Ans: A Newton is that force which acts on a body of mass of 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.

Q17: It is easier to push an empty box than to push the box full of books. Give reason.
Ans: This is because the empty box has less inertia of rest than the box full of books.

Q18: Is force a scalar quantity or a vector quantity?
Ans: Force is a vector quantity. It has both magnitude and direction.

Q19: What is inertia?
Ans: The natural tendency of an object to resist a change in its state of rest or of uniform motion is called 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: Steel ball because the density of steel is maximum. For the same shape and volume, the steel ball will have the maximum mass.

Q21: No force is required to move an object with constant velocity. Why?
Ans: For an object moving with a constant velocity, acceleration is 0, so force is 0 as well.

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 backward, 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 of mass 20 kg will have more inertia, as inertia is a measure of mass.

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

Q26: State Newton’s third law of motion.
Ans: To every action, there is an equal and opposite reaction, and they act on two different bodies.

Q27: What is momentum?
Ans: The momentum of an object is the product of its mass and velocity and has the same direction as that of the velocity. The SI unit is kg m/s. (p = mv)

Q28: If a man jumps out from a boat, the boat moves backwards. Why?
Ans: When a man jumps out of a boat to the bank of the river, he supplies a force in the forward direction. Due to the reaction of this, the boat moves backwards.

Q29: What decides the rate of change of momentum of an object?
Ans: The rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of force.

Q30: Why action and reaction do not cancel each other?
Ans: Action and reaction act simultaneously but on different objects. Hence, they do not cancel each other.

Q31: What is balanced force?
Ans: When forces acting on a body from the opposite direction do not change the state of rest or of motion of an object, such forces are called balanced forces.