CBSE – 9th Standard Science

Class 9 Question Bank Chapter 9 – Force and laws of Motion


Q1. What is the First Law of Motion?

Ans. 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.

Q2. Give an example of first law of motion?

Ans. When a motorcar makes a sharp turn at a high speed, we tend to get thrown to one side.

Q3. What is Inertia?

Ans. The tendency of undisturbed objects to stay at rest or to keep moving with the same velocity is called inertia.

Q4. What is the another name of First law of Motion?

Ans. The first law of motion is also known as the law of inertia.

Q5. Which objects offer larger Inertia?

Ans. Heavier or more massive objects offer larger inertia.

Q6. What is the relation between Inertia and mass?

Ans. Inertia is the natural tendency of an object to resist a change in its state of motion or of rest. The mass of an object is a measure of its inertia.

Q7. What is 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 applied unbalanced force in the direction of force.

Q8. Give an example of second law of motion?

Ans. A small mass, such as a bullet may kill a person when fired from a gun.

Q9. What is Momentum?

Ans. The momentum, p of an object is defined as the product of its mass, m and velocity, v.

Q10. What is the S.I unit of momentum?

Ans. The SI unit of momentum is kilogram-meter per second (kg m s-1).

Q11. What do you mean by one unit of force?

Ans. One unit of force is defined as the amount that produces an acceleration of 1 m s-2 in an object of 1 kg mass.

Q12. What is the unit of force?

Ans. The unit of force is kg m s-2 or Newton which has the symbol N.

Q13. What is the Third law of motion?

Ans. The third law of motion states that 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.

Q14. Give an example of third law of motion?

Ans. When a sailor jumps out of a rowing boat. As the sailor jumps forward, the force on the boat moves it backwards

Q15. What is the law of conservation of momentum?

Ans. The total momentum of the two objects is unchanged or conserved by the collision.

Q16. How to relate force of friction and motion of object?

Ans. Force of friction always opposes motion of objects.

Q17. Where there is no external force what will happen to total momentum?

Ans. Where there is no external force, the total momentum remains conserved.

Q18. What do you mean by recoil of the gun?

Ans. When a gun is fired, it exerts a forward force on the bullet. The bullet exerts an equal and opposite reaction force on the gun. This results in the recoil of the gun.

Q19. What are the two opposing forces?

Ans. The two opposing forces are known as action and reaction forces.

Q20. What do you say about action and reaction forces?

Ans. The two forces are always equal in magnitude but opposite in direction.

Try these:-

Q.1   A girl of mass 40 kg jumps with a horizontal velocity of 5 m s-1 onto a stationary cart with frictionless wheels. The mass of the cart is 3 kg. What is her velocity as the cart starts moving? Assume that there is no external unbalanced force working in the horizontal direction.

Q.2   Two hockey players of opposite teams, while trying to hit a

hockey ball on the ground collide and immediately become entangled. One has a mass of 60 kg and was moving with a velocity 5.0 m s–1 while the other has a mass of 55 kg and was moving faster with a velocity 6.0 m s–1 towards the first player. In which direction and with what velocity will they move after they become entangled? Assume that the frictional force acting between the feet

of the two players and ground is negligible

Q.3   A constant force acts on an object of mass 5 kg for a duration of 2 s. It increases the object’s velocity from 3 m s–1 to 7 m s-1. Find the magnitude of the applied force. Now, if the force was applied for a duration of 5 s, what would be the final velocity of object?

Q.4   A motorcar is moving with a velocity of 108 km/h and it takes 4 s to stop after the brakes are applied. Calculate the force exerted by the brakes on the motorcar if its mass along with the passengers is 1000 kg.

Q.5   A force of 5 N gives a mass m1, an acceleration of 10 m s–2 and a mass m2, an acceleration of 20 m s-2. What acceleration would it give if both the masses were tied together?