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NCERT Solutions for Class 9 Science Chapter 9
Class 9 – Science – Physics – NCERT Answers
Force and Laws of Motion – Extra questions with answers
- Question 1:
A body of mass 1 kg is kept at rest. A constant force of 6∙0 N starts acting on it. Find the time taken by the body to move through a distance of 12 m.
- Answer 1:
- Question 2:
A feather of mass 10 g is dropped from a height. It is found to fall down with a constant velocity. What is the net force acting on it?
- Answer 2:
- Question 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 to 7 m/s. Find the magnitude of the applied force. Now if the force were applied for the duration of 5 s, what would be the final velocity of the object.
- Answer 3:
10 n ; 13 m/s
- Question 4:
A motorcar is moving with a velocity of 108 km/h and it takes 4 second to stop after the brakes are applied. Calculate the force exerted by the brakes on the motorcar if the mass along with the passengers is 1000 kg.
- Answer 4:
- Question 5:
A girl of mass 40 kg jumps with a horizontal velocity of 5 m/s onto a stationery cart with friction-less 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.
- Answer 5:
- Question 6:
A bullet of mass 20 gram moving with a velocity of 300 m/s gets embedded in a freely suspended wooden block of mass 880 gram. What is the velocity acquired by the bock?
- Answer 6:
- Question 7:
A machine gun fires 25 g bullets at the rate of 600 bullets per minute with a speed of 200 m/s. calculate the force required to keep the gun in position.
- Answer 7:
- Question 8:
A truck of mass 2500 kg moving at 15 m/s collides with a car of mass 1000 kg moving at 5 m/s in the opposite direction. With what velocity would the two move together?
- Answer 8:
9∙3 m/s in the direction of the truck
- Question 9:
According to the third law o motion, when we pushed on an object, the object pushes back on us with an equal and opposite force. If the object is a massive truck parked along the roadside, it will probably not move. A student justifies this by answering that the two opposite and equal forces cancel each other. Comment on this logic and explain why the truck does not move.
- Answer 9:
When we push a massive truck parked along the roadside, it does not move. The justification given by the student that the two forces and equal forces cancel each other is totally wrong. This is because forces of action and reaction never act on one body. There is no question of their cancellation. The truck does not move because the push applied is far less than the force of friction between the truck and the road. We also do not move because the force of reaction acting on us (due to our pushing the truck) is less than force of friction between us and the road.