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Home › Grade9 › Grade 9 CBSE › Physics › Force and laws of motion › Force and laws of motion Worksheet-8

Force and laws of motion Worksheet-8

Force and laws of motion Worksheet-8

 

  1. Why is it difficult to walk on a slippery road ?

 

  1. Explain why, a runner presses the ground with his feet before he starts his run.

 

  1. A 60 g bullet fired from a 5 kg gun leaves with a speed of 500 m/s. Find the speed (velocity) with which the gun recoils (jerks backwards).

 

  1. A 10 g bullet travelling at 200 m/s strikes and remains embedded in a 2 kg target which is originally at rest but free to move. At what speed does the target move off ?

 

  1. A body of mass 2 kg is at rest. What should be the magnitude of force which will make the body move with a speed of 30 m/s at the end of 1 s ?

 

  1. A body of mass 5 kg is moving with a velocity of 10 m/s. A force is applied to it so that in 25 seconds, it attains a velocity of 35 m/s. Calculate the value of the force applied.

 

  1. A car of mass 2400 kg moving with a velocity of 20 m S–1 is stopped in 10 seconds on applying brakes. Calculate the retardation and the retarding force.

 

  1. For how long should a force of 100 N act on a body of 20 kg so that it acquires a velocity of 100 m/s ?

 

  1. How long will it take a force of 10 N to stop a mass of 2.5 kg which is moving at 20 m/s ?

 

  1. The velocity of a body of mass 10 kg increases from 4 m/s to 8 m/s when a force acts on it for 2 s.

(1) What is the momentum before the force acts ?

(2) What is the momentum after the force acts ?

(3) What is the gain in momentum per second ?

(4) What is the value of the force ?

 

  1. A gun of mass 3 kg fires a bullet of mass 30 g. The bullet takes 0.003 s to move through the barrel of the gun and acquires a velocity of 100 m/s. Calculate :

(i) the velocity with which the gun recoils.

(ii) the force exerted on gunman due to recoil of the gun

 

  1. Name the law involved in the following situation :

a body of mass 5 kg can be accelerated more easily by a force than another body of mass 50 kg under similar conditions

 

  1. Name the law involved in the following situation :

When person A standing on roller skates pushes another person B (also standing on roller skates) and makes him move to the right side, then the person A himself gets moved to the left side by an equal distance.

 

  1. Name the law involved in the following situation :

The sum of products of masses and velocities of two moving bodies before and after their collision remains the same.

 

  1. Name the law involved in the following situation :

If there were no friction and no air resistance, then a moving bicycle would go on moving for ever.

 

  1. Why are car seat-belts designed to stretch somewhat in a collision?

 

  1. Why are car seat-belts designed to stretch somewhat in a collision?

 

  1. The troops are equipped to be dropped by parachutes from an aircraft are called paratroopers. Why do paratroopers roll on landing ?

 

  1. Why would an aircraft be unable to fly on the moon?

 

  1. Explain why it is possible for a small animal or creature to fall from a considerable height without any injury being caused when it reaches the ground.

 

Answer:

  1. –

 

  1. –

 

  1. 6 m/s

 

  1. 0.99 m/s

 

  1. 60 N

 

  1. 5 N

 

  1. -2 m/s2; -4800 N

 

  1. 20 s

 

  1. 5 s

 

  1.   Put the values of u,v,m and t and get the answers.

(1) 40 kg.m/s              (2) 80 kg.m/s

(3) 20 kg.m/s2            (4) 20 N

 

  1. (i) 1 m/s; (ii) 1000 N

 

  1. newton’s second law of motion

 

  1. newton’s first law of motion

 

  1. Low of conservation of momentum

 

  1. Newton’s first law of motion

 

  1. By stretching somewhat the seat-belts allow the large momentum of a passenger to reduce gently, and the passenger is prevented from being thrown forward violently, and injury is prevented during a collision or accident.

 

  1. By stretching somewhat the seat-belts allow the large momentum of a passenger to reduce gently, and the passenger is prevented from being thrown forward violently, and injury is prevented during a collision or accident.

 

  1. –

 

  1. An aircraft needs air because

(i) air moving under the wings of aircraft is strong enough to hold it up and

(ii) air burns the fuel in aircraft engines.

Since there is no air on moon, an aircraft cannot fly.

 

  1. P = mv. Since momentum is directly proportional to mass so less momentum is produced or

the small animal falls to the ground with less momentum, less opposing force of ground (less force of reaction) acts on the small animal and hence no injury is caused