## शनिवार, 29 जुलाई 2017

### FORCES AND LAWS OF MOTION PART-I

(Page No. 118)
1. Which of the following has more inertia:

(a) a rubber ball and a stone of the same size?

(b) a bicycle and a train?

(c) a five rupees coin and a one-rupee coin?

Ans. (a) a stone of the same size will have more inertia than a rubber ball.

(b) A train will have more inertia than a bicycle.

(c) A five rupees coin will have more inertia than a one-rupee coin.

2. In the following example, try to identify the number of times the velocity of the ball changes:

“A football player kicks a football to another player of his team who kicks the football towards the goal. The goalkeeper of the opposite team collects the football and kicks it towards a player of his own team”.

Also identify the agent supplying the force in each case.

Ans. In the given example the velocity of football changes four times. As described below:

(i) when the football player is supplying the force when he kicks the football to another player.

(ii) when the other player kicks football towards the goal.

(iii) When the goalkeeper of other team stops the ball.

(iv) When the goalkeeper kicks the football towards player of his team.

3. Explain why some of the leaves may get detached from a tree if we vigorously shake its branch.

Ans. Some of the leaves may get detached from a tree if we vigorously shake its branch because the some of the leaves due to property of inertia remain at rest while we vigorously shake branch of the tree as a result those leaves detach and fall off.

4. Why do you fall in the forward direction when a moving bus brakes to a stop and fall backwards when it accelerates from rest?

Ans. when a moving bus brakes to a stop we fall in the forward direction because we are also moving with the speed of bus due to the inertia of motion and when suddenly it puts brakes i.e. comes to rest the lower half of our body also comes to rest but the upper half of our body not being in close contact with bus is still in the phase of motion so we fall in the forward direction.

When the bus accelerates from rest, we are also at rest being on the resting seat as the engine applies force in forward direction we fall backwards due to the inertia now.

(Page No.126)

1. If action is always equal to the reaction, explain how a horse can pull a cart.

Ans. With a balance force the overall impact is absence of movement but with unbalanced forces, the resultant or the bigger force causes the motion. Same is true in the case where a horse pulls a cart. Horse exerts more force on the cart than the cart exerts to resist its movement hence this is an unbalanced force and the cart moves in the direction of horse’s pull.

2. Explain, why is it difficult for a fireman to hold a hose, which ejects large amounts of water at a high velocity.

Ans. It is difficult for a fireman to hold a hose, which ejects large amounts of water at a high velocity because of the third law of newton when the hose ejects large amounts of water at a high velocity in forward direction the water coming out pushes the hose pipe in backward direction and it becomes difficult to hold it.

3. From a rifle of mass 4 kg, a bullet of mass 50 g is fired with an initial velocity of. Calculate the initial recoil velocity of the rifle.

Ans. Initial Momentum of rifle and bullet (before fire)

= mv

= 0

Final Momentum of rifle and bullet (after fire)

where (50 g = 0.05 kg)

= – 0.44 m/s

4. Two objects of masses 100 g and 200 g are moving along the same line and direction with velocities of and, respectively. They collide and after the collision, the first object moves at a velocity of . Determine the velocity of the second object.

Ans. Given,

= 100g = 0.1 kg

= 200g = 0.2 kg

velocity of the first object = = 2 m/s

velocity of the second object = = 1 m/s

Momentum before the collision =

= 0.4 kg m/s

since velocity of first object after collision = 1.67m/s

If we suppose that velocity of second object = v

As per law of conservation of momentum,

= (initial momentum = final momentum)

0.4 = 0.167 + 0.2v

0.2v = 0.4 -0.167

v = 0.2333/0.2 = 2.33/2 = 1.16 m/s