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Answer 1

3) Two identical bullets are fired one by one by a light rifle and another by a heavy rifle with the same force, which rifle will hurt the shoulder more and why?

Answer:  

 As both the bullets are identical and are fired with the same force. So, to reduce recall velocity, we use, heavy rifle so that its heavy mass compensate the large momentum. But for light rifle, recoil velocity will be large due to small mass thus light rifle will hurt more to the shoulder.

4) A horse continues to apply a force in order to move a cart with a constant speed. Explain.

Answer

According to Newton's First Law a body remains in its state of motion or rest until an external force acts on it.

But in this case the horse has to continuously exert the force to move the cart, this is because the force of friction is acting against the motion of the cart which us opposing its motion. So the horse has to apply force to overcome this force of friction.

5) Suppose a ball of mass m is thrown vertically upward with an initial speed v. Its speed decreases continuously till it becomes zero. Thereafter, the ball begins to fall downward and attains the speed v again before striking the ground. It implies that the magnitude of initial and final momentums of the ball are same. Is this an example of conservation of momentum?

Answer: - Sometimes

Law of conservation of momentum is applicable to isolated system (no external force is applied).

In this case, the change in velocity is due to the gravitational force of earth.When the ball is thrown up, negative force of gravity is applied and so the speed becomes zero.After that due to gravitational force the ball falls down with same initial velocity.