According to law of conservation of momentum; the momentum of bullet will be equal to the momentum of rifle. In case of light rifle; velocity will be more than the velocity of heavier rifle so that momentum (product of mass and velocity) for both shall be equal. Due to this, the lighter rifle will hurt the shoulder more. 

When a cart is moving on the road, it has to encounter friction. To maintain a constant speed, some force need to be applied continuously to overcome the friction. Hence, the horse needs to continuously apply a force in order move the cart with a constant speed. 

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. Yet, it is not an example of conservation of momentum. Explain why?

Ans. 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.

Separation between them will increase. Initially the momentum of both of them are zero as they are at rest. In order to conserve the momentum, the one who throws the ball would move backward. The second will experience a net force after catching the ball and herefore will move backwards that is in the direction of the force.

The working of the rotation of sprinkler is based on third law of motion. As the water comes out of the nozzle of the sprinkler, an equal and opposite reaction force comes into play. So, the sprinkler starts rotating. 

Acceleration can be given as follow:

Acceleration can be given as follow: 

When mass is doubled and force is halved;

So, acceleration becomes one-fourth.