Question

State the law of conservation of momentum. A stone of 1 kg is thrown by a boy of mass 30 kg with a velocity of 20m/s across a frozen surface of a lake and comes to rest after travelling a distance of 50m. What is the force of friction between the stone and ice? What is the velocity of the boy just after he throws the stone? ​

Answer 1

Before we proceed we have to make a free body diagram of the stone-ice system. In the figure, we are seeing that frictional force is acting in leftward while the stone in moving rightward direction. One thing we have to fix in our mind that frictional force will always act opposite to the direction of motion of the body.

Now on resolving the forces,

f = ma ------ (i)

also form the equation of motion v² = u²+as,here final velocity is zero,so v = 0

u² = -2as

a = -4²/2s

= -20²/2*50

= -4*m/sec²

on substituting the given data in equation(1)

f = 1*(-4) = 4N