Why is the water at the bottom of a waterfall warmer than the water at the top?

When a stone of mass $m$ falls through a vertical height $d$,the decrease in the gravitational potential energy is:

$A$ boy is moving on a straight road against a frictional force of $5\, N$. After travelling a distance of $1.5\, km$ he reaches a roundabout (Fig.) of radius $100\, m$. He moves on the circular path for one and a half cycles and then moves forward up to $2.0\, km$. Calculate the total work done by him against the frictional force.

$A$ spring which has been kept compressed by tying it is allowed to be dissolved in an acid. What happens to the potential energy of the spring?

State whether the following statement is true or false:
When energy changes from one form to another,the energy that disappears from one form reappears in an exactly equivalent amount in the other form.

An automobile engine propels a $1000 \,kg$ car $(A)$ along a levelled road at a speed of $36 \,km \,h^{-1}$. Find the power if the opposing frictional force is $100 \,N$. Now,suppose after travelling a distance of $200 \,m$,this car collides with another stationary car $(B)$ of the same mass and comes to rest. Let its engine also stop at the same time. Now car $(B)$ starts moving on the same level road without its engine started. Find the speed of the car $(B)$ just after the collision.