$A$ certain force acting on a $20 \, kg$ mass changes its velocity from $5 \, m \, s^{-1}$ to $2 \, m \, s^{-1}$. Calculate the work done by the force. (in $J$)

$(i)$ Define the term potential energy. Write the $SI$ unit of potential energy.
$(ii)$ $A$ body of mass $50 \, kg$ is situated at a height of $10 \, m$. What is its potential energy? (Given $g = 10 \, m s^{-2}$)

State whether the following statement is true or false:
When an arrow is released from a bow,potential energy changes into kinetic energy.

$A$ ball is dropped from a height of $10\, m$. If the energy of the ball reduces by $40\%$ after striking the ground,how much high can the ball bounce back (in $,m$)? $(g = 10\, m \,s^{-2})$

$A$ light and a heavy body have the same momentum. Which one will have greater kinetic energy?

$A$ rocket is moving up with a velocity $v$. If the velocity of this rocket is suddenly tripled,what will be the ratio of the initial kinetic energy to the final kinetic energy?