A body covers a semi-circle of radius $10 \mathrm{cm}$ in $2 \mathrm{seconds}$. Find its speed.

A particle is moving in a circle of diameter $5 \mathrm{m}$. What is the magnitude displacement when it completes $1\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.$ revolutions? 

Look at the figure above:

Name the force that keeps the stone in its path.