A block of mass \(m\) is being lowered by means of a string attached to it. The system moves down with a constant velocity. Then:
                                          

A rigid body of mass \(m\) is moving in a circle of radius \(r\) with constant speed \(v.\) The force on the body is \(\dfrac{mv^2}{r}\) and is always directed towards the centre. The work done by this force in moving the body over half the circumference of the circle will be:
1. \(\dfrac{mv^{2}}{rπ}\) 
2. \(mr^{2} \pi\)
3. zero
4. \(2 mv^{2} \pi\)$\mathrm{}$

The kinetic energy of a body is increased by 21%. The percentage increase in the magnitude of linear momentum of the body will be:

1.  10%

2.  20%

3.  Zero

4.  11.5%