Chapter 11, Problem 7A

To kick a football so that it doesn’t rotate through the air, where should it be kicked relative to its center of mass?

To determine

The place where the football should be kicked relative to its center of mass so that it does not rotate.

Answer to Problem 7A

It should be kicked in line with its center of mass.

Explanation of Solution

Introduction:

The torque acting about an axis of rotation due to the force $F$ is given by,

  $\begin{array}{c}\tau =F\times \left(r\right)\\ =F\text{r}\sin \theta \end{array}$

Where,

  $r=$ denotes the shortest distance of the point of application of force from the axis of rotation.

  $\theta =$ angle between the force and the distance

Now to avoid the ball from rotation, it should be kicked in a way that the torque on the ball be zero. Hence, it should be kicked in the line of the center of mass which will make the angle between the force and distance as $0°$ and thereby will reduce the torque.

Conclusion:

Hence, the ball should be kicked in a way to make the torque on the ball as zero in order to stop it from rotating.