Question

When does a body rotate? State one way to change the direction of rotation of the body. Give a suitable example.

Answer 1

A force applied on a body can rotate the body. When this happens we say that the force creates some torque on the body due to which the body undergoes a rotational motion. This can be understood by an example of a see-saw.

Complete answer:
From Newton's first law of motion, we know a body moves when an unbalanced force acts on the body. This motion of the body is called translation motion.
Other than translation motion, a body can have a rotational motion also.
A force acting on a body cannot only produce translation motion of the body. Rather it can also rotate the body.
The rotational motion of a body is understood by a quantity called torque. When a force rotates a body, we say that a torque is generated on the body.
Therefore, a body rotates when an unbalanced torque is applied on the body.
The torque on a body or the rotation of a body is always about an axis of rotation. If the body is fixed to a point or an axis, then the body will only rotate about that point or axis.
Suppose a body has a fixed axis of rotation. We apply a net torque on the body and the body rotates about the fixed axis. To reverse the direction of the body, we can apply a greater torque in the opposite direction such that the body accelerates in the opposite direction. Due to this the body will slow down to stop. Then it rotates in the other direction.
A simple example can be taken of a see-saw. The see-saw rotates in that direction where the weight is greater because this weight creates greater torque. When the weight on the other side becomes greater, it rotates in the other direction.

Note:
Note that torque and force are not the same quantities. Torque is analogous to force. Meaning, the features of torque in rotational motion are similar to force in translational motion. Also, note that even if the net force on a body is zero, the net torque can be non-zero.