TORQUE
We can tie a ball to a string and make it undergo circular motion, but how would
we make that ball itself spin? We could simply palm the ball and rotate our hand or
we could put our hands on opposite sides of the ball and push one hand forward
and the other backward. In both cases, in order to make an object’s center of mass
accelerate, we need to exert a force. In order to make an object spin, we need to
exert a torque.
Torque is the measure of how effectively a force can make an object spin or rotate.
(More precisely, it’s the measure of a force’s effectiveness at making an object
accelerate rotationally.) If an object is initially at rest, and then it starts to spin,
something must have exerted a torque. And if an object is already spinning,
something would have to exert a torque to get it to stop spinning.
All systems that can spin or rotate have a “center” of turning. This is the point that
does not move while the remainder of the object is rotating, effectively becoming
the center of the circle. There are many terms used to describe this point, including
pivot point and fulcrum.
The illustration above is a bird’s-eye view of a door. The hinge (pivot point) is
located on the left-hand side. (You can try some of theses examples at home on a
door in your house to get a better understanding.) In this first scenario, we want to
close the door:
SCENARIO 1