A student exerts a force of 50 N on a lever at a distance 0.4 m from its axis of rotation. The student
pulls at an angle that is 60º above the lever arm. What is the torque experienced by the lever arm?Let’s plug these values into the first equation we saw for torque:
This vector has its tail at the axis of rotation, and, according to the right-hand rule, points out of
the page.
Newton’s First Law and Equilibrium
Newton’s Laws apply to torque just as they apply to force. You will find that solving problems
involving torque is made a great deal easier if you’re familiar with how to apply Newton’s Laws
to them. The First Law states:
If the net torque acting on a rigid object is zero, it will rotate with a constant angular velocity.
The most significant application of Newton’s First Law in this context is with regard to the
concept of equilibrium. When the net torque acting on a rigid object is zero, and that object is not
already rotating, it will not begin to rotate.
When SAT II Physics tests you on equilibrium, it will usually present you with a system where
more than one torque is acting upon an object, and will tell you that the object is not rotating. That
means that the net torque acting on the object is zero, so that the sum of all torques acting in the
clockwise direction is equal to the sum of all torques acting in the counterclockwise direction. A
typical SAT II Physics question will ask you to determine the magnitude of one or more forces
acting on a given object that is in equilibrium.
EXAMPLE