Cracking the SAT Physics Subject Test

There’s no special name for this unit; it’s just called a newton-meter. Because it is
not in newtons, torque is NOT a force, but rather is a property of a force: torque
merely tells us how effective a force is at making something rotate. In Scenario 2,
Situation 3, a force was being applied directly into the pivot point. The magnitude
of force did not suddenly disappear, but it was ineffective at closing the door.
Torque is the rotational equivalent of force in trying to make something accelerate
rotationally.

Torque problems usually involve putting systems in equilibrium.

Another Instance of Newton’s Second

From the equations for the center of mass, we see that

Fnet = Macm

In other words, the net (external) force acting on the system causes the

center of mass to accelerate according to Newton’s second law.

There’s another way to determine the value of the torque. Of course, it gives the
same result as the method given above, but this method is often easier to use.

Instead of determining the distance from the pivot point to the point of application
of the force, we will now determine the (perpendicular) distance from the pivot
point to what’s called the line of action of the force. This distance is the lever arm