All AP Physics C: Mechanics Resources
Example Question #1 : Understanding Newton's Laws
A paint bucket hangs from the right end of a meter stick, oriented horizontally. The left end of the meter stick rests on a fulcrum so that it may rotate about that point. A rope is to be attached from the left end, so that the system does not rotate. What is the minimum force this rope must support, assuming the meter stick is massless?
Determine the clockwise torque caused by the bucket.
Write an expression for the counterclockwise torque caused by the rope.
Combine the torque of the rope and the torque of the bucket into the net torque equation.
Since the system has no angular acceleration, net torque must be zero, allowing us to solve for the force of the rope.
Example Question #2 : Understanding Newton's Laws
An object at rest will remain at rest unless acted on by a(n) __________.
The correct answer is external force. External forces applied to an object will result in non-zero acceleration, causing the object to move. In contrast, internal forces contribute to the properties of the object and do not result in acceleration of the object.
Either positive or negative forces can result in the acceleration of an object. The sign on the force simply conveys information about its relative direction.