9.3

Net Torque and Angular Motion

Student Notes

In this simulation one or two forces are applied to a rod. The rod can rotate freely about its center. How will the angle and point of application of the forces determine the angular acceleration of the rod? To investigate this, first note that the red force is initially zero. Only the blue force is active, and it is at right angles to the rod. Try moving the blue force back and forth along the rod using the blue position control. Note how the torque readout changes as you move the point of application of the force. Click the start button to see how the rod moves under the influence of that torque.
Using only the blue force, how can you maximize the applied torque? Check it out using the simulation.
Find two different ways to have a zero torque with a non-zero blue force.
You can also change the angle of the blue force. Try changing this now. Compared to the initial conditions, find two different ways to reverse the sign of the torque using only the blue force. If the torque has changed sign, the rod will rotate in the opposite direction.
Set the blue force back to +2 m position, 5 N magnitude and 90 angle. Now introduce the red force. Set it to 5 N, and place it at -2 m, or 2 m left of the pivot point. How will the additional torque affect the motion of the bar? Make a prediction and run the simulation to see.
Move the red force to -1.9 m. How will the rod move now? Make a prediction, and check it out.

Self-test

Find two different force arrangements that will maximize the angular acceleration of the rod.
Arrange both forces so they point in the same direction. Does this mean that the torque from the red force also acts in the same direction as the torque from the blue force?

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