May 1, 2012 Issue

Physics To Go 124 - Balance & torque

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Physics in Your World

Hyperphysics: Torque image
Image credit: Jupiter405; image source; larger image

Hyperphysics: Torque

Think about the forces on this sailboat: The force of the wind on the sail (perpendicular to the fabric of the sail), tends to rotate the boat. A force that can rotate an object is called a torque. In this case, if the torque of the wind isn't balanced, it will tip the boat over. The weight of the sailor, and also the weight of the hull that's out of the water, both create torques in the opposite sense, to balance the torque of the wind.

For more on torques, see Hyperphysics: Torque, and also this other Hyperphysics page.

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Physics at Home

Balancing Act

Security note:
Once you have clicked on the "simulation" link below, be sure to read the Java Security Advisory before running the simulation: To do that, click the "Read now" button on the yellow band near the top of the PhET page.

Go to Balancing Act (and click "Run now!") for a simulation that lets you move objects of different mass different positions on a teeter totter to make it balance.

To learn more, visit this Hyperphysics page.

(This feature was updated on May 6, 2013.)


From Physics Research

Newton's 3rd Law image
image credit: U.S. Air Force; image source; larger image

Newton's 3rd Law

What is the purpose of the small rotor on the back of this helicopter? In fact, it's needed because of Newton's third law--for every action force there is an equal and opposite reaction. The helicopter puts a force on the main rotor shaft to make it turn, and the rotor shaft puts an equal and opposite force back on the helicopter. For an example of reaction forces, see this video.

This reaction force creates a torque that, by itself, would make the helicopter spin in the opposite direction as the rotor. To counter this torque, the small rotor on the tail creates an opposing torque, so the two are balanced.

(This feature was updated on September 22, 2013.)

Worth a Look


Check out this PBS video to use the concept of torque to explain how one dancer can balance on top of the head of another--and on point!

(This feature was updated on December 28, 2013.)

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