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Torque is a Rotational Force

by Ron Kurtus (revised 31 October 2015)

A torque is a special form of force that turns an axle in a given direction. It is sometimes called a rotational force.

You can create a torque by pushing on a rod or lever that rotates an axle. Likewise, a torque on an axle can result in a linear force at a distance from the center of the axle.

Questions you may have include:

This lesson will answer those questions. Useful tool: Units Conversion



Relationship of torque and force

A force is defined as a push in a specific direction that moves an object. It is considered linear or moving in a straight line. On the other hand, a torque is a force that rotates an axle or wheel around its center.

The relationship between torque and force is:

T = FR
or
F = T/R

where

R is also sometimes called the moment arm. The force, F, is applied perpendicular to the radius, lever or moment arm.

Creating a torque

A requirement for creating a torque is that the object must be able to rotate about some center point. Some examples are a wheel that can rotate on an axle, a bolt that is constrained to turn in a hole, and the axle of a lever or teeter-totter.

By applying a force on the edge of a wheel, you create a torque that rotates the wheel. Likewise, by applying a force on a wrench, you can rotate a bolt within its hole.

Force on wrench creates torque on bolt

Force on wrench creates torque on bolt

If the force on the wrench is 10 pounds and the length of the wrench is 6 inches, the torque created is 10 pounds times 0.5 foot = 5 foot-pounds.

If the force on the wrench is 0.5 newtons and the length of the wrench is 20 centimeters, the torque created is 0.5 newtons times 0.2 meters = 0.1 newton-meters.

Creating force from torque

A torque on the axle of a wheel can be transferred into a force on the circumference of the wheel.

Torque applied to wheel results in force on edge

Torque applied to wheel results in force on edge

You can demonstrate this force by placing a bicycle up-side-down and spinning one of its wheels. Touching the outer surface of the wheel, you can feel the force pushing your hand in the direction of rotation. (Obviously, don't be so foolish to stick your fingers in the spokes of the spinning wheel.)

Summary

A torque is a special form of force that turns an axle in a given direction. Torque equals force times moment arm. Pushing on a rod that rotates an axle can create a torque on that axle. Likewise, a torque on an axle can result in a linear force at a radius from the center.


Keep a positive attitude


Resources and references

Ron Kurtus' Credentials

Websites

Physics Resources

Books

The Science of Forces by Steve Parker; Heinemann (2005) $29.29 - Projects with experiments with forces and machines

Glencoe Science: Motion, Forces, and Energy, by McGraw-Hill; Glencoe/McGraw-Hill (2001) $19.32 - Student edition (Hardcover)

Top-rated books on Physics of Force


Questions and comments

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