Torque in Physics: Equation, Examples & Problems

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  • 0:04 What Is Torque?
  • 0:56 Equation
  • 1:58 Example Problem
  • 3:15 Lesson Summary
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Lesson Transcript
Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

After watching this video, you will be able to explain what torque is and use an equation to calculate torque in simple situations. A short quiz will follow.

What Is Torque?

Archimedes said, 'Give me a lever long enough and a fulcrum on which to place it, and I shall move the world.' And he wasn't wrong. He was talking about torque. That's t-o-r-q-u-e, not t-a-l-k.

Have you ever noticed that it's easier to push open a door near the handle than it is to push it near the hinges? No, well give it a try. The reason is that the amount that something rotates isn't just affected by the strength of the force you apply, but also by the size of the lever you use.

Torque is a twisting force that tends to cause rotation. A bigger force does mean a bigger torque. But a bigger distance away from that force -- a bigger lever -- also means a bigger torque. If you try to lift a heavy object by putting a lever underneath it, a longer lever will make the job easier.

Equation

Mathematically, torque is described by this equation: torque equals force (F) times perpendicular distance (d). For example, if the force is you pushing on a door, then the distance to the hinge is the perpendicular distance -- the line to the hinge is 90 degrees to the force arrow (the direction you're pushing). Since force is measured in newtons and distance is measured in meters, torque is measured in newton-meters. So, a larger force is a greater torque, and a larger distance is also a greater torque.

The other thing to note is that these torques can be clockwise or counter-clockwise. If one person pushes on a door one way, and another person pushes on the same door the other way, and the two torques happen to be equal, they'll balance out and nothing will happen. The overall torque, otherwise known as the net torque, is what decides what happens to the object itself.

Example Problem

Let's go through an example of how to use the equation. Let's say you have a seesaw of length 4 meters hinged right in the middle. A child of mass 25 kilograms sits on one end of the seesaw. What torque does the child apply to the seesaw? (Note that the force of gravity can be calculated from the equation force equals mass times gravity, where the value of gravity is 9.8.)

So, what do we do first? Well, first of all we should write down what we know. We know that the mass is 25 kilograms. The length of the seesaw is 4 meters, but that's not the distance we put into the torque equation. That's because the pivot point, or hinge, is in the middle. So, the perpendicular distance from the pivot point to the force of the child is half of that -- 2 meters.

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