# The Centripetal Force Requirement: Definition, Examples & Problems

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• 0:04 What Is Centripetal Force?
• 1:01 Examples of Centripetal Force
• 2:21 Centripetal Force Equation
• 3:14 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.

What keeps a roller coaster in a loop? Learn about centripetal force, what causes a centripetal force, and how to calculate it. Complete a quiz to test your knowledge.

## What Is Centripetal Force?

Whenever an object moves in a circle, it has a centripetal force. But what exactly is that? A centripetal force is a force pointed towards the center of a circle that keeps an object moving in that circle. Without it, an object would fly out of the circle, continuing in a straight line.

We know this because of Newton's First Law which says that an object in motion stays in motion, and an object at rest stays at rest unless acted upon by an unbalanced force. Let's say you rolled a ball along the floor. The force of gravity pulls the ball toward the ground, but the normal force counteracts gravity and points away from the ground. The two forces are balanced. In the absence of any other forces, objects continue in a straight line at a constant velocity.

The entire structure of circular motion means the forces can't possibly be balanced. For circular motion to occur, the net force, or sum of all forces, must be inward, pointing toward the center of the circle. When this happens, those forces are called centripetal.

## Examples of Centripetal Force

The centripetal force isn't some magical thing. There's always a specific, real-life force that is acting as the agent of the centripetal force. For example,

• If you whirl a ball on a string above your head, the centripetal force is the force of tension.
• If you're in a spacecraft orbiting the earth, the centripetal force is the force of gravity.
• If you drive a car in a circle, the centripetal force is the force of friction between the road and the tires.

In a force diagram, the force should always be labeled with the specific force that's acting. Sometimes things do get a bit more complicated. Sometimes the centripetal force can be provided by two forces. Let's look at an example.

Have you ever been on a roller coaster? Particularly a roller coaster that goes upside down? If so, you've experienced a centripetal force during the loop.

But the source of that force was different in different parts of the loop. At the bottom, the force of gravity was pointing away from the center of the loop. The normal reaction force (the force that acts against gravity and supports an object on a surface) between you and the track pointed the opposite direction. At the bottom of the loop the force of gravity and the normal force were balanced.

But at the top of the loop, gravity was pointing towards the center of the circle. So at the top, gravity and the normal force were together pointing down, resulting in a net inward force, or centripetal force. The more forces that are present in a situation, the more complicated things can get.

## Centripetal Force Equation

We can calculate centripetal force using this equation:

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