Up until now we've focused on acceleration that is parallel to the direction of motion. This type of acceleration will speed an object up or slow the object down. But this is not the only kind of acceleration. Acceleration can actually cause the direction of an object's motion to change. In this lesson, we will use Newton's Laws of Motion to explain one of the most misunderstood concepts in physics: centripetal acceleration.
Watch this cool demonstration of centripetal acceleration at work:
Newton's 1st Law & Centripetal Acceleration
Newton's 1st Law tells us that an object in motion will tend to remain in motion (in a straight line and at a constant velocity) until it is acted on by an outside force. Imagine a ball on a string, being swung in a circle. The direction of the ball's motion is clearly changing, since it is not traveling in a straight line. There must be some force acting on the ball.
The force must have been acting through the string, because as soon as the string was cut, the ball's path immediately became a straight line.
Newton's 2nd Law and Centripetal Acceleration
Newton's 2nd Law tells us that Force = mass x acceleration. Since there is some kind of force acting on the ball and causing it to move in a circle, the ball must be accelerating. The force and the acceleration must be in the same direction. But which direction is that, exactly? We have two possibilities: inward (toward the center of the circle), or outward.
Most people assume the acceleration is outward, because of what they see in the first video on this page. Forces appear to be pushing the water toward the outside of the circle, because it doesn't fall out of the cup. They often call this "centrifugal force" or "centrifugal acceleration". These people are incorrect.
The true direction of acceleration is inward.
Newton's 3rd Law and Centripetal Acceleration
So if centripetal acceleration is causing an inward force, why doesn't the water fall out of the cup? Newton's 3rd Law tells us that forces cause equal and opposite reactions. Because of Newton's 3rd Law, we see paired forces. Centripetal force is pulling inward on the cup, which means that the cup is pushing the water with an inward force. The water reacts by pushing outward on the cup. That's why it doesn't fall out of the cup, and many people mistakenly think that the imaginary "centrifugal force" is at work.
After you have completed this part of the lesson, you can check the associated box on the main course page to mark it as complete
