Almost any place can become a “lab” for exploring forces. Your group is going to experiment with the force they feel when pressed against the side of a car that is going around a curve really fast. They’ll explore a similar situation, an object that stays in a mini-bucket as it is swung overhead.
Centripetal force is a force associated with a rotating system, such as a merry-go-round on a playground. It is a force that acts on a body moving in a circular path and is directed toward the center around which the body is moving. Another way to understand this force is to consider Isaac Newton’s First Law, “that an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by another force.” When you are pressed against a car door as the car goes around a curve, you may feel pushed outward, much as you would on a merry-go-round. Really, there is no force pushing you outward. Your body wants to go in a straight line, but the car holds you in, acting as the other force to make you go in a curved line. On the merry-go-round, you are holding yourself onto the ride, exerting a force that keeps you from flying off to travel in a straight line.
To begin this activity, ask kids to imagine they are at an amusement park. They’re watching people get on a ride and trying to decide whether they should get on it, too. It looks like a big round cage. People are standing along a round wall, facing towards the center. When the ride starts to spin, slowly one side tilts up into the air. It keeps tilting! Ask kids, “Do you think the people will fall out? What will happen?”
Now introduce the concept of force. Ask, “Do you know what a force is?” (A force is a push or pull.) Then ask, “What happens when you are riding in a car and the driver brakes really hard?” (Answers will vary.) Explain that when they ride in a car, their body moves really fast with the car. Even though a driver may press on the brakes to make the car stop, their body doesn’t stop right away. It tries to keep moving in the same direction. The seat belt helps to stop them.
Now tell the kids, “Let’s keep thinking about the idea of an object in motion staying in motion in a straight line, unless something changes it.”
Make a mini-bucket in front of your group—it takes no more than five minutes (see diagram on the Fun With Forces sheet). Then do a demo. Put a ball or a penny in the bucket and swing it in a small circle, so that the cup goes upside down like a loop-the-loop roller coaster. Swing it fast enough to keep the item from falling out.
Ask kids: “Why didn’t the ball fall out? What would happen if I swung the mini-bucket more slowly?” (Gravity would pull the item toward the ground). Ask kids, “Now that we tried it with a ball or penny, what do you think will happen with a ping pong ball or water?” Your kids don’t need to know the term centripetal force. The point of this activity is to get them to observe, ask why, and experiment.
Now do the activity with the group:
When the group comes back together, discuss together the questions posed on the student sheet: Why didn't the penny and water fall out when you whipped the cup around over your head? How is this similar to a loop-de-loop roller coaster? What outside is at work in this experiment and on roller coasters? You could tie this in to other experiments and conversations about gravity if you're looking for related activities.
Who says science can't be fun? An amusement park uses the principles of science to make exciting and sometimes terrifying rides. Park rides that move in circles and steep curves make you feel like you're being pushed outward. That's centripetal force. Do you think that's the same force that keeps you from falling out of a roller coaster when it goes upside down? Find out with this experiment.