Centripetal and centrifugal. Well those are big words. Don’t worry, though, by the time we’re through you’ll be running circles around those words. Seriously though, we’ll be doing a couple of experiments to show how things moving in a circular fashion act on a physical level.
Experiment 1: Centripetal acceleration vs. Centrifugal Force
- Toilet paper cardboard tube, about four-five feet of string (kite string works well), paper clips, strong tape, washers (if none, you may substitute with other small items to weight things down), whiffle ball or small object you can tape or tie to a string
Note: Do this outside or someplace where a flying object like a whiffe ball won’t do any damage!
- Tie a paper clip to one end of the string
- Feed the other end through the cardboard tube
- Tape or securely attach a whiffle ball or other small object to the end of the string without the paper clips. Duct tape will secure the ball the best
- Hold the paper clip string with one hand
- Hold the cardboard tube with the other hand and start to swing the ball end of the string over your head until you get the feel for the swinging
- Now stop the swinging and this time attach an object to the other end of the string onto the opened paper clip. Not too heavy! (I used a sandal)
- Now start swinging the ball again. You don’t have to hold onto the paper clip end this time
- Keep the weighted end in the same spot. If you swing too hard, the weight will lift up really quickly. Note how fast the ball is swinging
- Stop the swinging and now add more weight to the paper clip end. Make a guess: How will you have to swing the ball now?
- Test different weights on the lower end to see how hard you have to swing the ball around.
You have just learned a little of centrifugal force and centripetal acceleration. As you swing the ball around your head, the other end lifts up. The ball moves outward as you swing it and that is called centrifugal force. The faster you swing the ball, the more force it has. Centripetal acceleration moves the opposite way: inwards.
Experiment 2: Water, water not everywhere.
- Empty bucket with handle (or just an empty plastic cup), water, your arm
Note: You might get wet! Do this outdoors or in a place you don’t mind getting wet
- Practice the motion you will use with a full bucket first: Hold the handle of the bucket and do a complete circular swing up from the ground to behind your back. Practice until you can do this is one smooth motion every time. If you don’t have a bucket, you can use a small plastic cup by holding your hand over the top of the cup.
- Fill the bucket partway with water
- Do step number two again, this time with the water in the bucket!
- As you swing the bucket around your body you will notice the water stays inside the container. It will not dump onto your head (unless you stop swinging the bucket halfway around!). This tendency to push outwards is called centrifugal force. The water and the bucket both move outwards during the swing. This is what happens when you drive in a car and go around a sharp turn. You will notice your body push against the direction of the turn. Centrifugal force!
- Try different objects in the bucket. Can you keep a bunch of objects in the bucket as you swing? How much until you can’t do it anymore? Why is it that more objects are harder to keep inside?
Steve Davala is a high school chemistry and physics teacher who likes to write. He’s got two kids of his own and subjects them to these science activities as guinea pigs. Follow him on Twitter @sdavala or email him at firstname.lastname@example.org.