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Three Fun Ways to Demonstrate Centripetal Force


Today's Snack: Since centripetal force is all about spinning, Pepperoni Spinners go well with today's activity!


1 C. flour

tsp. salt

1 pkg. quick active dry yeast

1 T. vegetable oil

2/3 C. very warm water

of a 3-oz. size pkg. sliced pepperoni

C. shredded mozzarella cheese

tsp. oregano

Pizza sauce


Stir flour, salt and yeast in a medium bowl. Stir in oil and water. A soft dough will form. Put the dough on a lightly-floured surface. Pat flour on your hands and shape it into a ball.

Knead the dough by pushing it with the palms of your hands, folding it, and making a quarter-turn. Repeat. Knead for 5 minutes.

Now put the bowl over the dough and let it rest for 5 minutes.

Grease a round pan, 9" x 1", with vegetable shortening, butter, or spray. Press the dough with your floured hands or a floured rolling pin into a 9" square on a floured surface. Put the pepperoni on the dough. Sprinkle with cheese and oregano.

Tightly roll up the dough. Pinch the edges to seal.

Cut the roll into nine 1" slices. Put them slightly apart in the greased round pan. Cover with a dish towel, and let rise in a warm place for about 30 minutes. The dough should double in size.

Preheat oven to 375 degrees. Bake your Pepperoni Spinners for 25 to 30 minutes, or until golden brown. Serve warm with pizza sauce heated in the microwave. Makes 9.






1. A bucket with a handle and some water


2. For each student, two 2-foot lengths of string, one 7- or 8-oz. paper cup, a hole puncher, ice pick or ballpoint pen to poke holes, one penny, one marble and one small ball


3. Ping-pong ball, Scotch tape, rubber band, scissors




Have you ever been riding in a car when the driver took a sharp turn? You feel like your body lurches in the opposite direction from the turn. It's as if your body wants to keep on going in the direction it WAS going. That's centripetal force.


For the same reason, you can be traveling fast on a roller coaster that loops you upside down, but you don't fall out. What? You're defying gravity and you're not even in a spacesuit? Again, it's because of centripetal force.


Centripetal force keeps something that's rotating moving in a circle. Pronounced "sen TRIP uh tull," centripetal force is what keeps the planets revolving around the Sun. Without it, the planets would go spinning off into space.


Think about a ball with an attached string. If you whirl the ball around your head by the end of the string, it will fly overhead in a circle. But if you let go of the string, it will fly off in a straight line - no longer in a circle. That's because when you let go of the string, you took away the tension that was creating the centripetal force that was making the ball fly in a circle.


Up until recently, people thought of "centrifugal" force (pronounced "sen TRIFF uh gull") as the thing that keeps the ball flying in a circle. And that's true - if your frame of reference is what's happening with the ball. It's flying around on the outside radius of the circle from your hand. Centrifugal force is the outward pressure, to move away from the center. In science and manufacturing, we use a lot of centrifuges, which are devices that spin around and separate two fluids, for example.


But in physics, our frame of reference is on the thing that keeps the ball flying around. The pressure that we're talking about is really centripetal force -- the inward force that is causing the ball to move in a circular path.


Centripetal force is used in many ways. It's important in gyroscopes, which have rapidly spinning wheels that are useful in compasses, automatic pilot devices, and missile guidance systems.


Centripetal force also is used in adjusting the throttle when an automotive engine changes speeds . . . in devices such as chain saws and go-karts to allow the engine to start and idle without moving into action . . . and in manufacturing, such as "spin casting," a method of molding parts so that they are exactly the same.


Here are three activities to help demonstrate centripetal force:


1. Bucket Swing


Better do this outside, not too close to any bystanders or they may get wet!


Fill a bucket with water about three-fourths full. Take a hold of the handle, and swing the bucket from front to back, gradually higher and higher.


When you think you can do it quickly, without spilling, then smoothly and speedily swing the bucket all the way around in a circle.


Even though the water is momentarily upside down, it doesn't fall out of the bucket. Why? (answer: gravity is momentarily not as strong as centripetal force while the bucket is moving fast)



2. Defying Gravity


With an ice pick, paper punch or ballpoint pen, poke or punch two holes near the top of the cup, opposite each other, right under the rim of the cup.


Then thread one end of one length of string through one hole, make a loop, and tie it with a knot thick enough so that it won't go through the hole. Do the same with the other side.


Now put a penny in your cup, take a hold of the ends of the two strings, straighten your arm out, and spin the cup really fast around in a circle, so that the cup goes upside-down like a loop-de-loop roller coaster.


Did the penny stay inside? Why? (gravity)


Now try spinning the cup really slow - as slowly as you can. Does the penny fall out? What's the difference? (Speed, or momentum)


Now try the same experiment with a small ball or a couple of marbles. Try it slow and fast again. Any change?



3. Ping Pong Challenge


Have you ever wondered how a communications satellite stays up, instead of falls down, as it orbits the Earth? Here's an experiment to show you the different forces that keep satellites revolving around us.


First, cut the rubber band so that it is one long piece. Tape one end of it to the ping-pong ball.


Stand several feet away from other students or objects so that you won't hit anybody or anything.


Slowly swing the ping-pong ball with the rubber band in a circular motion. Be sure to notice how far away the ball is from your hand as you get started and swing slowly.


Now increase the speed of the circular swing, again noting the distance from your hand.


What happens? Does the ball revolve farther away from you when the speed of the revolutions is increased?


But as long as you hold on to one end of the rubber band, and the tape stays stuck, the ball won't revolve any further away after a certain point. The stretchiness of the elastic band is maxed out. But the ball won't come any closer, either.


This shows the balance between the force of gravity, which tends to pull the ball toward the Earth, and the centripetal force, which forces the ball away from your hand to fly in a circle.


That's how satellites work! They revolve around the Earth "stuck" between the two forces, gravitational and centripetal. The satellites are free to fly around the Earth instead of falling down, but not SO free that they revolve away into space.


By Susan Darst Williams Experiments 07 2009***

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