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# Defy Gravity! Centripetal Force

Media Type:
Video

Running Time: 2m 54s
Size: 8.7 MB

or

Source: ZOOM

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Collection Credits

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In this video segment adapted from ZOOM, cast members use centripetal force to demonstrate that a ball set in motion can remain inside an open container even when the container is held upside down. They succeed in generating centripetal force, but they also reveal that this force alone is not enough to completely offset the force of gravity.

## Supplemental Media Available:

Defy Gravity! Centripetal Force (Document)

Background Essay

According to Newton's law of inertia, an object already moving will continue to move in a straight line at a constant speed unless acted on by an outside force. Thus, to make an object move in a circular path, an outside force must act on the object. Centripetal force is the force that pushes or pulls an object inward so that it will move in a circular path. The word centripetal means to seek the center.

When you whirl a stone tied to a string in a circle, you must constantly pull on the string to keep the stone from flying off in a straight line. The force the string applies to the object is the centripetal force. Centripetal force operates in other scenarios as well, acting, for example, on a car to make it travel around a curve. In this case, the force is produced by the friction between the tires and the pavement. Likewise, Earth's gravity exerts a centripetal force on a communications satellite that prevents it from flying off into space by keeping it in orbit.

In the initial demonstration of this segment from ZOOM, involving a cylindrical container, as the ball rolls around the inside of the cylinder walls, a centripetal force continually redirects it to the center of the cylinder. If you were to create an opening in the cylinder through which the ball could escape, the ball would fly outward along a line tangent to the cylinder and would then continue in the same direction of travel as at the moment it exited the cylinder. When this container is held upside down, the centripetal force is not strong enough to counteract gravity, which pulls the ball downward and out of the cylinder.

With the second container, a pitcher with a wide body that curves inward near the lip, the ball does not drop out of the opening. As with a cyclist riding on a banked racetrack, the downward force of gravity is matched by the upward force provided by the surface over which the ball (or the cyclist) travels. Centripetal force acts on the ball as it rolls around inside the pitcher, putting the ball in position to be supported by the pitcher's curved surface, which is what really keeps it from falling out.

Discussion Questions

• What is the problem these kids are trying to solve?
• How does "centripetal force" keep the ball from immediately falling out of each pitcher?
• If the ball doesn't fall out (as in the second demonstration with the wide body pitcher), even though gravity is pulling it down, there must be an upward force acting on it. Where is that force coming from?
• Describe a problem from everyday life that is similar to this situation.

• Transcript

(humming) (gasps)

RACHEL: Hey, ZOOMers, do you think you can defy gravity? Well, Erica R. of Brooklyn, New York, wants us to try. She wants us to see if we can carry a ball in a pitcher. But the trick is the pitcher has to be upside down like this. It's best to use a plastic pitcher for this challenge.

KENNY: How are we going to do this?

RACHEL: It's hard.

KENNY: If you put it in tip it upside down it's going to fall over... fall out.

RACHEL: Whoops.

KENNY: So there has to be a way we can keep it up in...

RACHEL: This is hard, because there's no, like little thing it can, like, hook on or something.

KENNY: What if we... ran really fast? That's not going to...

RACHEL: Yeah, that would work. You know, like a hula hoop, kind of... Okay, say I was moving it. If you keep spinning, it will stay on you. You will stay in it.

Well, I just thought of a really cool idea. This pitcher is a hula hoop and this is, like, me or you. And so it could be like this. You could start going like that. Like a hula, and then after it really starts to twirl we can... whoops.

KENNY: Tip it upside down?

RACHEL: We can go like this and it will still be swirling. We just have to keep going like this and we can walk around.

KENNY: Let's try it.

RACHEL: Okay.

KENNY: Whoa, I was doing it for a second!

RACHEL: You really have to get it going, though and really spin it.

KENNY: I did it!

RACHEL: You did?

RACHEL: (gasping) Oh, my gosh, it works! First I have to do it, though.

KENNY: Good idea.

RACHEL: Thanks.

KENNY: Let's see if we can get it to work now. Ready, watch. I did it!

RACHEL: You did?

KENNY: Yup, for a little bit.

RACHEL: Ah, I turned as soon as you did it. Here, do it again.

KENNY: Okay.

RACHEL: These balls are bouncy.

KENNY: (humming fanfare)

RACHEL: Oh, my gosh, Kenny, you did it! It works!

KENNY: I know, cool.

RACHEL: Good job.

KENNY: We thought it was pretty hard to get the ball to stay in this pitcher. But we found that it worked really well with this wide pitcher. Watch this.

The wide pitcher is easier than the narrow pitcher because when I swirl it the ball gets trapped in this wide space and it's harder for it to fall out. Try it at home.

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