| Carlos and Dr. Doreen discuss matters: |
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When I let one ball bang into the others, why does only one ball fly off the other end? |
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Because the balls are obeying the 
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Equal what?! |
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When you push, kick, or throw something, you give it some  . You have put some of your energy into the object--say, it's a soccer ball. As the ball travels, it slows down because some of its is used to move air molecules out of the way, or to push blades of grass aside, or to knock down your opponent! Er, well . . . in the case of your opponent, what's left of the soccer ball's won't have much effect!
Now, if you were to throw a 15-pound bowling ball at the same speed, that ball will definitely have more of an effect on your opponent! So, the amount of something has depends on how heavy it is and how fast it is going.
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Oh, I get it. So, if I throw a baseball really hard, it will have more than if I throw my cat's ball of yarn just as hard. |
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Right! And if the baseball meets up with your neighbor's window, it's more likely to go right through it than if the yarn ball hits the window. |
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Because the baseball has more ! |
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Yes! Now, what if instead of a baseball and a yarn ball, you have two baseballs. You throw one and your little sister throws one. You are bigger and stronger than she, so you can throw harder, which makes the ball go faster. Suppose you can throw your ball twice as fast as your sister can throw hers. Both balls weigh exactly the same. Which ball will have more ? |
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Well, if depends on weight and speed, my ball will have twice as much . |
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Absolutely right! |
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So, what about the contraption we built? |
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What about it? |
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Hmmmm. OK, it's as if I throw one ball at the rest of the balls that are all hanging in a row. The moving ball has a certain amount of , depending on its weight and its speed. All the balls are alike. When the first ball hits the second ball in the row, it moves and hits the third, which hits the fourth, which hits the fifth. The fifth ball is the last one in the row and doesn't have anything to hit, so it just flies off into space. So the of the first ball just gets passed through the other balls and ends up in the last ball. |
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Excellent! So  !
The more scientific word for is  . So we say is saved--or conserved. None is lost. Some of the energy that gives the ball speed might get a little spread out if the balls aren't exactly lined up with each other. Or if the balls were made of some soft material, some of the energy would go into squishing the balls a little.
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OK, so what about when I let go of two balls or three balls? |
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What about it? |
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Hmmmm. You are making me think very hard! OK, so if I let go two balls, they have twice as much . . . er, as one ball. This will be enough to send two balls flying off the other end. But with three balls, it gets really weird! The middle ball is moving with the other two when it hits the two non-moving balls, yet the middle ball itself joins them in flying off the other end! |
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So three balls have more weight than two balls. Since must be saved, the same amount of material has to keep moving to obey the
By the way, scientists use the term "mass" rather than weight for the amount of material in an object. Weight is really just how hard gravity is pulling on something. In space, where gravity is very weak, both the soccer ball and the bowling ball would be weightless, but the bowling ball would still have a lot more mass.
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So, what if the balls refuse to obey the
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Do the Momentum Police come and give them a ticket? (hee-hee-hee)
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Ha! It's not that kind of law. Finding the "laws of nature" is our way making sense of the world. If something in nature seems to "disobey" one of the laws of nature, it just means we just didn't quite understand the law.
Now, would you like to learn about some really weird things scientists are doing in space to learn more about the laws of nature?
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