Talk:Work (physics)/Archives/2009/May

Catcher does negative work?

If the catcher stays still as he catches the ball, then he won't displace the ball any. So the displacement vector will be zero, and the work will be zero. What am I doing wrong? --76.102.149.170 (talk) 14:54, 14 May 2009 (UTC)

Actually this Talk page is for discussing how the content of the article can be improved. Your request is to teach you elementary physics; that's not what this Talk page is for.

If the catcher's hand moves only a very small distance then the force must be very large. The change in kinetic energy (of the baseball) is given: the smaller the distance traveled, the larger the corresponding force to get the work done. Zero distance would mean an infinite force, which is fysically impossible. Mathematically any scenario with infinities is ill defined. --Cleonis | Talk 18:17, 14 May 2009 (UTC)

If you're actually interested in improving the article, you'd be interested in parts of the article that seemed confusing to someone who was unfamiliar with the material. I'm part of the target audience! It's true that teaching me elementary physics isn't what the talk page is for. But it is what the article is for, and frankly, it wasn't doing a very good job. If you insult me again, then I won't come back and the article can remain crappy.

Based only on the fact that work is equal to the dot product of force and displacement, I'm not sure that the work done by the catcher is necessarily negative. Let's say the catcher accelerates his glove forward as he catches the ball, so as to catch it more quickly and throw it to first base. The ball is being displaced toward the pitcher, and the force exerted by the catcher's glove is also in that direction. Therefore the work must be positive. Because of this, I recommend you either remove the catcher remark or add a note that his glove must be moving backward as it yields to the ball.--76.102.149.170 (talk) 04:39, 18 May 2009 (UTC)

The work-energy theorem tells us that the work done on the ball by the catcher is equal to the change in kinetic energy of the ball. The ball arrives at the catcher's glove at high speed and therefore high kinetic energy. A fraction of a second later the catcher has controlled the ball and brought it to rest. The kinetic energy of the ball has fallen from a high value to zero. Therefore the work done on the ball was negative.

If the ball arrives at the catcher's glove with a high kinetic energy, and the catcher does positive work on it, then it has even higher kinetic energy and must leave the catcher's glove at a higher speed than its speed at arrival. Dolphin51 (talk) 07:19, 18 May 2009 (UTC)

OK, I think maybe I'm starting to understand how to reconcile the two definitions of work. The catcher's glove is made of leather and therefore yields to the ball. So the work on the ball is negative, because the ball is being displaced into the catcher's glove... AND its energy is going down. You can imagine that if the ball was hitting a wall instead, the wall would be displaced much less... but the ball would also retain a greater amount of kinetic energy, and it would bounce off.

I probably shouldn't write about this in the article, should I? It was helpful to me, but it doesn't seem very Wikipedia-ish.--76.102.149.170 (talk) 04:39, 18 May 2009 (UTC) —Preceding unsigned comment added by 153.18.17.22 (talk)