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#21
11-23-2005, 03:49 PM
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Re: More silly physics fun
[ QUOTE ]
Friction keeps your feet from moving on the board but that energy is transferred down to the wheels and, since perfect bearings, the board moves backward. [/ QUOTE ] Exactly. Think about having 2 blocks, one velcro'd on top of the other, sitting on a sheet of ice. If you push the top block, the whole system slides. 
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#22
11-23-2005, 04:37 PM
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Re: More silly physics fun
[ QUOTE ]
[ QUOTE ] Friction keeps your feet from moving on the board but that energy is transferred down to the wheels and, since perfect bearings, the board moves backward. [/ QUOTE ] Exactly. Think about having 2 blocks, one velcro'd on top of the other, sitting on a sheet of ice. If you push the top block, the whole system slides. [/ QUOTE ] Ha. I'm not stupid, despite my efforts to prove otherwise in the plane thread.
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#23
11-23-2005, 04:44 PM
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Re: More silly physics fun
+ is right and up.
[ QUOTE ] Case 1: Detached wall, we don't catch the balls As you threw the balls off you and the card would move in the opposite direction under the laws of conservation of momentum. You would gain speed m(ball)/m(cart)*vball for each ball. [/ QUOTE ] Correct.  [ QUOTE ] Case 2: Detached wall, catching the ball. You would gain the above velocity each time you threw a ball, and again each time you caught one. [/ QUOTE ] Correct.  [ QUOTE ] Case 3 Attached wall, not catching the ball. You would move opposite the direction if your initial throws. The ball bounces off the wall and heads off in the other direction, behinid you. You will move opposite the final motion of the ball. [/ QUOTE ] Incorrect. You'd remain still.  [ QUOTE ] Case 4 Attached wall, catching the ball After you do everything you will be again sitting on the cart with a bunch of motinoless balls. You must also be motionless. [/ QUOTE ] Incorrect, you'd move in my + direction. 
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#25
11-23-2005, 05:19 PM
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Re: More silly physics fun
In cases 3 and 4 you are complicating things too much, think about them simply in terms of initial and final momentum of the system. They have to be the same right? (let's take the case of just one ball, it's simpler and the same)
In every case the system starts with p = 0 (p is momentum). So in every case it must end with p = 0. In case 3, the final state has the tennis ball going backwards with some velocity x, since the tennis ball is part of the system p(tennis ball)+p(rest of system) must = 0. Since p(tennis ball) isn't zero then p(rest of system) can't be either. It must be equal and opposite. Case 4 is the same question except at the end of the experiemnt the tennis ball isn't moving, so neither can the cart/person. In case 4 what you are claiming is that our system is going to have a spontaneous increase in momentum without the introduction of an outside force. Doesn't that strike you as odd.
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#26
11-23-2005, 05:28 PM
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Re: More silly physics fun
why aren't these threads in SMP?
Melch
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#27
11-23-2005, 06:05 PM
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Re: More silly physics fun
[ QUOTE ]
why aren't these threads in SMP? Melch [/ QUOTE ] Meh. I don't like The Trainwreck and Ray put the original (plane) thread in here. Plus, it's much more fun in here. I would seriously hope that the plane thread wouldn't have made it past one page in The Trainwreck.
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#28
11-23-2005, 06:14 PM
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Re: More silly physics fun
You're wrong about the force the wall puts on the ball. It's -2 x f if f = force imparted by you throwing the ball.
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