More silly physics fun

Patrick del Poker Grande · #1 11-23-2005, 02:36 PM

Inspired by Daryn's post in this awesome thread:

This is a popular question in freshman physics. Imagine this set-up (because I don't want to take the time to MS Paint it): You have a wall at the front end of a cart and you're standing on the back end of this cart facing the wall and you have a bunch of perfectly elastic tennis balls. You throw the balls against the wall and they bounce off. Describe the motion of the cart, assuming perfect bearings and a smooth, no-slip condition between the flat ground and the wheels.

If you can't figure out what I'm talking about, imagine Daryn's fan on skateboard picture and replace the fan with a little person throwing tennis balls against the "sail".

pokerdirty · #2 11-23-2005, 02:39 PM

don't make OOT think during the holidays

Patrick del Poker Grande · #3 11-23-2005, 02:40 PM

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don't make OOT think during the holidays

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Bah!

daryn · #4 11-23-2005, 02:39 PM

and no more "hand waving"

draw diagrams!! give reasoning!

kurosh · #5 11-23-2005, 02:39 PM

After you throw the ball, the cart will roll backwards. When the ball hits the wall, the cart will roll forwards. If you catch the ball, the cart will stop.

Eh, I think this might be right.

#6 11-23-2005, 02:41 PM

haha nice...OOT is hopeless on this one.

MrWookie47 · #7 11-23-2005, 02:42 PM

In white? Eh, I guess so.<font color="white">

In the case of tennis balls, they'll bounce off the wall and will end up traveling in the direction I'll call -x. Since the system started with no momentum, and now a portion of the system has momentum in the -x direction, the cart/you/wall unit must gain momentum in the +x direction.

In the case of the fan system in the picture, though, the air molecules will be traveling in the +x direction as the air flows around the wall. Thus, the cart will gain momentum in the -x direction. </font>

Edit: <font color="white">In the brief time between when you release the ball and when the ball hits the wall, you travel in the -x direction. </font>

slickpoppa · #8 11-23-2005, 03:34 PM

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Edit:In the brief time between when you release the ball and when the ball hits the wall, you travel in the -x direction.

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scratch what i just said

mmbt0ne · #9 11-23-2005, 03:36 PM

[ QUOTE ]
[ QUOTE ]

Edit:In the brief time between when you release the ball and when the ball hits the wall, you travel in the -x direction.

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Thats not true, unless you are standing on roller skates or something. When you throw the ball in the +x direction you do not have any momentum in the -x direction because that momentum is absorbed by the friction between your feet and whatever you are standing on.

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You're close, but you need to take one more step to get there. [img]/images/graemlins/smile.gif[/img]

CrazyEyez · #10 11-23-2005, 03:42 PM

[ QUOTE ]
[ QUOTE ]
[ QUOTE ]

Edit:In the brief time between when you release the ball and when the ball hits the wall, you travel in the -x direction.

[/ QUOTE ]

Thats not true, unless you are standing on roller skates or something. When you throw the ball in the +x direction you do not have any momentum in the -x direction because that momentum is absorbed by the friction between your feet and whatever you are standing on.

[/ QUOTE ]

You're close, but you need to take one more step to get there. [img]/images/graemlins/smile.gif[/img]

[/ 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. (Then it stops when the ball hits the wall.)
Right?