More silly physics fun

[Patrick del Poker Grande]

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]

don't make OOT think during the holidays

[daryn]

and no more "hand waving"

draw diagrams!! give reasoning!

[kurosh]

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.

[Patrick del Poker Grande]

[ QUOTE ]
don't make OOT think during the holidays

[/ QUOTE ]
Bah!

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

[MrWookie47]

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>

[Shajen]

how large is the wall?

How big is the shopping cart?

How hard am I throwing the ball against the wall?

What is the air speed of an unladen swallow?

[Matt Flynn]

[ QUOTE ]
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".

[/ QUOTE ]


I assume you mean the wall never deforms, there is no friction or drag of the cart apparatus interacting with the ground, etc. such that this is a pure first-approximation problem.

Further assume the ball thrower both throws and catches the ball accurately every time, and that he throws such that the ball hits level with his throw.

In that case, the cart herky-jerks but the net is it moves forward - quite quickly after enough throws, unless you assume there is no air resistance, in which case it herky-jerks but does not move.

Bonus question: Does gravity affect the motion of the described ball-and-cart?

[Patrick del Poker Grande]

I knew this one would donk. Woe is me... back to the plane thread!