Friction

[chiddler]

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You pull a tablecloth so fast from under a table that the silverware above it does not move very much.

How does the physics of that work?

 

[syoung]

v--> c, so the items on top just stay there

 

[chiddler]

v--> c, so the items on top just stay there

i'm not sure what you mean. can you please explain? what is c? how does it answer my question?

 

[syoung]

Sorry was just trolling on that one a bit. v->c (speed of light), leaves everything behind.

i don't know why the stuff stays there. But it does move a little.
Or maybe it overcomes the static friction so much that it becomes sliding friction? And a combination of change in momentum?
delta p = 0, the cloth m << than silverware m, so when you pull the cloth at high v the silverware don't move so much?

 

[chiddler]

Sorry was just trolling on that one a bit. v->c (speed of light), leaves everything behind.

i don't know why the stuff stays there. But it does move a little.
Or maybe it overcomes the static friction so much that it becomes sliding friction? And a combination of change in momentum?
delta p = 0, the cloth m << than silverware m, so when you pull the cloth at high v the silverware don't move so much?

ok sure. it does move a little.high enough speeds can bypass static friction, using kinetic friction instead.

i understand these, but having trouble forming a tangible understanding of what is going on.

 

[syoung]

http://www.mythbusterstheexhibition.com/science-content/tablecloth-chaos/

 

[milski]

The force acting on the dishes is friction from the table cloth. It's fairly limited - it depends on the cloth, speed that you're pulling and mass of dishes. If that force is F, the total impulse transferred to the dishes is F.t where t is the time that it takes to pull the table cloth away. If t is small enough, the impulse is small enough too and the dishes are not disturbed.

 

[chiddler]

The force acting on the dishes is friction from the table cloth. It's fairly limited - it depends on the cloth, speed that you're pulling and mass of dishes. If that force is F, the total impulse transferred to the dishes is F.t where t is the time that it takes to pull the table cloth away. If t is small enough, the impulse is small enough too and the dishes are not disturbed.

for such instances, momentum is usually constant, yes? so changing t will change f, not momentum. therefore, p=Ft, a smaller t equals a larger F.

why am i wrong?

to increase efficacy of an airbag for example, it's better to increase time of impact in order to reduce F on the driver.

 

[milski]

for such instances, momentum is usually constant, yes? so changing t will change f, not momentum. therefore, p=Ft, a smaller t equals a larger F.

why am i wrong?

to increase efficacy of an airbag for example, it's better to increase time of impact in order to reduce F on the driver.

Impulse is constant for the airbag case because you have known initial and final velocities and you can calculate the momentums for each of them.

In the tablecloth case you don't have a predefined final velocity for the dishes - the more momentum you can transfer from the table cloth to them, the higher that velocity will be. You can minimize the impulse here by either decreasing t or F. F is more or less fixed*, but you can decrease t significantly.

*F actually increases as the velocity of the tablecloth increases but you can decrease t a lot faster than F will increase.

 

[chiddler]

Ok. Impulse is constant because momentum is fixed in that case. Momentum not fixed with tablecloth because you can transfer different amounts depending on what you do.

And then F is fixed so the only other thing to change is t to reduce p.

Thanks very much for your help.

 

[milski]

Ok. Impulse is constant because momentum is fixed in that case. Momentum not fixed with tablecloth because you can transfer different amounts depending on what you do.

And then F is fixed so the only other thing to change is t to reduce p.

Thanks very much for your help.

Some very minor nit-picks, in case anyone reads this:

- the first case where impulse is constant is the airbag of a crashing car.
- F is not exactly fixed but you can easily say that it's less than some relatively small F, so for all practical purposes, it can be considered fixed. The only reason I'm bringing it up is that the fact that dynamic friction depends on speed. We have considered this and the force is still limited.