# Major Inertia Problem! (at least for me!)

physicsvirgin

## Main Question or Discussion Point

By my username, it's apparent I'm not very proficient in Physics! I'm on my 2nd day of Intro in college...and I am already clueless!

Today, our professor gave us a demonstration of inertia by pulling a tablecloth from a table covered with dishes. It was a great stunt, but I'm totally blank. I realize that the dishes did not come crashing down because of inertia - the dishes were at rest and tend to stay at rest. However, if he had pulled the tablecloth SLOWLY, the dishes WOULD have moved. In fact, they even jiggled a little in class! Why is this? Why does speed matter? If inertia is a LAW, why don't the dishes stay put no matter how quickly or slowly you pull the cloth?

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quasar987
Homework Helper
Gold Member
Here's my modest opinion:

Matter stays put unless a force acts on it. And when there is a force acting on it, it is accelerated. If the cloth is pulled very quickly, the force (of friction) on the dishes is only very brief, and thus they are only very poorly accelerated. If the cloth is pulled slowly, the static friction force is not "beaten" and the dishes get dragged along with the cloth.

Chi Meson
Homework Helper
To continue...

In the tablecloth trick, when you pull the cloth quickly, the plates would have to accelerate quickly in order to stay with the cloth (because the plates have to go from zero speed to high speed in very short time). The only way things can accelerate quickly is if they have a large force on them. The only horizontal force that acts on the plates is from friction, but this amount of friction is too small to cause the required acceleration, therefore the plates move only a small amount as the cloth slips out from underneath.

ZapperZ
Staff Emeritus

Recall that F = dp/dt, or the rate of change momentum. If you are pulling on the tablecloth slowly, you are trying to change the momentum of the objects on the table over a long period of time, i.e. dt is large. This means that only a smaller force F is need, and this is sufficiently provided by the friction between the object and the tablecloth. We can assume that the static and kinetic friction force between them is relatively constant.

However, if you yank on the table cloth real quickly, you are trying to make a momentum change over a VERY short period of time. To be able to do this, F needs to be quite large. But in this case, F is fixed, or already have a limit, based on the frictional forces. So the table cloth slides more easily under the object before they can gain sufficient momentum change.

Zz.

physicsvirgin
Thanks!

I really appreciate the explanations! Thanks, guys!

But it seems to me that it is more of an acceleration/friction issue than it is inertian (is that a word?). I think I am just analyzing too much, eh? Is it simply that the force was so short and quick, the "force" of intertia/gravity/state of rest was 'stronger' than the force applied to the tablecloth? In other words, inertia didn't know that the event had occurred and kept 'doing its thing?'

I really think I am going to enjoy the class despite my fears of science! I was so entertained today, it was almost like watching a poor man's David Copperfield! Now only if they can get Mr. Wizard to come in to demonstrate and I'll be set!!!

ZapperZ
Staff Emeritus
physicsvirgin said:
I really appreciate the explanations! Thanks, guys!

But it seems to me that it is more of an acceleration/friction issue than it is inertian (is that a word?). I think I am just analyzing too much, eh? Is it simply that the force was so short and quick, the "force" of intertia/gravity/state of rest was 'stronger' than the force applied to the tablecloth? In other words, inertia didn't know that the event had occurred and kept 'doing its thing?'

I really think I am going to enjoy the class despite my fears of science! I was so entertained today, it was almost like watching a poor man's David Copperfield! Now only if they can get Mr. Wizard to come in to demonstrate and I'll be set!!!
Actually, in classical physics that you are studying, inertia, force, momentum, etc. are ALL intimately connected. The so-called three Newton's Laws are actually a description of the same phenomena. So the First Law, which is essentially about inertia, is really F=dp/dt when F=0. It means then that dp/dt=0, or p=constant. This IS the First Law, a description of inertia.

So yes, they are about force, acceleration, inertia, friction, etc.

Zz.