why does a glass stay still when the sheet underneath it is pulled really fast?

Question

can any one explain the physics behind it??

Answer 1

The glass actually does move slightly; but the quicker you pull the sheet, the less the glass will move.

Step by step, here's what happens:

As you start to pull the sheet, the sheet exerts a sideways force on the glass, due to friction. However, the "coefficient of static friction" (μs) puts an upper limit on how much frictional force the sheet can provide, which in turn puts an upper limit on how much it can accelerate the glass. In numbers: the static friction cannot make the glass accelerate faster than this:

A_max = g·μs

(BTW, the value of μs depends on the materials of the sheet and glass.)

To put it another way, if you accelerate the sheet any faster than A_max, the static friction won't be strong enough to drag the glass along; i.e., the sheet begins to "slip" along the bottom of the glass.

If you accelerate the sheet by _less_ than that amount, the friction _will_ continue to drag the glass along without slipping. That's why it's important to give it a good initial yank.

Once the sheet is slipping from the glass, the character of the friction changes, and you have to use a different number, the "coefficient of kinetic friction" (μk). This coefficient also depends on the materials of the sheet and glass; and is invariably less than μs. The kinetic (sliding) frictional force is actually not dependant on the sheet's speed at all. It causes the glass to accelerate at this rate:

A_k = g·μk

So, the glass continues to accelerate sideways at that rate until you manage to get the sheet out from under it. That's why it's important to pull fast so the sheet is out of the way as quickly as possible.

If you assume that you accelerate the _sheet_ at a constant rate (which is never quite the case, but makes the math easier), then you can actually calculate how much the glass will move:

If A_sheet < g·μs, then static friction drags the glass all the way to the end of the table. Your trick has failed. :-(

If A_sheet > g·μs, then the glass travels by this amount:

L/(A_sheet/(g·μk) - 1)

where "L" is the length of sheet that you have to move out of the way.

Answer 2

There are basically two types of friction. Static friction and kinetic friction. Start pushing a heavy box on the floor. You push and push, but at first it does not move. This is due to static friction. Once it starts going you don't have to push as hard, this is because the kinetic friction, the actual friction between two moving bodies, is typically less than the static friction between them, when they are not moving. This is why you have to pull really hard and fast on the sheet. If you pull to slow, then the dominating force is static friction, and the glasses will move. If the friction forces are small enough, the inertia of the glasses will keep them from falling over.

Answer 3

the force of friction on the glass is almost non-existent because of the speed at which the sheet is being pulled

Answer 4

It is called inertia. An object in motion tends to stay in motion, and an object at rest tends to stay at rest. In order to start an object moving, you must apply force. If you pull the sheet too quick, you fail to provide sufficient force to start the object in motion.

Answer 5

I guess it is the force, the weight of the glass, which greatly opposes the friction force of the cloth. Ask Bekki B

Answer 6

simple inertia. And the friction index is less when already in motion, the trick is a quick start.

Answer 7

it's Newton's First Law:
An object at rest, or in constant motion, will remain in rest or in constant motion unless an unbalanced force acts on it.