Pulling a Tablecloth from Under a Steel Ball

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In summary, the key to pulling a tablecloth from under a steel ball without knocking it over is understanding the concept of inertia. The type of tablecloth and steel ball used can affect the success of the experiment, with smooth and slippery materials being ideal. This experiment can also be done with other objects with high inertia, but it may be more challenging. The specific technique involves pulling the cloth quickly and smoothly in a horizontal direction. With proper precautions and adult supervision, this experiment can be safely done at home.
  • #1
GingerBread27
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Ok so I'm guessing my TA made up this problem on his own so I don't know how to start, considering there are no numbers and lol I just love having numbers to work with. Anyways here goes.

A tablecloth covers a horizontal table and a steel ball lies on top of it. The tablecloth is pulled from under the ball (in a perfectly horizontal fashion), and friction causes the ball to move and roll. What is the ball's speed on the table when it reaches a state of rolling without slipping? Assume that the table is large enough that the ball does not fall off of it.

Help! :eek:
 
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  • #2
As the table cloth is pulled from under it, friction makes the ball start to roll. Assuming this takes place long enough, the ball will start spinning with the same speed as the table cloth is pulled. Once the cloth is gone, it will touch the table and slow down a little because its now slipping. Once its slow enough for friction to get it moving, it should move with a little less than the speed at which the table cloth was pulled. I am guessing though.
 
  • #3
thanks for your thoughts, hope someone else has an idea
 
  • #4
You must be primarily concerned of the system where 1) friction allows the ball to accelerate without slipping, or where the slipping involves either 2) a typical critical frictional "breakaway" from static to kinetic friction or 3) the sphere experiences a quantity of torque from the tablecloth divided by the moment of inertia that exceeds the acceleration at point of contact.
 
  • #5
well as of now I'm assuming that the speed the cloth is pulled is equal to the speed of the rolling ball, but from there I'm lost.
 
  • #6
If the ball is rolling without slipping then wouldn't the angular momentum just be equal to Vcm/R?
 
  • #7
This is an interesting question. Can somebody please set a question related to this one. Anything will do. I just want to see how this question could be asked. My final exam is impending. I'm thinking that my lecturer might design all the weird questions for us. Thanks
 

1. How is it possible to pull a tablecloth from under a steel ball without knocking it over?

The key to this seemingly impossible feat is to understand the concept of inertia. The steel ball, being a solid object, has a high inertia and will resist any changes to its state of motion. When the tablecloth is pulled quickly and smoothly, the cloth will slip out from under the ball before the ball can react and fall off the table. This is possible because the cloth has a much lower inertia and can be moved with less force.

2. Does it matter what type of tablecloth and steel ball are used?

Yes, the type of tablecloth and steel ball can affect the success of this experiment. The tablecloth should be smooth and slippery, with minimal friction between the cloth and the table. The steel ball should also be smooth and round, with a uniform weight distribution. This will allow for the cloth to slide easily and the ball to remain stable on top of the cloth.

3. Can this experiment be done with other objects besides a steel ball?

Yes, this experiment can be done with other objects as long as they have a high inertia. Some examples include a glass, a book, or a heavy vase. However, it may be more challenging to find the right balance between the object's inertia and the cloth's smoothness to successfully pull off the trick.

4. Is there a specific technique to pulling the tablecloth?

Yes, there is a specific technique that should be used to successfully pull off this experiment. The cloth should be pulled quickly and smoothly in a horizontal direction, rather than straight down. This will minimize any friction between the cloth and the table, allowing for a clean and swift removal of the cloth.

5. Is this experiment safe to do at home?

Yes, this experiment can be safe to do at home as long as proper precautions are taken. Make sure the tablecloth is not too heavy and the object on top is stable. Also, be sure to practice the technique before attempting the trick with a valuable or delicate object on top. It is always best to have adult supervision when conducting science experiments at home.

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