How Far Do Dishes Move in the Tablecloth Trick?

In summary, the magician pulls a tablecloth out from under some dishes. The tablecloth exerts a force on the dishes that is not included in the system.
  • #1
sona1177
173
1
For the following question:

A magician pulls a tablecloth out from under some dishes. *How far do the dishes move during the .25 seconds it takes to pull out the tablecloth? The coefficient of kinetic friction between the cloth and the dishes is .12

I think I got the answer right by considering the force of kinetic friction (to the right, normal up, weight down) so since kinetic friction is = to .12 * N then .12mg=ma giving .12g=a then I used displacement=.5at^2 and got .037 meters. *Now that I'm reviewing the problem I'm wondering why in the free body diagram you don't include the force of the tablecloth pulling out from under the dishes. *Sorry if this sounds like a dumb question but I looked online and though I can't check my answer in the back of the book, all other examples don't consider the table cloth pulling on the dishes so my question is why?
 
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  • #2
sona1177 said:
*Now that I'm reviewing the problem I'm wondering why in the free body diagram you don't include the force of the tablecloth pulling out from under the dishes. *Sorry if this sounds like a dumb question but I looked online and though I can't check my answer in the back of the book, all other examples don't consider the table cloth pulling on the dishes so my question is why?
If you draw your free body diagram correctly, the force that the tablecloth exerts on the dishes should be part of it. There are two pieces of the Universe that interact with the dishes, (a) Earth, which exerts a force straight down and (b) tablecloth which exerts a force that has two components (i) the normal force opposite to the Earth force and (ii) the force of friction which is perpendicular to the normal force and in the direction of motion of the dishes.

What you should not include is the force that the magician exerts on the tablecloth; it is not part of your system, which is just the dishes.
 
  • #3
But as the magician pulls on the tablecloth doesn't that tablecloth also pull on the dishes?
 
  • #4
Yes, that's what I said. The tablecloth not the magician is what exerts the force on the dishes. The magician is not in any way touching the dishes, is he? Therefore he does not exert a force on the dishes.
 
  • #5
Ok yes, I think I'm forgetting what a free body diagram is, lol. Only the forces that touch the object, with the exception of gravity, I assume, are included. Thanks for all your help!
 

FAQ: How Far Do Dishes Move in the Tablecloth Trick?

What are Newton's Laws?

Newton's Laws are three fundamental laws of motion that describe the behavior of objects in motion. The first law states that an object will remain at rest or in motion at a constant velocity unless acted upon by an external force. The second law explains the relationship between an object's mass, acceleration, and force. The third law states that for every action, there is an equal and opposite reaction.

What is the "tablecloth trick" demonstration of Newton's Laws?

The "tablecloth trick" is a demonstration of Newton's first law of motion. It involves pulling a tablecloth out from under a set of dishes without disturbing them. This shows that the dishes, or objects in motion, will remain in motion unless acted upon by an external force, in this case, the force of friction between the tablecloth and dishes.

How does the "tablecloth trick" relate to Newton's second law?

The "tablecloth trick" also demonstrates Newton's second law of motion. The force required to pull the tablecloth out from under the dishes depends on the mass of the dishes and the acceleration at which the tablecloth is pulled. This can be seen in the equation F=ma, where F is the force, m is the mass, and a is the acceleration.

What factors can affect the success of the "tablecloth trick"?

The success of the "tablecloth trick" can be affected by several factors. These include the type and texture of the tablecloth and dishes, the force applied to the tablecloth, and the friction between the tablecloth and dishes. The weight and arrangement of the dishes can also impact the outcome of the trick.

Can the "tablecloth trick" be performed with any type of dishes?

The "tablecloth trick" can be performed with various types of dishes, but it is more successful with certain types. Dishes with a smooth, flat bottom and a low center of mass, such as plates or bowls, are easier to perform the trick with. Dishes with a curved or uneven bottom, such as cups or glasses, may be more difficult to keep in place during the trick.

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