Question

What happens when a tablecloth is pulled out from underneath a table setting, and everything on the table stays put?

Asked by: K. Devereux

Answer

In that demonstration, the tableware is exhibiting Newton's laws of motion.

Newton's first law says that the tableware will remain motionless unless acted upon by an outside force. To set the objects on the table into motion, the horizontal force acting upon them in this case is the frictional force between them and the table cloth.

Newton's second law says the amount of force needed to accelerate the tableware is directly related to the rate of acceleration. When the tablecloth is pulled slowly, the tableware matches its low rate of acceleration and moves along with it. When the cloth is pulled quickly, the tableware needs to accelerate at a greater rate to move along with it; but the force needed to make that happen is greater than the frictional force available an it lags behind. If the cloth is pulled very rapidly, the tableware 'stays put.'

Answered by: Paul Walorski, B.A. Physics, Part-time Physics Instructor



This question touches on one of the most mysterious things in our universe! The simple answer to your question is inertia. In other words, when you pull the table cloth off the table full of dishes the dishes stay put because of inertia. Inertia is the property of matter that describes its resistance to any change in its motion. When you are running and you try to turn you notice that it is harder to turn the faster you run or the bigger you become the harder it is to change direction. These are other examples of inertia. When you are riding in a car and the driver turns a corner, the feeling that you are being pushed into the door or over to the driver is another example of inertia.

But, what exactly, IS this tendency of things to 'want' to stay as they are? Why does all matter have this property? Why does the amount of matter affect the amount of inertia? Did you know that you could measure the mass of an object by its inertia? In fact, the inertial mass of any object exactly equals the gravitational mass of the object. Einstein himself could not explain this relationship!

If your mother is like mine and will not let you pull a table cloth out from under her dishes you could use a sheet of paper and some coins. If you place the sheet of paper on the edge of a table with three or four inches hanging off the edge you can lick your finger and quickly strike the paper with a downward motion which will slide the paper out from under the coins without the coins moving. This is even more fun when you try to see how many coins you can stack up or how many coins you can put on their edges and have still standing.

I would like to end with one quick word of warning. I have learned from hard experience that if you do try the table cloth demonstration, be sure that the table cloth does not have a raised hem around the edge! I hope this answer helped and I hope it raised more questions!

Answered by: Tom Young, M.S., Science Teacher, Whitehouse High School

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