Why Does an Object at Rest Tend to stay at Rest?
An object at rest (including an object at rest on top of a tablecloth) tends to stay at rest unless acted on by an external force. One way to prove this is to pull a cloth out from under the object. This can be done more simply at first or more elaborately as you build your confidence in the law of physics.
What You Need
- tablecloth (one with low friction is best—tweedy fabrics, gravy spots, and spilled soda can increase friction and compromise the intended results)
- objects to place on the tablecloth such as bowls, bottles, lit candles, and your physics textbook
- spare roast turkey, stuffing, and cranberry sauce, if you actually attempt to do this at your Thanksgiving dinner
- Place the cloth on the table, so at least several inches extend beyond the edge of the table.
- Carefully place the objects on the cloth. If the objects are closer to the edge of the table this is easier, but allow for at least a few inches for the objects to slide.
- At this point, all you need to do is pull the tablecloth out from under the objects on the table. As with a band-aid, the faster the better. Don't hesitate and be tentative with your pull because that increases the chances for the objects to topple. (It doesn't hurt to create suspense by pretending you never did this before and have every reason to expect it to fail. The more you do this, the greater level of acting skills this may require.)
- You can also do this with just one beaker and a cloth. Perhaps this is a less-dramatic start, but it still proves the same point.
Note: This works best when objects placed on the tablecloth have smooth bottom surfaces. Bowls with a circular lip tend to catch on the tablecloth. Pottery with a felt bottom or mounted silicon rubber resting points can also lead to humiliation and ridicule if they hang up during this demonstration. The objects should have a short vertical "moment arm," which means bowls are safer than bottles and partially filled bottles are safer than empty bottles. Bottles with liquid should be at room temperature to avoid condensation on the outside of the bottle, which can increase friction.
The cloth is removed and the objects-at-rest sitting on the tablecloth tend to stay at rest in approximately the same position they were originally placed. Most likely, there will be some sliding and even teetering before the objects come to rest.
The criterion for stability is that the height to diameter ratio for cylindrical objects be less than the coefficient of static friction between the cloth and the object.
Why It Works
This is basically a fun experiment, but there is a good bit of physics to learn here. The objects retain their positions on the table due to Newton's first law, which states that an object in motion tends to stay in motion unless acted upon by an external force. (An object at rest tends to stay at rest unless acted upon by an external force.) If excessive friction exists between the cloth and the bottom of the objects, there will be an external force and the objects will move. The table must have low enough friction so the tablecloth can be pulled out smoothly, but enough friction so the objects don't slide too far after the cloth is removed. The small frictional force that occurs when the cloth is pulled out exerts a torque that can rotate the object, especially one whose center of mass is relatively high above the table. The frictional force exerted by the table on the bottom surface of the objects opposes this rotational motion and helps stabilize tall objects, such as bottles and candlesticks.
Other Things to Try
As a way to get in touch with your inner nerd, you can draw diagrams, called free-body-diagrams, showing all the forces present in this project. You can learn a lot of physics by doing this.
One aspect of Newton's first law is that an object at rest tends to stay at rest. This can be seen in the reluctance of the objects on the table to be moved as the cloth is pulled out from under them. Some frictional force exists between the cloth and the objects, which exerts a torque that, if strong enough, will rotate and topple the object.
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