Sunday, 30 January 2011
Why is one wheel faster than the other?
Place both wheels at the top edge of the angled plane and release them simultaneously
The same wheel will always win the race. Can you find any differences between the wheels?
The wheels are the same size, the same weight and are made from the same materials. The only difference is the placement of the weight. One of the wheels has a heavy centre and the other wheel’s weight is located toward the outer rim. For this reason, the wheels have different moments of inertia.
The moment of inertia signifies the inertia of the object as it rotates, or, in other words, its ability to resist any change in the rotational speed. The wheel whose weight is located around its outer rim has a higher moment of inertia. Therefore, it accelerates slower as it rolls and falls behind the other wheel whose weight is located closer to its axis of rotation.
Many public bathrooms have very large toilet paper rolls in their stalls. Due to their high moment of inertia, these rolls spin slower as the paper is being pulled off the roll. If the paper is pulled too quickly, it will tear too soon. A knowledge of physics is an asset in this situation as well. In order to get a suitable amount of paper, it should first be pulled off of the big roll slowly. Then the paper can be torn off with a rapid tug without needing to use your other hand to hold the roll steady, as is necessary with a smaller roll.