#2             83. WHEELS AGAIN             


Karen and Tammy were watching the great bike race.

The riders slowed as they approached a tricky bend, and then powered off again.

“Look at the bicycle wheels, Tammy,” said Karen. “You can see the spokes at the bottom, but not at the top. I wonder why?”

“Perhaps they are moving faster at the top,” laughed Tammy.

“Don”t be silly,” answered Karen. “The wheels aren’t falling apart! It must be an optical illusion.”

What do you think?
HINT 1

Think about Tammy’s suggestion. Might she be right?
HINT 2

Think about a diameter of the wheel which passes through the contact point with the ground. How does this move? Does one part move faster than another?
SOLUTION

Every part of the wheel revolves around the axle at a uniform speed (although not all points at the same speed!). However, when the bicycle is in motion, the uppermost part of the wheel always moves faster through space than the lower part. In fact, the wheel is instantaneously pivoting about its point of contact with the ground.
EXTENSIONS

1. Suppose the wheel is rotating about its axle. Think about the relative speeds of different points on the wheel.

2. We have talked about speed relative to the ground. Think about other cases where this relativity comes into play. For example, you sit in a carriage of one of two adjacent trains. The other train starts to move. Or is it yours? How does relative motion fit in here? Think of some other examples.