Greenwood press

ROTATIONS
111

---

by movement in the axle of a car. The rotation of the wheels and the friction
between the wheels and the road enable a car to move forward. 
Circular gears with wedges help support rotation in engines and machines.
For example, a bicycle uses gears to change the amount of force needed to move
the pedals. As the gear rotates, its teeth grab onto the chain and move it forward
in order to spin the wheels on the bicycle. Gears with a smaller radius require less
force, since the chains move a smaller distance. As the bicycle builds speed, the
gears rotate more quickly, making it more difficult to pedal in lower gears. By
shifting the chain to a higher gear with a greater radius when the bicycle
increases speed, the pedals will slow down, since the chain has a greater distance
to move, making it easier to maintain a higher speed. When the person on the
bicycle slows down, the gears should be shifted down to a smaller radius so that
pedaling becomes easier. 
Several amusement-park rides rotate to create a spinning effect. An object
will feel like it is moving more quickly around a circle if it sits further away from
its center of rotation. In this situation, the object has to travel a further distance
around a circle than an object closer to the center, but also during the same time
period. Rotational motion with ice skaters changes, because angular momentum
is conserved. Angular momentum is determined by the product of the radius of
the arm length and the skater’s angular velocity. If his or her arm radius decreases
so that the arms are closer to the body, his or her angular velocity will increase.
As a result, skaters will spin faster when they move their arms closer to their bod-
ies. Conversely, skaters can slow down their spinning motion by spreading their
arms out from their bodies.
The earth rotates around an axis that passes through the two poles. The radius
of the earth is 3,963 miles. Therefore every object at the equator is moving at a