0.14 Phy1110: force -- moments, torque, couple, and equilibrium  (Page 3/13)

After experiencing that once or twice, the child comes to learn that it is not the high downward velocity that hurts. Instead, it is the very high negativeacceleration when the child makes contact with the ground that hurts.

Position relative to the fulcrum

Children also quickly learn that when two children of different body masses play on a see saw, the one with the greatest body mass must sit closer to thefulcrum. Otherwise, the see saw will only be in equilibrium when one end is touching the ground and the other end is high in the air.

Children learn that if each child sits just the right distance from the fulcrum with the more massive child closer to the fulcrum, they can cause theboard to balance and be in equilibrium, at least for a short period of time. They also learn that is the seating position that provides the smoothest up anddown motion.

Each child exerts a downward force

Although the children probably don't realize it, each child exerts a downward force on the board equal to the body mass of the child multiplied by theacceleration of gravity. Those forces are commonly referred to as the weights of the children.

Balance and equilibrium

When the see saw is in equilibrium (with both ends off the ground), the fulcrum exerts an upward force on the board that is equal to the combined weights of the children.Otherwise, the see saw and the children would either fly off into space, or sink into the earth.

In addition, when the see saw is in equilibrium with both ends off the ground, the product of each child's weight and the distance of the child from the fulcrum is equal to the product ofthe other child's weight and the distance of the other child from the fulcrum. Children usually figure out how that works but I doubt that they understand whyit works.

The crank

I have an emergency light that has a crank on the side. When I turn the crank, an electrical generator inside the light turns, which generateselectrical power that illuminates a light bulb in the end of the light.

There are many other examples of cranks in common use. For example, I have an automobile jack with a crank that is used to raise the automobile in order toreplace a flat tire.

Sailboats have hand-operated windlasses with cranks that are used to shorten the length of ropes to raise the sails. Awnings have cranks that are used toraise or lower the awning to create more or less shade from the sun.

A toggle switch on the wall is a small crank with about a 30-degree angle of travel that is used to turn the lights on and off. Pushing the handle in onedirection or the other causes an axel to turn inside the switch causing an electrical contact to be closed or open.

The turning effect of a crank

When analyzing a crank, there are at least two factors that must be considered. One factor is the force exerted on the handle. The second is thelength of the crank arm. The turning effect depends on the product of the force and the length of the crank arm.

Moments or torques

Cranks are often used, (along with other mechanical devices such as screws) to provide mechanical advantage. The same turning effect (torque) can beachieved with a long crank arm and a small force, or a short crank arm and a large force. (Remember, the torque is the product of the two.) For example, muchmore torque is required to raise my automobile off the ground than is required to open or close the electrical contacts inside a toggle switch. Therefore, thecrank arm on my automobile jack is much longer than the crank arm on a toggle switch.