Thread: How things work? View Single Post
#20
Wednesday, September 09, 2009
 dr.janxeb Senior Member Join Date: Jan 2009 Location: 24°51′36″N 67°0′36″E Posts: 141 Thanks: 3 Thanked 108 Times in 48 Posts
Simple Machines

SIMPLE MACHINES

You ride your bicycle to school. You run the flag up the flagpole. You use a shovel to dig. In all of these actions, you are using simple machines.

When you think of a machine, you probably imagine something that is made of metal and has moving parts, such as a car or washing machine. However, anything that changes the force, or effort, needed to do work is a machine.

There are four kinds of simple machines: levers, pulleys, wheels and axles, and inclined planes. No matter how complicated any machine is, it is made up of these four simple machines.

WHY ARE SIMPLE MACHINES USEFUL?

Simple machines help us by reducing the force necessary to move something. They reduce the force by increasing the distance over which the force is applied. If you want to open the lid of a paint can, for example, you wedge a screwdriver into the edge and pry the lid up. To lift the edge of the lid just a tiny bit, you must move the handle of the screwdriver a much greater distance.

LEVER

How easy do you think it would be to lift a friend 3 feet (1 meter) off the ground? Pretty difficult? But there is a simple way. Put your friend on a seesaw.

A seesaw is a type of lever. It is a rigid bar or plank with a central point, called a fulcrum. If you push down on one end of the lever, you get a force pushing up at the other end. The closer the load is to the fulcrum, the easier it is to lift the load. So if your friend weighs more than you, he or she should sit nearer to the middle of the seesaw.

Other examples of levers are nutcrackers, scissors, shovels, and tweezers. Even your arm is a lever—its fulcrum is your elbow.

PULLEY

A pulley is a wheel with a groove in the edge to guide a rope or cable. A single pulley does not decrease the force needed to lift something. It only changes the direction of the force. The pulley on a flagpole, for instance, lets you raise the flag by pulling down instead of up.

Two pulleys combined can decrease the force necessary to move something. Builders use a system of pulleys to lift heavy items such as bricks to the tops of buildings.

WHEEL AND AXLE

A bicycle wheel turns around a rod at its center, called an axle. Imagine trying to spin the front wheel of a bike by twisting the axle. It would be much easier to spin the wheel by grabbing its outside edge. By applying force on a wheel, you move the load at its axle. It takes less force (over more distance) to turn a wheel than it does to turn its axle directly.

Wheels and axles are found in many everyday objects, such as doorknobs, bathroom taps, and ceiling fans.

INCLINED PLANE

If you wanted to raise a car 3 feet (1 meter), how would you do it? Lifting the car straight up would be extremely difficult. Driving it up a ramp would be easy. A ramp is a slanted surface called an inclined plane.

Inclined planes decrease the effort needed to lift a load by increasing the distance over which the load is moved. Instead of moving the car 3 feet straight up, the car moves several feet forward as it goes up the ramp—farther, but easier.

A wedge is a double inclined plane. When you split wood, you apply downward force. The wedge-shaped axe head changes redirects the force sideways against the wood.

SIMPLE MACHINES WORK TOGETHER

All complex machines are made of combinations of simple machines. A car includes hundreds of simple machines. Even a can opener contains three. The hinged handle is a lever. The turning knob is a wheel and axle. And the sharpened cutting disk is a wedge.
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