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Simple machines make work easier by multiplying, reducing, or changing the direction of a force. The scientific formula for work is w = f x d, or, work is equal to force multiplied by distance. Simple machines cannot change the amount of work done, but they can reduce the effort force that is required to do the work! As you can see by this formula, if the effort force is reduced, distance is increased.
There are six types of simple machines: pulleys, wheels and axles, inclined planes, levers, wedges, and screws. Wedges and screws are both a type of inclined plane; pulleys and wheels and axles are both a form of lever.
An inclined plane is a board or other flat surface set at an angle to the horizontal. Since the force needed to push an object up an inclined plane is less than the force needed to lift the same object, inclined planes reduce the amount of force necessary to do a job. A ramp is an example of an inclined plane.
A wedge is shaped like two inclined planes placed back to back. Its sides slant down to a point, so that it can be driven into an object by an effort force. An axe is an example of a wedge.
Screws are basically an inclined plane wrapped around a middle section. The more 'threads' (twists) on a screw, the less force required to drive it into something. You might want to allow your children to experiment by driving different types of screws into a board. Discuss with them which screws are easiest to turn, and explain how the number of threads and the diameter of the screw affects this.
Levers are a bar or board that turn on a fixed support called a fulcrum. Levers raise and lower an object as force is exerted upon the lever. An example of a lever is a bottle-opener: the handle acts as a lever arm, and the pivot that fits under the rim of the cap acts as a fulcrum.
A pulley is a wheel with a grooved rim that is used to reduce the amount of force and change the direction of force needed to do work. Pulleys are modified levers; their fulcrum is at their center. A fixed pulley is used to change the direction of force needed to do work; in order to hoist up a load with a pulley and rope, force is exerted downward on the rope. Since it is easier to pull down using your own weight than to pull upwards, fixed pulleys are commonly used. A moveable pulley is attached to a load and is used to reduce the amount of force needed to do work. It slides along a rope, rather then a rope sliding along it. A block and tackle is a combination of fixed and moveable pulleys, and is used to both change the direction and the amount of the force needed to do work.
An example of a wheel and axle is an old-fashioned well, where a wheel is turned in order to crank the connected axle around, and the rope that the water pail is attached to then wraps around the axle. A wheel turns because of effort force and resistance pushing against it; the force can be exerted on either the axle, or the wheel. The example of an old well is a case of the force turning the wheel. An example of the force being applied to the axle is on a Ferris Wheel: the wheel spins because of the force on the axle.
The term mechanical advantage is used to describe the number of times a simple machine multiplies the effort force applied. The mechanical advantage is the ratio of the load force to the effort force, or, MA= F load / F effort. This ratio gives an idea of the effectiveness of a simple machine in reducing work.
All more complex machines use at least one form of simple machine. A good activity for demonstrating this is to look at some common household machines with your children, and see if they can identify any of the simple machine parts. The screw and the wheel and axle on a hand powered drill, and the screw and lever on a car jack, are examples of simple machines that are used in more complex ones.