Welcome to 6th Grade! » Science » Simple Machines
*Source: Motion, Forces, and Energy by Prentice Hall Publishers, Science Explorer
There are six basic kinds of simple machines: the inclined plane, the wedge, the screw, the lever, the wheel and axle, and the pulley.
An inclined plane is a flat, sloped surface. A ramp is an example of an inclined plane. An inclined plane allows you to exert your input force over a longer distance. As a result, the input force needed is less than the output force.
You can determine the ideal mechanical advantage of an inclined plane by dividing the length of the incline by its height.
A wedge is a device that is thick at one end and tapers to a thin edge at the other end. Some examples of wedges include tacks, forks, axes, knives, and zippers. When you use a wedge, instead of moving an object along the inclined plane, you move the inclined plane itself.
The ideal mechanical advantage of a wedge is determined by dividing the length of the wedge by its width.
Like a wedge, a screw is a simple machine that is related to the inclined plane. A screw can be thought of as an inclined plane wrapped around a cylinder. This spiral inclined plane forms the threads of the screw.
The ideal mechanical advantage of a screw is the length around the threads divided by the length of the screw.
A lever is a rigid bar that is free to pivot, or rotate, on a fixed point. The fixed point that a lever pivots around is called the fulcrum.
The ideal mechanical advantage of a lever is determined by dividing the distance from the fulcrum to the input force by the distance from the fulcrum to the output force.
Levers are classified according to the location of the fulcrum relative to the input and output forces. The three classes of levers differ in the positions of the fulcrum, input force, and output force.
A wheel and axle is a simple machine made of two circular or cylindrical objects fastened together that rotate about a common axis. The object with the larger radius is called the wheel and the object with the smaller radius is called the axle.
You can find the ideal mechanical advantage of a wheel and axle by dividing the radius of the wheel by the radius of the axle. (A radius is the distance from the outer edge of a circle to the circle’s center.)
A pulley is a simple machine made of a grooved wheel with a rope or cable wrapped around it. You use a pulley by pulling on one end of the rope. This is the input force. At the other end of the rope, the output force pulls up on the object you want to move.
There are two basic types of pulleys. A pulley that you attach to a structure is called a fixed pulley. Fixed pulleys are used at the tops of flagpoles. If you attach a pulley to the object you wish to move, you use a movable pulley. Construction cranes often use movable pulleys. By combining fixed and movable pulleys, you can make a pulley system called a block and tackle.
The ideal mechanical advantage of a pulley is equal to the number of sections of rope that support the object.
Simple Machines in Your Body
Most of the machines in your body are levers that consist of bones and muscles.
When you bite into an apple, you use your sharp front teeth, called incisors. Your incisors are shaped like wedges to enable you to bite off pieces of food.
A compound machine is a machine that utilizes two or more simple machines. The ideal mechanical advantage of a compound machine is the product of the individual ideal mechanical advantages of the simple machines that make it up.
An apple peeler like the one shown below is a compound machine. Four different simple machines make it up. The handle is a wheel and axle. The axle is also a screw that turns the apple. A wedge peels the apple’s skin. To hold the machine in place, a lever can be switched to engage a suction cup.
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