There are three types of levers, identified as first class, second class,

and third class. The difference is where the fulcrum and applied force

are, in relation to the load. Each class works differently, and is used

to do different jobs

• The first class lever uses a fulcrum in between, and the applied force and load are at opposite ends.

The diagram at the left shows a first class lever set up to move a heavy load with a small applied force. The force must be applied over a long distance, in order to make the heavy load move just a small amount.

By adjusting how far the fulcrum is from the load, you can control the mechanical advantage. The closer it is to the load, the more force is applied.

• Eg scissors

• The second class lever is one where the fulcrum is at one end, and the applied force at the other. The load that is to be moved is between them.

This lever is different in how it works ... it causes the load to move in the same direction as the force you apply.

Just as with a first class lever, how close the load is to the fulcrum determines by how much your force will be multiplied. If you want to move a very large load with a small applied force, you must put the load very close to the fulcrum.

• Eg. Wheel barrow

• The fulcrum is once again at one end of the lever, but this time the load is at the other end, and you apply a force in between.This lever can not give any mechanical advantage. Regardless of where you apply the force, the force you apply must always be greater than the force of a load.If you were using this lever to lift an object at a distance, it would require less force to just stand above it and lift it up ... using the lever will require more force!So why use a third class lever at all?The answer lies in the fact that the load moves in the same direction as the force you apply, which is convenient. So is the application of force between the load and the fulcrum

• Eg. Your arm

http://www.worsleyschool.net/science/files/lever/page2.html

• When a device reduces the effort needed to lift a load, there is an advantage.

• Levers and pulleys are examples of simple machines.

• Two single pulleys can be used to construct a multiple pulley system.

• Pulley systems can be described using diagrams.

• There is a relationship between the number of ropes supporting a load in a pulley system and the effort required to lift a load.

• There is a cost associated with the advantage in a pulley system.

• A pulley is an object that is usually round with a smooth groove around its outside edge.

• A pulley transfers a force along a rope without changing its magnitude.

• assume that the rope through the groove of a pulley moves smoothly and evenly, without catching. They say it moves without friction.

• When two rough surfaces are rubbed together (like two wooden blocks), they become warm; the heat is caused by friction. If the two surfaces were slicked with oil and then rubbed together, they would move much more smoothly and very little heat would be generated. There is much less friction.

• also assume that the pulley and rope weigh very little compared to the weight on the end of the rope, so they can ignore these two weights and make their calculations with only the heavy weight on the end of the rope.

There is a force (tension) on the rope that is equal to the weight of the object. This force or tension is the same all along the rope. In order for the weight and pulley (the

system) to remain in equilibrium, the person holding the end of the rope must pull down with a force that is equal in magnitude to the tension in the rope. For this

simple pulley system, the force is equal to the weight, as shown in the picture. The mechanical advantage of this system is 1! The output force is the weight to be held in

equilibrium and the input force is the applied force.

In the second figure, the pulley is moveable. As the rope is pulled up, it can also move up. The weight is attached to this moveable pulley. Now the weight is supported by both the rope end attached to the upper bar and the end held by the person! Each

side of the rope is supporting the weight, so each side carries only half the weight (2 upward tensions are equal and opposite to the downward weight, so each tension is equal to 1/2 the weight). So the force needed to hold up the pulley in this example is

1/2 the weight! Now the mechanical advantage of this system is 2; it is the weight (output force) divided by 1/2 the weight (input force).

The mechanical advantage of each system is easy to determine. Count the number of rope segments on each side of the pulleys, including the free end. If the free end is to

be pulled down, subtract 1 from this number. This number is the mechanical advantage of the system! To compute the amount of force necessary to hold the

weight in equilibrium, divide the weight by the mechanical advantage! In the third figure, for example, there are 3 sections of rope. Since the applied force is downward,

we subtract 1 for a mechanical advantage of 2. It will take a force equal to 1/2 the weight to hold the weight steady. The fourth figure has the same two pulleys, but the

rope is applied differently and it is pulled upwards. The mechanical advantage is 3, and the force to hold the weight in equilibrium is 1/3 the weight. Each additional

figure shows another possible pulley configuration and lists the force necessary to lift and hold the weight still. The mechanical advantage for the system will be the number

in the denominator of the force.

• These systems are known as simple pulley systems because they use the same rope throughout the system. If the pulleys were attached with several different ropes (not one continuous rope), the system would be a complex pulley system. The force necessary to hold a complex pulley system in equilibrium would have to be computed using other Statics methods. Once it was known, however, the mechanical advantage of the system would still be computed by dividing the weight to be held by the force applied to hold it!

• http://www.swe.org/iac/lp/pulley_03.html