Simple Machines

-Force, Work, & Power

Section 1: Force, Work & Power Even though you might not feel it, a

force is puling on you right now. It is, of course, the force of gravity. The size of this gravitational force is

equal to your weight. Fortunately you are usually balanced by

another force For example, your legs if you are

standing; your chair if you are sitting.

This force acts in the opposite direction from the gravitational force

And is supplied by the ground or the seat of the chair.

In these situations, the forces are balanced so there is no movement.

Section 1: Force, Work & Power

When a force is not balanced, there is always some kind of change.

For example, if you push hard enough on the side of a chair, the chair will probably move.

This is because the force you supply is not balanced by an opposing force

Section 1 Force, Work & Power

Work: Force over Distance

To a scientist Work means using a force to move an object through a distance.

Thus, work is related to motion. In everyday language, we might say that sitting at a

desk reading this lesson is “work” But in a scientific sense, we could not call this work In order for work to be done, some object must be

moved through a distance.

Think about the last time you played tug of war.

When you win a game of tug of war, you do work in the scientific sense.

The purpose of the game is, of course, to move an object (the other team) through a distance (across the center line).

Work: Force over Distance

Your team has to apply a force (your pulling on the rope) to move the other team.

From your teams perspective, we call this force the effort.

At the same time the opposing team is resisting your force, so we call their force the resistance.

As long as the effort force is equal to the resistance force, both forces are balanced and nothing moves.

Work: Force over Distance

Since neither team is moving, nobody has done any work.

You may be pulling and straining, yet you are not doing any work in the scientific sense.

By using more force, however, your team can move the opposing team across the center line.

Your effort overcomes the resistance and causes the other team to move

Now work is being done!

Work: Force over Distance

Would you like to know how much work your team did in winning the game?

To find out, we have to know 1.how much force your team used to move its opponents and 2.how far you moved them.

You could use a force meter attached to the rope to measure the actual force and a meter stick to measure the distance.

Work: Force over Distance

Any work that was done is equal to the force applied multiplied by the distance over which it was applied.

This can be expressed by the equation: Work = Force x Distance

or W = Fd

Work: Force over Distance

Since force is measured in newtons, and distance is measured in meters, the unit of work is the newton-meter (N.m)

This unit is also given the name joule (J). The term Joule is used is used to describe a

force of 1 N acting through a distance of 1 m, which is 1 N.m

Work: Force over Distance

Lets assume for the moment that your team applied a force of 10,000 N to overcome the resistance of the other team and that your team pulled the other team a distance of 3 m.

The work your team did can be calculated as follows: W = F x d W = 10,000 N x 3 m W = 30,000 N.m or 30,000 J

Therefore, your team did a total of 30,000J of work to win the tug of war.

Work: Force over Distance

Section 2: Simple Machines

By now it should be obvious that human power alone is not very effective in doing large amounts of work quickly.

This is why machines were invented. Simple machines are used to change the size

or direction of a force.

For example, moving a heavy boulder probably requires more force than you could normally supply alone.

However, by applying the same force to a pole or bar placed under the boulder, you may find you can move it.

By using a simple lever, you can produce a large force to move a heavy load.

Simple Machines

You may know that complex mechanical systems are made up of simpler machines.

Among the simple machines are wheel-and-axles, pulleys, screws, and wedges.

But all simple machines can be reduced to just two basic machines--- the lever and the inclined plane.

Simple Machines

Simple Machines

Lever

Wheel & Axle

Pulley

Inclined Plane

Screws

Wedges

It might seem like a lever can supply more work than you put into it. But this is not true.

The work done in moving one end of a lever is always equal to the work done on the load at the other end.

This means that a lever only transfers work. It does not increase the amount of work

done, but allows you to do the same work with less force.

Simple Machines: The Lever

Remember, that work depends only on the force applied and the distance moved

Of course, the distance over which you must apply the force is increased.

This is a fundamental feature of all machines.

To decrease the force needed to accomplish work you must increase distance.

On the other hand, if you want to decrease distance you must increase the force.

Simple Machines: The Lever

A lever always turns on a fixed point called a fulcrum. This is the place where the lever is supported.

The part of the lever between the fulcrum and the load is called the resistance arm

From the fulcrum to the end of the lever where the force is applied is called the effort arm.

Simple Machines: The Lever

Simple Machines: The Lever

When the effort arm is longer than the resistance arm, applying a small force to the effort arm results in a larger force at the end of the resistance arm.

This is because the effort force acts over a greater distance than the resistance force.

Simple Machines: The Lever So far we've been discussing a first

class lever-one in which the fulcrum is between the resistance and the effort.

A first class lever can change the size of the force and reverse its direction as well.

By changing the position of the fulcrum you can create a second or third class lever.

Second and third class levers change the size, but not the direction of the force.

Simple Machines: The Lever

Simple Machines: The Wheel and Axle Have you ever tried to turn the shaft of a

doorknob when the handle was missing? If so you know how much harder it was to open the door.

Could you imagine trying to make a car turn a corner without a wheel on the steering column?

A doorknob and a steering wheel are examples of the wheel and axle , a machine that is similar to the lever.

Simple Machines: The Wheel and Axle

A wheel and axle has a fulcrum at the center of the axle.

Its resistance arm is the radius of the axle.

The effort arm is the radius of the wheel.

A small effort applied to the wheel results in a large force turning the axle.

Simple Machines: The Wheel and Axle

Have you noticed that large trucks and tractors have big steering wheels?

It is the size of the wheel that determines the amount of force applied to the axle.

The larger the wheel, the more distance it covers in turning the axle.

The force needed to turn that distance is thereby reduced, while the force applied to the axle is increased.

This makes it much easier to turn a heavy truck.

Simple Machines: The Pulley

A pulley is similar to a wheel and axle. But the axle on a pulley does not turn.

A rope runs over the pulley with the resistance hanging down on one side and the effort applied to the other side.

Simple Machines: The Pulley Because the effort and

resistance are the same, there is no gain in force.

A single fixed pulley simply changes the direction of the force.

Such pulleys are useful because it is often easier to pull downward than to lift upward.

Simple Machines: The Pulley With a single moveable pulley you can

lift a resistance with half the effort you would have to supply to a fixed pulley.

With the fixed pulley all the resistance hangs on one rope, while the resistance is supported by two ropes in the case of a moveable pulley.

It, therefore, takes half as much effort to pull up on one rope using a moveable pulley

Simple Machines: The Pulley

Pulleys can be combined to multiply force even more.

Moveable pulleys and fixed pulleys work together in a block and tackle.

By moving the end of the rope a long distance with a small force, a very large force can be applied to lift a heavy object a small distance.

Simple Machines

Lever

Wheel & Axle

Pulley

Inclined Plane

Screws

Wedges

Simple Machines: The Inclined Plane An inclined plane is a simple machine with no

moving parts and works on a basic principle. An inclined plane simply increases the

distance over which an applied force acts. Since it forms a sloping surface, such as a

ramp, you can use an inclined plane to move a load with less force than would be needed to raise it up to the same height.

Simple Machines: The Inclined Plane

Remember the relationship between force and distance that you saw in the lever and its related machines? It also holds true for the inclined plane.

As the length of the plane increases, the force needed to move the load decreases.

If you shorten the length of the inclined plane, more force is required.

Simple Machines: The Inclined Plane Again, the work done in either case is the

same. It is only a matter of which is more important

in getting the job done- the amount of force you must use or the distance you must travel.

Simple Machines: The Screw

Though they are considered simple machines in their own right, screws are actually forms of the inclined plane.

A screw is actually an inclined plane wrapped around a cylinder.

Simple Machines: The Screw

Screws are very useful because the length of the inclined plane can be spread over a large distance.

The more tightly wound the screw is, the larger the distance.

It will take more turns to fully tighten the screw, but it will take less force to do it.

Simple Machines: The Wedge Two inclined planes back to back make a

wedge. When a wedge is used to split a log the

two inclined planes move the resistances apart on each side.

You could imagine that a short, wide wedge would split a log without being driven very far.

However, the force with which you pound the wedge would be much greater.

Simple Machines: The Wedge

A long, narrow wedge, though it would need to be driven further, would be a lot easier to pound.

There are probably many wedges on your kitchen counter. A knife is actually a vary narrow wedge.

Summary

Machines were invented to overcome the limitations of human power.

The lever and the inclined plane are the two most basic machines, from which all other simple and complex machines are built.

Simple machines are used to change the size or direction of a force.

They do not increase the amount of work done.

Summary

To decrease the force needed to accomplish work, distance must be increased

In contrast, if distance decreases, force must be increased