Work and Machines

What is Work?

The Meaning of Work

In science, you do work on an object when you you exert a force on the object that causes the object exert a force on the object that causes the object to move some distanceto move some distance.

If you push a child on a swing, you are doing work on the childchild.

If you pull your books out of your book bag, you do work on the booksbooks.

If you lift a bag of groceries out of a shopping cart, you are doing work on the groceriesgroceries.

No Work Without Motion

In order for you to do work on an object, the object must the object must move some distance as a result of your forcemove some distance as a result of your force.

If the object does not move, no work is done no matter no work is done no matter how much force is exerted.how much force is exerted.

For example, if you are asked to hold a piece of wood while you are helping on a construction project, you definitely exert a force to hold the wood in place, so it might seem as if you do work.

But because the force you exert does not make the wood move, you are not doing any work on it.

Only Force in the Same Direction

How much work do you do when you carry your heavy books to school? You may think a lot, but actually you don’t.

In order to do work on an object, the force you exert must be in the same direction as the object’s motion.

When you carry an object at constant velocity, you exert an upward force to hold the object so that it doesn’t fall to the ground.

The motion of the object, however, is in the horizontal direction.

Since the force is vertical and the motion is horizontal, you don’t do any work on the object as you carry it.

Take a look…

Turn to page 107 and look at Figure 3. When you pull a sled with a rope, not all of not all of

your force does wok to move the sled.your force does wok to move the sled. You are pulling horizontally and upward so the You are pulling horizontally and upward so the

force that is pulling horizontally is the doing force that is pulling horizontally is the doing the work.the work.

The force that is pulling upward does not help The force that is pulling upward does not help do the workdo the work.

Calculating Work

If you exert a force of 100 N to lift a potted tree a meter off the ground is that more or less work than if you exerted 200 N to lift a heavier plant the same distance?

Is it more work if you lift a tree from the ground to a wheelbarrow or from the ground to the top story of a building?

Using your Common Sense

It only makes sense that you do more work if you lift a heavier object then a lighter object and if you move an object a longer distance.

The amount of work that you do depends on both both the amount of force you exert and the distance the the amount of force you exert and the distance the object moves.object moves.

Work = Force x DistanceWork = Force x Distance The amount of work done on an object can be

determined by multiplying force times distance.

Joule

When force is measured in NewtonsNewtons and distance is measured in MetersMeters, the SI unit of work is newton x meternewton x meter or joulejoule in honor of James Prescott JouleJames Prescott Joule, a physicist who studied work in the middle 1800s.

One joule joule is the amount of work you do when you exert a force of 1 newton1 newton to move an object a distance of 1 meter.

Practice

Look at the sample problem on page 108. Work = Force x Distance Work = 20N x 10m Work = 200n(m) which is 200j The answer is 200j. Lets solve the practice problems on the

bottom of the page.

The Tree

If you exerted 100N and lifted the tree 1 meter off the ground, you would have exerted 100j. (100N x 1 m)

If you exerted 200N and lifted the tree 1 meter off the 200N and lifted the tree 1 meter off the ground, you would have exerted 200jground, you would have exerted 200j. This shows that This shows that when lifting something heavier, you do more workwhen lifting something heavier, you do more work.

Now you need to lift the same tree to the top floor of a building 40 meters tall. The work that you did lifting the tree into the wheelbarrow was 100j. Suppose you went on an elevator and the elevator lifted the tree 40 meters the elevator lifted the tree 40 meters to the top of the building, the work done would be 100N x to the top of the building, the work done would be 100N x 40m or 4,000j.40m or 4,000j. The elevator elevator would have done much more work then you did lifting the tree 1 meter into the wheelbarrow.