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# Work and Machines - PowerPoint PPT Presentation

Work and Machines. What is Work?. The Meaning of Work. In science, you do work on an object when you exert a force on the object that causes the object to move some distance . If you push a child on a swing, you are doing work on the child .

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## PowerPoint Slideshow about ' Work and Machines' - cybill

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Presentation Transcript

### Work and Machines

What is Work?

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

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

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

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

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

• If the object does not move, no work is done no matter 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.

• 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.

• Turn to page 107 and look at Figure 3.

• When you pull a sled with a rope, not all of your force does wok to move the sled.

• You are pulling horizontally and upward so the force that is pulling horizontally is the doing the work.

• The force that is pulling upward does not help do the 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?

• 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 the amount of force you exert and the distance the object moves.

• Work = Force x Distance

• The amount of work done on an object can be determined by multiplying force times distance.

• When force is measured in Newtons and distance is measured in Meters, the SI unit of work is newton x meter or joule in honor of James Prescott Joule, a physicist who studied work in the middle 1800s.

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

• Look at the sample problem on page 108.

• Work = Force x Distance

• Work = 20N x 10m

• Work = 200n(m) which is 200j