Work & Machines

What is work?

• If you push or pull something, that is NOT work.

• Work is the transfer of energy that happens when a force makes an object move.

• Remember – force is a push or a pull on an object

• Key - for work to be done, the object has to move.

Doing Work

• Two things have to happen in order to consider it work…

1. The object has to move2. The motion of the object has to be

in the direction the force was applied.

Doing Work Example

• The bell rings and you need the books under your desk.

• Lifting up the book is doing work…why?

1. The books move and…

2. There is a force applied, by your arms, in the direction that the books move.

Doing Work Example

• Your arms are doing the work.• Now that they are in your hands, you

start to walk with them…• It is now your legs that are doing the

work to move you and your books, not your arms.

Work and Energy

• When work is done, energy is always transferred.

• Think about carrying a box with your kid brother up the stairs. When you are done, your arms will be sore.

• Remember – when you increase the height of an object you increase its potential energy.

Work and Energy

• So, by taking the box-n-brother up the stairs, you have increased its height, therefore, increasing its potential energy and you transferred energy to it.

Work and Energy

• Remember – Energy is the ability to cause change.

• AKA – Energy is the ability to do work.

• If something has energy, it can transfer energy to something else.

Work and Energy

• When you do work on something, you increase its energy.

• IF you do work on something, you decrease your energy.

• Energy is always transferred from the object that is doing the work to the object that has the work done on it.

Calculating Work

• Would you do more work if you lifted the books from the floor to your waist or from the floor to over your head?

• Over your head…• You do more work when you exert a

greater force and when you move an object a greater distance.

Calculating Work

• The amount of work done depends on two things.

1. The amount of force exerted2. The distance the object is

moved

Calculating Work

• When a force is exerted and an object moves in the direction of the force, the amount of work done can be calculated…

• Work = Force x distance• W=Fd

Calculating Work• If Maria applies a

force of 25N to a baseball for a distance of .1m, how much work did she do?

• W = ?• F = 25N• d = .1 m

Remember -Newton x meter =

Joule&

Work is always measured in Joules

•W=Fxd•W = 25 N x .1m

•W = 2.5 J

Calculating Work• Force is measured in

– Newtons (N)• distance is measured in

– meters (m)• Newton (N) times meters (m) = Joule• Work, like energy is measured in

– Joules (J)

• One joule is about the amount of work required to lift a baseball a vertical distance of 0.7m

When is work done?

• Push a book across a table a distance of 1m.

• The distance you use to calculate the work you did is how far the book moved while the force is being applied, or while your hand is in touching the book.

• The distance in the formula, W=Fd, is the distance that your hand was in contact with the book