1. Unit 8-4 & 8-5 Work and Machines

2. I. Work
In science, the word work has a different meaning than you may be familiar with.
The scientific definition of work is: using a force to move an object a distance when both the force and the motion of the object are in the same direction.

3. What does it mean? There is no work without motion.
No work without forces going in the same direction

4. Work or Not?
According to the scientific definition, what is work and what is not?
a teacher lecturing to her class
a mouse pushing a piece of cheese with its nose across the floor

5. 5

6. II. Direction and force
Horse does not work on the rider – force not in same direction
Work is NOT being done.
c. Horse does work on the cart - force and motion are going in the same direction
Force on person
Force on cart
motion
motion

7. III. What is work?
A scientist delivers a speech to an audience of his peers. No
A body builder lifts 350 pounds above his head.
Yes
A mother carries her baby from room to room.
A father pushes a baby in a carriage.
A woman carries a 20 kg grocery bag to her car?

8. IV. Calculating Work Formula: Work = Force x Distance
Formula: W = F x D
The unit of force is newtons
The unit of distance is meters
The unit of work is newton-meters
One newton-meter is equal to one joule
So, the unit of work is a joule

9. W=FD Work = Force x Distance
Calculate: If a man pushes a concrete block 10 meters with a force of 20 N, how much work has he done?

10. W=FD Work = Force x Distance
Calculate: If a man pushes a concrete block 10 meters with a force of 20 N, how much work has he done? 200 joules
(W = 20N x 10m)

11. V. How Machines Make Work Easier
A machine is a device that helps make work easier to perform by accomplishing one or more of the following functions:
transferring a force from one place to another,
changing the direction of a force,
increasing the magnitude (amount) of a force, or
increasing the distance or speed of a force.
A machine does not change the amount of work done. It can simply make the work easier for you.

12. VI. Reasons Why Machines are NOT 100% Efficient
Machines have moving parts so there is friction
Some of the work done is used to overcome friction