1. How do machines do work?
12-2

2. What is a machine?
A device that allows you to do work in a way that is easier
By changing
Amount of force you exert
Distance over which you exert your force
Direction you exert your force

3. Ex. Shovel to move dirt
If you use a shovel to move dirt compared to using just your hands, does this make less work for you?

4. Input and output forces
Input force
Force you exert on the machine
Moves the machine a certain distance, called the input distance
Output force
Force that the machine exerts on the object
The work the machine does by exerting a force over another distance, output distance

5. Input and Output Work Input work Output work
Input distance times input force
Output work
Output distance times output force
When using a machine the amount of output work can never be greater than the input work
Don’t get more out of the machine that you put in

6. Making work easier Changing force
With some machines output force is greater than input force
This means there is an increase in the distance over which the force is applied

7. Ramp
Length is greater than height, input force is exerted over a greater distance
The longer the ramp, the less of an incline there is, and the easier it is to push something up the ramp

8. Faucet knob
Handle of faucet has greater diameter than pipe it is turning, so force is exerted over a greater distance.
It is much easier to turn something that has a large diameter than something with a small diameter

9. Change force
Some machines have an output force that is less than the input force
Allows you to exert your input force over a shorter distance
Must have a greater input force

10. Hockey Stick
Your hands don’t move much, but you have put in a strong force. This causes the stick to move a greater distance than your hands, hitting the hockey puck

11. Chopsticks
Your fingers don’t move very much, but this causes a larger movement in the chopsticks. The force you put in is larger than the output force

12. Changing direction
Some machines don’t change either the force or distance
They change the direction in which the force is exerted

13. Weight machine
Allows you to pull the weight at an angle, making it easier for you to move the weights

14. Mechanical Advantage
Number of times a machine increases a force exerted on it
Ratio of output force to input force gives you mechanical advantage
Mechanical advantage = output force / input force

15. Changing Force
Output force is greater than input force, mechanical advantage is greater than 1
Exert force of 10N on hand held opener, opener exerts force of 30 N
Calculate the MA for the can opener:

16. Changing Distance
Output force is less than input force, so mechanical advantage is less than 1
Input force is 20N and machine’s output force is 10N
Calculate the MA for this machine:

17. Changing Direction
If only the direction changes, the mechanical advantage will always be 1
Input and output forces are equal, only the angle of force changes

18. Efficiency of Machines
Efficiency of a machine compares the output work to the input work. Expressed as a percent
Higher percent, the more efficient the machine is
Efficiency = output work / input work x 100

19. What is more efficient, a pair new scissors or rusty scissors? Why?

20. Real and ideal Machines
A machine with an efficiency of 100% would be an ideal machine
Do you think these kinds of machines exist? Why?