1. Machines make jobs easier
Work and machines
Machines make jobs easier

2. Machine –
a device that helps do work by either changing the size (amount or distance)of a force or changing the direction of the force

3. Total work does not change
Work = Force X Distance
Force and distance have an inverse (opposite) relationship
If a machine allows you to apply a smaller force the distance will increase
If a machine reduces the distance the force will increase
If a machine allows you to apply a force in a different direction it doesn’t change how much force you have to apply or how far you move it

4. Making work easier NOT Machines Do change the amount of work done 5 N
Change the amount of force you exert and the distance the force is exerted
Do change the amount of work done
output force
5 N
Input force
10 N
NOT

5. Making work easier NOT Machines
Change the direction in which you exert force
Do change the amount of work done
50N , 3 m
Input force
50N , 3 m
Output force
NOT

6. Work Input
The work done on a machine
W = F x d

7. Work Output
The work done by a machine
W = F x d

8. Mechanical advantage
The number of times a machine multiplies the force
Mechanical ad. = Output force
Input force
If M.A. > 1 the machine output force is greater than the input force – helpful for lifting heavy objects
If M.A. < 1 the machine reduces force needed but distance increases (fishing pole)

9. Calculating Mechanical Advantage Problem
A mover uses a handcart to lift a refrigerator. He applies an input force of 50 N. The handcart applies an output force of 350N on the refrigerator. What is the mechanical advantage of the handcart?

10. Write the equation
MA = Output force
Input force

11. Fill in the values from the problem
MA = 350 N
50 N

12. Solve the problem MA = 350 N 50 N MA = 7
The output force of the handcart is 7 times the input force of the man, but the distance of the output force is only 1/7 of the distance of the input force, so the work stays the same

13. Efficiency Efficiency compares the output work to the input work (%)
Cannot be greater than 100%

14. Machines are not 100% efficient because of friction
A force that opposes motion – will slow machines down
Occurs when 2 surfaces touch or rub against each other
May cause heating of machine
The less work that the machine has to do to overcome friction, the more efficient it is
Work output is always less than work input
No work is lost
Work output + work to overcome friction = work input

15. There can’t be an ideal machine
An ideal machine would have 100% efficiency
Since any time 2 surfaces rub together there is friction, this is impossible
Technologies exist to increase efficiency by decreasing friction
Ideas?

16. Increasing Efficiency