1. Chapter 5 Work and Machines

2. WORK
The transfer of energy that occurs when a force makes an object move
In order for work to be done a force must make an object move

3. 2 Conditions of work 1. The object must move
2. Movement must be in the same direction as the force applied

5. Work and Energy When work is done there is always a transfer of energy
Energy is the ability to do work
When you do work on an object, you increase its energy

6. Calculating work Work (J) = Force (N) x distance (m) W = Fd
Example: How much work is done when a force of 33N pulls a wagon 13 meters?
W = (33N) x (13m) = 429 J

7. Work Examples
A couch is pushed with a force of 75 N and moves a distance of 5 m across the floor. How much work is done in moving the couch?

8. Work Examples
W = f x d
W = 75 N x 5 m
W = 375 J

9. Work Examples
The brakes on a car do 240,000 J of work in stopping the car. If the car travels a distance of 50m while the brakes are being applied, what is the force the brakes exert on the car?

10. Work Examples
W = f x d
240,000 J = f x 50m
f = 4800 N

11. Power The rate at which work is done SI unit for power is the Watt (W)
P (watts) = Work (J)/Time (s)
Usually expressed in kilowatts

12. Power Examples
In lifting a baby from a crib, 50J of work are done. How much power is needed if the baby is lifted in 2.0 s?

13. Power Examples
P = w/t
P = 50 J/ 2.0 s
P = 25 W

14. Power Examples
The power produced by an electric motor is 500 W. How long will it take the motor to do 10,000 J of work?

15. Power Examples
P = w/t
500 W = 10,000 J/ t
t = 20 s

16. A device that makes doing work easier
Machine
A device that makes doing work easier

17. 2-Forces Involved in Work Done by Machines
1. Input Force: The force that is applied to the machine. (effort force)
Given the symbol Fin
Output Force: The force applied by the machine.
Given the symbol Fout

18. Mechanical Advantage The ratio of the output force to the input force.
Can be calculated using the following equation.

19. Calculating Mechanical Advantage
Calculate the mechanical advantage of a hammer if the input force is 125 N and the output force is 2,000 N.

20. Calculating Mechanical Advantage
MA = Fout/Fin
MA = 2,000 N / 125 N
= 16

21. Conserving Energy A machine cannot create energy, so…
Wout can never be greater than Win

22. Which means that… Wout will always be less that Win …
because friction changes some of the energy to heat.

23. A machine without friction. Win = Wout
Ideal Machine
A machine without friction.
Win = Wout

24. Efficiency
A measure of how much of the input work put into a machine is change into useful output.

25. Efficiency Cont… The efficiency of an ideal machine is 100 percent.
The efficiency of a real machine is always less than 100 percent.

26. How can machines be made more efficient???
by adding a lubricant, such as oil or grease, to surfaces that rub together.

27. Simple Machines
A Machine that does work with only one movement of the machine
6 types of simple machines: lever, pulley, wheel and axle, inclined plane, screw, and wedge

28. Lever A bar that is free to pivot or turn around a fixed point
The fixed point is called a fulcrum
There are 3 classes of levers

29. Ex. – Screwdriver opening a paint can
Ex. – Wheelbarrow
Ex. – Baseball Bat

30. Pulley
A grooved wheel with a rope, chain, or cable running along the groove
The axle of the pulley acts as the fulcrum
Fixed and movable
Example - Elevators

31. Wheel and Axle consists of a shaft or axle attached to a larger wheel.
The wheel and axle rotate together
Examples: Pencil sharpeners, door knobs, screw drivers, faucet handles

32. Inclined Plane A sloping surface, such as a ramp.
Reduces the amount of force required to do work

33. Screw An inclined plane wrapped in a spiral around a cylindrical post
Example – Lid of a peanut butter jar

34. Wedge An inclined plane with one or two sloping sides
It changes the direction of the input forces
Example - Knife

35. Compound Machine Two or more simple machines that operate together
Examples – car, can opener

36. Compound Machines