1. Work and Machines Chapter 14

2. Work and Power
Work – occurs when a force causes an object to move in the direction of the force.
How do you know if work is being done?
The object moves as a force is applied.
The direction of the object’s motion is the same as the direction of the force being applied.

3. Example:
You exert an upward force on the suitcase. But the motion of the suitcase is forward. Therefore you are not doing “work” on the suitcase.

4. Question:
If you pulled a wheeled suitcase instead of carrying it, would you be doing work on the suitcase? Why or why not?

5. Answer:
Pulling a wheeled suitcase is doing work because the force applied and the motion of the suitcase are in the same direction.

6. Calculating WORK Work = Force x Distance Remember:
Force in measured in Newtons
The SI unit for distance are meters
The units for work are newton-meters or joules (J)

7. Work depends on force and distance
The force needed to lift an object is equal to the gravitational force on the object – in other words, the objects weight.

8. Cont’d
Increasing the amount of force increases the amount of work done.

9. Cont’d
Increasing the distance also increases the amount of work done.

10. POWER Def: the rate at which work is done. Power= Work time Remember:
Is increased as more work is done in a given amount of time.
Power= Work
time
Remember:
Unit for work =Joule
Unit for time= second
J/s = Watts (W)

11. What is a machine?
Def: a device that helps make work easier by changing the size or direction of a force.

12. Input and Output Work Input – the work that you do to a machine.
Work Output – the work done by the machine.
OUTPUT FORCE
INPUT FORCE

13. Do machines save work?
NO! Machines make work easier but do not get rid of the work to be done.

14. Mechanical Advantage
Def: tells you how many times the machine multiplies force.
Compares the input and output forces
Mechanical Advantage (MA)= output force
input force

15. Example: MA = 500 N 50 N MA = 10 OUTPUT FORCE = 500 N INPUT FORCE

16. Cont’d
Because the MA of the hand truck is 10, that means that the output force is 10 times bigger than the input force.
The larger the MA , the easier a machine makes your work.
BUT as your MA increases, the distance that the output force moves the object decreases.

17. Mechanical Efficiency
Def: a comparison of a machine’s work output with the work input.
Mechanical Efficiency= work output
work input
*the 100 means that the answer is a percentage.
X 100

18. REVIEW
Work is done on a ball when a pitcher throws it. Is the pitcher still doing work on the ball as it flies through the air? Explain.
No, the pitcher is no longer doing work on the ball as it flies through the air because he is no longer exerting a force on it.
However, work is being done on the ball by the Earth, which exerts a force on the ball and pulls it back toward the ground.

19. Review: 2. Explain the difference between work and power?
Work occurs when a force causes an object to move in the direction of the force.
Power is the rate at which work is done.
*the more work you do the more power you get.

20. Review:
3. You lift a chair that weighs 50 N to a height of 0.5 m and carry it 10 m across the room.
*How much work do you do on the chair and when does the work occur?
Work is done on the chair only when it is picked up, not when it is carried across the room.
W=50 N x 0.5 m
W= 25 J

21. Review 4. Explain how using a ramp makes work easier
Using a ramp makes work easier because it allows you to apply a smaller input force than you would have to apply when lifting a load straight up.
However, the smaller force has to be exerted over a longer distance.

22. Review: 5. Why can’t a machine be 100% efficient?
A machine can’t be 100% efficient because some of the work input is used to overcome friction.
Therefore, work input is always greater than work output.

23. Review:
6. Suppose you exert 15 N on a machine, and the machine exerts 300 N on another object. What is the machine’s mechanical advantage? MA = 300 N 15 N MA = 20

24. The Types of Simple Machines
There are 6 types of simple machines:
Levers
Inclined Planes
Wedges
Screws
Wheel and Axle
Pulleys

25. LEVERS
Def: a simple machine consisting of a bar that pivots at a fixed point, called a fulcrum.
Levers are used to apply force to a load.

26. First Class Levers
The fulcrum is between the input force and the load.
Ex: hammer pulling a nail out, seesaw

27. Second Class Levers
The load is between the fulcrum and the input force.
Ex: wheelbarrow, bottle cap opener

28. Third Class Levers
The input force is between the fulcrum and the load.
Ex: using dumb bell weights, Using a hammer

29. Mechanical Advantage of Levers
MA = length of input arm / length of output arm

30. Inclined Planes
Def: a simple machine that is a straight, slanted surface.
Ex: a ramp, a door stop

31. Mechanical Advantage of Inclined Planes
MA = length of slope / height of slope

32. Wedges Def: a double inclined plane that moves.
Ex: knives, an axe blade

33. Screws
Def: an inclined plane that is wrapped in a spiral.
Ex. screw

34. Wheel & Axle
Def: a simple machine consisting of two circular objects of different sizes.
Ex: Ferris wheels, the steering wheel on a car

35. Mechanical Advantage of Wheel and Axle
MA = radius of wheel / radius of axle

36. Pulleys
Def: a simple machine consisting of a grooved wheel that holds a cable or rope.
Two types:
Movable
Fixed
*Many combined pulleys are called a block and tackle

37. Fixed Pulley
EFFORT FORCE
LOAD

38. Movable Pulley
EFFORT FORCE
LOAD
EFFORT
FORCE
LOAD

39. Compound Machines
Def: machines that are made of two or more simple machines.
Ex: can opener (wedge, wheel & axle, 2nd-class lever), scissors (wedge, and two 1st –class levers

40. Review:
Identify the simple machines that make up tweezers and nail clippers.
2. What would you call this type of machine?

41. Answers:
Each side of the tweezers is a 3rd-class lever. The sharpened edges of the nail clippers are wedges, and the arm that activates the clipper is a 2nd-class lever.
2. Compound Machine

42. Review:
3. What is the unit for work? The joule (J)

43. Review: 4. Which of the following is NOT a simple machine?
A water faucet handle
b. A jar lid
c. A can opener
d. A seesaw
* The can opener – it is a compound machine

44. Review: 5. A machine can increase: Distance and the expense of force
b. Force at the expense of distance
c. Neither distance nor force
d. Both (a) and (b)
*d. Both (a) and (b)

45. Review:
6. What is the unit for power? The watt (W)

46. Review:
7. What is power? How fast work is being done.