2. Chapter Eight: Work
8.1 Work

3. 8.1 Work
In science, work is a form of energy you either use or get when a force is applied over a distance.
You do 1 joule of work if you push with a force of 1 newton for a distance of 1 meter.

4. 8.1 Work
When thinking about work, remember that work is done by forces that cause movement.
If nothing moves (distance is zero), then no work is done.

5. 8.1 Work
Force (N)
W = F x d
Work (joules)
Distance (m)

6. 8.1 Work and energy Doing work always means transferring energy.
The energy may be transferred to the object you apply the force to, or it may go somewhere else.

7. 8.1 Work and energy
You can do work to increase an object’s potential energy.
Then the potential energy can be converted to kinetic energy.

8. 8.1 Work
A raised object’s potential energy equals the amount of work it can do as it moves down.
The amount of kinetic energy an object has equals the amount of work the object can do by exerting force as it stops.

9. 8.1 Work
If force is equivalent to the weight of the object in newtons, and
height (h) is equivalent to distance (d),
Then multiplying the weight by height gives you the amount of work the object can accomplish as it moves down (as well as its potential energy).

10. 8.1 Work
Force A does no work because it does not cause the block to move.
Force B is applied at an angle to the direction of motion, so only part of force B does work.
The most effective force to move the block is force C.

11. Solving Problems
How much work is done by a person who pulls a cart with a force of 50 newtons if the cart moves 20 meters in the direction of the force?

12. Looking for: Given: Relationships: Solution Solving Problems
…work done by person
Given:
…force = 50 N (forward);
…distance = 20 m
Relationships:
Work = force x distance
Solution
50 N × 20 m = 1,000 joules.

13. Chapter Eight: Work
8.2 Efficiency and Power

14. 8.2 Efficiency and Power
Every process that is done by machines can be simplified in terms of work:
work input: the work or energy supplied to the process (or machine).
work output: the work or energy that comes out of the process (or machine).

15. 8.2 Efficiency and Power
A rope and pulley machine illustrates a rule that is true for all processes that transform energy.
The total energy or work output can never be greater than the total energy or work input.

16. 8.2 Efficiency 65% of the energy in gasoline is converted to heat.
As far as moving the car goes, this heat energy is “lost”.
The energy doesn’t vanish, it just does not appear as useful output work.

17. 8.2 Efficiency efficiency = Wo Wi
The efficiency of a machine is the ratio of usable output work divided by total input work.
Efficiency is usually expressed in percent.
Output work (J)
efficiency = Wo Wi
x 100%
Input work (J)

18. Solving Problems
You see a newspaper advertisement for a new, highly efficient machine. The machine claims to produce 2,000 joules of output work for every 2,100 joules of input work.
What is the efficiency of this machine?
Is it as efficient as a bicycle?
Do you believe the advertisement’s claim? Why or why not?

19. Looking for: Given: Relationships: Solution Solving Problems
…efficiency of machine
Given:
…Wi = 2100 J, Wo = 2000 J
Relationships:
% efficiency = Wo x 100 Wi
Solution
2000 J ÷ 2100 J x 100 = 95% efficient

20. 8.2 Power The rate at which work is done is called power.
It makes a difference how fast you do work.

21. 8.2 Power Michael and Jim do the same amount of work.
Jim’s power is greater because he gets the work done in less time.

22. 8.2 Power Power is calculated in watts.
One watt (W) is equal to 1 joule of work per second.
James Watt, a Scottish engineer, invented the steam engine.
James Watt explained power as the number of horses his engine could replace.
One horsepower still equals 746 watts.

23. 8.2 Power
Work (joules)
Power (watts)
P = W t
Time (s)

24. Solving Problems
Allen lifts his weight (500 newtons) up a staircase that is 5 meters high in 30 seconds.
How much power does he use?
How does his power compare with a 100-watt light bulb?

25. Solving Problems Looking for: Given: Relationships: Solution …power
Fweight= 500 N; d = 5 m, t = 30 s
Relationships:
W = F x d; P = W ÷ t
Solution
W = 500 N x 5 m = 2500 Nm
P = 2500 Nm ÷ 30 s = 83 watts
Allen’s power is less than a 100-watt light bulb.

26. Chapter Nine: Simple Machines
9.1 Types of Simple Machines

27. 9.1 Using Machines
A machine is a device with moving parts that work together to accomplish a task.
A bicycle is a good example.

28. 9.1 Using Machines
The input includes everything you do to make the machine accomplish a task, like pushing on the bicycle pedals.
The output is what the machine does for you, like going fast or climbing a steep hill.

29. 9.1 Forces in Machines
A simple machine is an unpowered mechanical device, such as a lever.

30. 9.1 Review of work
Every process that is done by machines can be simplified in terms of work:
work input: the work or energy supplied to the process (or machine).
work output: the work or energy that comes out of the process (or machine).

31. 9.1 Review of energy
A rope and pulley machine illustrates a rule that is true for all processes that transform energy.
The total energy or work output can never be greater than the total energy or work input.

32. Chapter Nine: Simple Machines
9.2 Mechanical Advantage

33. 9.2 Mechanical advantage
Mechanical advantage is the ratio of output force to input force.

34. 9.2 Mechanical advantage Machines multiply forces.
One person could lift an elephant—quite a heavy load—with a properly designed system of ropes and pulleys!

35. 9.2 Mechanical Advantage MA = Fo Fi Output force (N) mechanical
Input force (N)

37. 9.2 The Lever
A lever includes a stiff structure (the lever) that rotates around a fixed point called the fulcrum.

38. 9.2 The Lever
Levers are useful because you can arrange the fulcrum and the input arm and output arm to adjust the mechanical advantage of the lever.

40. 9.2 The Lever
Each class of levers is defined by the location of the input and output forces relative to the fulcrum.

41. 9.2 Gears
Many machines require that rotating motion be transmitted from one place to another.
Gears change force and speed.

42. 9.2 Designing Gear Machines
The gear ratio is the ratio of output turns to input turns.
You can predict how force and speed are affected when gears turn by knowing the number of teeth for each gear.

43. 9.2 Gear Ratios To = Ni Ti No Number of teeth on input gear
Turns of output gear
To = Ni
Ti No
Turns of input gear
Number of teeth
on input gear

44. 9.2 Tension Ropes and strings carry tension forces along their length.
If the rope is not moving, its tension is equal to the force pulling on each end.

45. 9.2 Rope and Pulley A rope turns around a round wheel called a pulley.
A rope and pulley multiply the force of the pull on the rope.

46. 9.2 Rope and Pulley
MA = Force of tension / number of pulleys

47. 9.2 Ramps
A ramp is a simple machine that allows you to raise a heavy object with less force than you would need to lift it straight up.
The mechanical advantage of a ramp is the ramp length divided by the height of the ramp.

48. 9.2 Screws A screw is a rotating ramp.
You find the mechanical advantage of a screw by dividing its circumference by the lead.

49. 9.2 Screws
A wedge is like a ramp that can work while in motion (a ramp is always stationary).
A wedge has a side that slopes down to a thin edge.
The mechanical advantage for a wedge is inversely related to the size of the wedge angle.

50. 9.2 Wheel and axle A wheel rotates around a rod called an axle.
The mechanical advantage is the ratio of the radius of the wheel to the radius of the axle.
The wheel and axle move together to move or lift loads.

51. Solving Problems
A crowbar is a type of lever that you use to pull a nail out of a piece of wood.
If the handle of a crowbar is 40 centimeters and the foot is 2 centimeters, what is its mechanical advantage?

52. Looking for: Given Relationships: M.A. = Length of input arm
Solving Problems
Looking for:
…mechanical advantage of lever
Given
…input arm = 40 cm; output arm = 2 cm
Relationships:
M.A. = Length of input arm
Length of output arm
Solution
M.A. = 40 cm ÷ 2 cm = 20

53. Chapter Nine: Simple Machines
9.3 Levers in the Human Body

54. 9.3 Levers in the human body
The human body is a complex machine that includes a number of simple machines—levers.
Your arms and legs, for example, work as levers to move and lift objects.

55. 9.3 Levers in the human body
A classic example of a third class lever is a broom.
A broom does not multiply force, but it does multiply speed.
Since your limbs are third class levers, they multiply speed to do tasks quickly.

56. 9.3 Levers in the human body
Human arms and legs are examples of third class levers because the input forces are between a fulcrum and the output force.
The output force is what you accomplish with your hands and feet.
Where is the input force and the fulcrum?

57. 9.3 Levers in the human body
In the human body, all bones act as levers and each joint can serve as a fulcrum.
When lifting your head, your neck works as a first-class lever.
When you stand on your toes, the feet act as second-class levers.
When biting, your jaw works as a third-class lever.