1. Chapter 14 Work, Power, and Machines
Physical Science

2. Resources CH 14 PPT slides w/ practice problems & assessment
Quizlet VOCAB Flashcards
Bozeman Science
Energy – Work – Power (3.5 min.)
Work & Power – TEd Ed (4.5 min.)
Other realworld examples of work & power (8 min)

3. Work and Power Work – “product of force and distance”
done when a force acts on an object in the same direction as the object moves.
POWER – work / time
rate of doing work
more power = faster

4. WORK Requires Motion
A man is not actually doing work when holding the barbell above his head
Force is applied to barbell but…
If no movement, no work done
They do no work
He does work

5. Work Depends On Direction
Scenario 1:
All force acts in same direction of motion
= all force work.
Scenario 2:
Part applied force acts in the direction of motion
= part force does work.
Scenario 3:
None of force in direction of the motion
= force does no work.

6. Work Depends on Direction Continued…
All of the force does work on the suitcase.
The horizontal part of the force does work.
The force does no work on the suitcase.
This force does work
Force
This force does no work
Force
Direction of motion
Direction of motion
Direction of motion
Force and motion in the same direction
Part of force in direction of motion
Lifting force not in direction of motion

7. Work = Force x Distance
W = Fd
Remeber Force = mass x acceleration  F = ma
Joule (J) = SI unit for work
Unit: J = N(m)
Named after James Prescott Joule (1818 – 1889)
Research work and heat
Example: If a model airplane exerts 0.25 N over a distance of 10m, the plane will expend 2.5 J.
Work = Fd
= 0.25(10)
= 2.5 J

8. What is Power? Rate of doing work More power = work at a faster rate
Size of engine often indicates power
Can work at a faster rate

9. Which requires more Power?
Because the snow blower can remove more snow in less time, it requires more power than hand shoveling does.

10. Power = Work/Time P= W/t Watt (W) = SI unit for Power Units: W = J/s
Example Power Problem:
You exert a vertical force of 72 Newtons to lift a box to a height of 1.0 meter in a time of 2.0 seconds. How much power is used to lift the box?
step 1: read and understand (givens?)
step 2: Plan and Solve
Step 3: Look Back and Check

11. 1.
Math Practice – Page 415 # 1 - 3

12. James Watt and Horsepower
Horsepower (hp) = another unit for power
Equals ~746 watts
Defined by James Watt ( )
Trying to describe power outputs of steam engines
Horses were most common used source of power in 1700s

13. James Watt and Horsepower
The horse-drawn plow and the gasoline- powered engine are both capable of doing work at a rate of four horsepower.

14. Work and Machines 14.2

15. Machines Do Work! Machine – device that changes force Ex. Car jack
You apply force  jack changes force  applies much stronger force to lift car
Jack increase force you exerted
Make work easier in THREE WAYS:
They Change…
Change magnitude: size of force needed
Change direction: direction of force
Chance distance: distance over which force acts

16. Increasing Force – Change magnitude
Small force exerted over a large distance = large force over short distance
Like picking books up one at a time to move them
trade offs = more distance but less force

17. Increasing Distance Changing Direction
Decreases distance for force exerted and increases amount of force required
Tradeoff: increased distance = greater force exerted
Changing Direction
Boat moves in this direction.
Input force
Input distance
Output force
Output distance 17

18. Work Input and Work Output
Work input to a Machine
Input Force – Force you exert on a machine
Oar = force exerted on handle
Input Distance – Distance the input force act thru
How far handle moves
Work Input – work done by the input force
F x d
Work Output of a Machine
Output Force - force exerted by machine
Output Distance – distance moved
Work output – F x d
Less than input work b/c of friction
All machines use some input work to overcome friction
<100% efficient

19. SMARTStarter: How does the input force change
SMARTStarter: How does the input force change? How does the Input distancechange? Complete the table.
Machine
Force
Distance
Tire Jack
Input < Output
Lug Wrench
Rowing Oar

20. A nutcracker is a machine that converts the input force applied to it into a larger force capable of cracking a nut.
Because it increases force, the nutcracker has a mechanical advantage greater than 1.

21. Mechanical Advantage
How does the actual mechanical advantage of a machine compare to its ideal mechanical advantage?
The mechanical advantage of a machine is the number of times that the machine increases an input force.
Because friction is always present, the actual mechanical advantage of a machine is always less than the ideal mechanical advantage.

22. Mechanical Advantage
Actual Mechanical Advantage
The mechanical advantage determined by measuring the actual forces acting on a machine is the actual mechanical advantage.
The actual mechanical advantage (AMA) equals the ratio of the output force to the input force.

23. Mechanical Advantage
A loading ramp is a machine used to move heavy items into a truck.
The mechanical advantage of a ramp with a rough surface is less than that of a similar smooth ramp because a greater force is needed to overcome friction.

24. Mechanical Advantage
Ideal Mechanical Advantage
The ideal mechanical advantage (IMA) of a machine is the mechanical advantage in the absence of friction.
Because friction reduces mechanical advantage, engineers often design machines that use low-friction materials and lubricants.

25. Calculating Mechanical Advantage

26. Calculating Mechanical Advantage
The cable supporting the gondola forms an inclined plane, a type of machine. The inclined plane is used to move people up to the top of the mountain.

27. Calculating Mechanical Advantage
The gondola uses the inclined plane formed by its supporting cable to more easily move people uphill.
The increased horizontal distance (input distance) is greater than the vertical gain in height (output distance).
The inclined cable gives the gondola a mechanical advantage greater than 1.

28. Calculating Mechanical Advantage
Calculating IMA
A woman drives her car up onto wheel ramps to perform some repairs. If she drives a distance of 1.8 meters along the ramp to raise the car 0.3 meter, what is the ideal mechanical advantage (IMA) of the wheel ramps?

29. Calculating Mechanical Advantage
Read and Understand
What information are you given?

30. Calculating Mechanical Advantage
Read and Understand
What information are you given?

31. Calculating Mechanical Advantage
Plan and Solve
What unknown are you trying to calculate?

32. Calculating Mechanical Advantage
Plan and Solve
What unknown are you trying to calculate?

33. Calculating Mechanical Advantage
Plan and Solve
What formula contains the given quantities and the unknown?
Replace each variable with its known value and solve.

34. Calculating Mechanical Advantage
Plan and Solve
What formula contains the given quantities and the unknown?
Replace each variable with its known value and solve.

35. Calculating Mechanical Advantage
Look Back and Check
Is your answer reasonable?
The IMA must be greater than 1 because the input distance is greater than the output distance. The calculated IMA of 6 seems reasonable.

36. Answer: IMA = Input distance/Output distance IMA = 3 m/0.5 m = 6
Calculating Mechanical Advantage
1. A student working in a grocery store after school pushes several grocery carts together along a ramp. The ramp is 3 meters long and rises 0.5 meter. What is the ideal mechanical advantage of the ramp?
Answer: IMA = Input distance/Output distance
IMA = 3 m/0.5 m = 6

37. Calculating Mechanical Advantage
2. A construction worker moves a crowbar through a distance of 0.50 m to lift a load m off of the ground. What is the IMA of the crowbar?

38. Calculating Mechanical Advantage
2. A construction worker moves a crowbar through a distance of 0.50 m to lift a load m off of the ground. What is the IMA of the crowbar? Answer: IMA = Input distance/Output distance
IMA = 0.5 m/0.05 m = 10

39. Calculating Mechanical Advantage
3. The IMA of a simple machine is 2.5. If the output distance of the machine is 1.0 m, what is the input distance?

40. Calculating Mechanical Advantage
3. The IMA of a simple machine is 2.5. If the output distance of the machine is 1.0 m, what is the input distance? Answer: Input distance = (IMA)(Output distance)
Input distance = (2.5)(1.0 m) = 2.5 m

41. Efficiency
Why is the efficiency of a machine always less than 100 percent?
The percentage of the work input that becomes work output is the efficiency of a machine.
Because there is always some friction, the efficiency of any machine is always less than 100 percent.

42. Efficiency
Efficiency is usually expressed as a percentage.
EXAMPLE:
If the efficiency of a machine is 75 percent, then you know that 75 percent of the work input becomes work output.

43. Efficiency
If a machine requires 10.0 J of work input to operate, then the work output is 75% of
10.0 J.

44. Efficiency
Reducing friction increases the efficiency of a machine.
Roller bearings reduce the friction of the rotating wheels because rolling friction is less than sliding friction.
To further reduce the rolling friction, the roller bearings are also lubricated with grease.

45. Efficiency
Engineers analyze the flow pattern of a smoke trail to determine the fluid friction forces (air resistance) acting on the vehicle. Engineers use these data to optimize a vehicle's shape for maximum fuel efficiency.

46. Lever Lab 6 simple Machines? 3 classes of levers Part A – Part B –
First Class
Second Class
Third Class
Part A –
Part B –
Part C –
How much FORCE (N) is 500g of mass?
AMA vs IMA -- calculations

47. Simple Machines
Ancient people invented simple machines that would help them overcome resistive forces and allow them to do the desired work against those forces.

48. Simple Machines The six simple machines are: Lever Wheel and Axle
Inclined Plane
Wedge
Screw
Pulley

49. Simple Machines
A machine is a device that helps make work easier to perform by accomplishing one or more of the following functions:
transferring a force from one place to another,
changing the direction of a force,
increasing the magnitude of a force, or
increasing the distance or speed of a force.

50. Mechanical Advantage
It is useful to think about a machine in terms of the input force (the force you apply) and the output force (force which is applied to the task).
When a machine takes a small input force and increases the magnitude of the output force, a mechanical advantage has been produced.

51. Mechanical Advantage
Mechanical advantage is the ratio of output force divided by input force. If the output force is bigger than the input force, a machine has a mechanical advantage greater than one.
If a machine increases an input force of 10 pounds to an output force of 100 pounds, the machine has a mechanical advantage (MA) of 10.
In machines that increase distance instead of force, the MA is the ratio of the output distance and input distance.
MA = output/input

52. No machine can increase both the magnitude and the distance of a force at the same time.

53. The Lever
A lever is a rigid bar that rotates around a fixed point called the fulcrum.
The bar may be either straight or curved.
In use, a lever has both an effort (or applied) force and a load (resistant force).

54. The 3 Classes of Levers
The class of a lever is determined by the location of the effort force and the load relative to the fulcrum.

56. To find the MA of a lever, divide the output force by the input force, or divide the length of the resistance arm by the length of the effort arm.

57. First Class Lever
In a first-class lever the fulcrum is located at some point between the effort and resistance forces.
Common examples of first-class levers include crowbars, scissors, pliers, tin snips and seesaws.
A first-class lever always changes the direction of force (I.e. a downward effort force on the lever results in an upward movement of the resistance force).

58. Fulcrum is between EF (effort) and RF (load) Effort moves farther than Resistance. Multiplies EF and changes its direction

59. Second Class Lever
With a second-class lever, the load is located between the fulcrum and the effort force.
Common examples of second-class levers include nut crackers, wheel barrows, doors, and bottle openers.
A second-class lever does not change the direction of force. When the fulcrum is located closer to the load than to the effort force, an increase in force (mechanical advantage) results.

60. RF (load) is between fulcrum and EF Effort moves farther than Resistance. Multiplies EF, but does not change its direction

61. Third Class Lever
With a third-class lever, the effort force is applied between the fulcrum and the resistance force.
Examples of third-class levers include tweezers, hammers, and shovels.
A third-class lever does not change the direction of force; third-class levers always produce a gain in speed and distance and a corresponding decrease in force.

62. EF is between fulcrum and RF (load) Does not multiply force Resistance moves farther than Effort. Multiplies the distance the effort force travels

63. Wheel and Axle
The wheel and axle is a simple machine consisting of a large wheel rigidly secured to a smaller wheel or shaft, called an axle.
When either the wheel or axle turns, the other part also turns. One full revolution of either part causes one full revolution of the other part.

64. Pulley
A pulley consists of a grooved wheel that turns freely in a frame called a block.
A pulley can be used to simply change the direction of a force or to gain a mechanical advantage, depending on how the pulley is arranged.
A pulley is said to be a fixed pulley if it does not rise or fall with the load being moved. A fixed pulley changes the direction of a force; however, it does not create a mechanical advantage.
A moveable pulley rises and falls with the load that is being moved. A single moveable pulley creates a mechanical advantage; however, it does not change the direction of a force.
The mechanical advantage of a moveable pulley is equal to the number of ropes that support the moveable pulley.

65. Inclined Plane
An inclined plane is an even sloping surface. The inclined plane makes it easier to move a weight from a lower to higher elevation.

66. Inclined Plane
The mechanical advantage of an inclined plane is equal to the length of the slope divided by the height of the inclined plane.
While the inclined plane produces a mechanical advantage, it does so by increasing the distance through which the force must move.

67. Although it takes less force for car A to get to the top of the ramp, all the cars do the same amount of work.
A B C

68. Inclined Plane
A wagon trail on a steep hill will often traverse back and forth to reduce the slope experienced by a team pulling a heavily loaded wagon.
This same technique is used today in modern freeways which travel winding paths through steep mountain passes.

69. Wedge
The wedge is a modification of the inclined plane. Wedges are used as either separating or holding devices.
A wedge can either be composed of one or two inclined planes. A double wedge can be thought of as two inclined planes joined together with their sloping surfaces outward.

70. Screw The screw is also a modified version of the inclined plane.
While this may be somewhat difficult to visualize, it may help to think of the threads of the screw as a type of circular ramp (or inclined plane).

71. MA of an screw can be calculated by dividing the number of turns per inch.