1. Work & Mechanical Advantage

2. Mechanical Work
the exchange of energy for some change in a physical system
symbolized by W and measured in joules

3. Mechanical Work
W = Fd
F is the force applied to the system parallel to the system’s displacement, in Newtons

4. Mechanical Work
W = Fd
d is the displacement of the system, in meters

5. Mechanical Work a scalar quantity
can be positive, negative, or zero (ex. in your text)

6. There are three conditions for work to occur:
Mechanical Work
There are three conditions for work to occur:
A force must be applied.
The object must move.
The force must be applied in the direction of motion.

7. When work is done on a system it gains energy.
Mechanical Work
When work is done on a system it gains energy.

8. Sample Problem 1 W = Fd W = (100 N)(7 m) W = 700 N•m = 700 J
Take a force of 100 N (about 22 pounds) and use it to move an object 7 m. How much work has been done?
W = Fd
W = (100 N)(7 m)
W = 700 N•m = 700 J

9. Sample Problem 2 W = Fd W = (157 N)(3.89247 m)
If a 157 N force is applied to an object, and this force causes the object to move m, how much work has been done?
W = Fd
W = (157 N)( m)
W = N•m = 611 J

10. The unit of power is the watt.
Power is the rate of doing work. It is how fast the energy is released.
power =
work
time
The unit of power is the watt.

11. Power
A machine doing more work in the same time or the same work in less time than another machine, means it has more power.

12. Power work power = time work = force • distance force • distance
Remember
work =
force • distance
force • distance

13. Sample Problem 3 W ΔT (550 N) • (300 m) 1650 s Power = = = 100 watts
If you weigh 550 N and run to the top of a skyscraper (300 m) in 1650 seconds, what would the power usage be? W ΔT
(550 N) • (300 m)
1650 s
Power = =
= 100 watts

14. Simple Machines Simple machines can help do work faster and easier.
They help reduce the toil of work caused by the curse on fallen man.
They do not change the amount of work necessary to do the job.

15. Simple Machines Simple machines can be classified as: levers
wheels and axles
inclined planes

17. http://www. angelfire. com/mb2/mbench/ch7/Pictures/wheel_axle_examples

19. Efficiency
Theoretically, a machine can be no more than 100% efficient.
Practically, a machine must be less than 100% efficient.
Efficiency is expressed as a percentage using the formula:
Wout
× 100
Win

20. Sample Problem 4
If a fan receives 352 J of electrical energy and produces 328 J of mechanical energy, what is its efficiency?
(3 SDs)
Wout
Win
× 100
328 J =
× 100
= %
352 J
= 93.2% efficient

21. Distance Principle How do machines make work easier? By reducing the
needed to do
amount of force
the work.

22. Distance Principle W F d = If W stays the same and F decreases,
what must happen to d?
It must increase.

23. So the force is reduced by exerting that force over a longer distance.
Distance Principle d F W =
So the force is reduced by exerting that force over a longer distance.

24. Mechanical Advantage (MA)
The reduction in force is called the Mechanical Advantage (MA).
There are two types of Mechanical Advantages:
Actual MA (AMA)
Ideal MA (IMA)

25. Mechanical Advantage (MA)
One way to calculate the AMA is:
weight
resistance Fr = =
force
effort Fe

26. Mechanical Advantage (MA)
In this example the force exerted, the effort, is 550 N.
The weight or resistance is 3,300 N.
550 N
3,300 N

27. Mechanical Advantage (MA)
resistance
3300 N
AMA =
= 6
3,300 N
550 N
550 N
effort
MA has no units.
Force is multiplied 6 times by exerting that force over 6 times the distance.

28. Mechanical Advantage (MA)
The AMA shows the helpfulness or usefulness of the machine.
The AMA includes the effects of friction.
We will often assume there is no friction. Then we can compute the IMA.
The AMA is always less than the IMA.

29. Mechanical Advantage (MA)
IMA is determined just from the dimensions of the simple machine.

30. Mechanical Advantage (MA)
Another way to compute the IMA of a simple machine is to divide the displacement of the effort by the displacement of the resistance.
IMA = d effort = 12 m
d resistance m
IMA = 6

31. T/F If you push against an object, but it does not move, work was done.

32. What is the formula for work?
W = f • d
W = f / d
W = f + d
W = f − d

33. What work is done when a force of 500 N moves an object 7.00 m?

34. What is the SI unit of work?
N•m J
Volt
Watt

35. Why use a machine?
It makes less work.
It changes the direction.
It uses less force.
It uses more mass.

36. How much power is generated if 2500 joules of work are done in 10 seconds?
2,490 watts
250 watts
25,000 watts
2,510 watts

37. What is the SI unit of power?
N•m J
Volt
Watt