1. Work and Power

2. What is work?
In science, the word work has a different meaning than you may be familiar with.
The scientific definition of work is: using a force to move an object a distance (when both the force and the motion of the object are in the same direction.)

3. Work or Not?
According to the scientific definition, what is work and what is not?
a teacher lecturing to her class
a mouse pushing a piece of cheese with its nose across the floor

5. What’s work?
A scientist delivers a speech to an audience of his peers.
A body builder lifts 350 pounds above his head.
A mother carries her baby from room to room.
A father pushes a baby in a carriage.
A woman carries a 20 kg grocery bag to her car?

6. What must happen in order for work to be done?
A force must be applied in the direction of the motion.
The object must move in the same direction as the force.

7. Work Concepts
Work (W) product of the force exerted on an object and distance the object moves in the direction of the force.
W is transfer of energy by mechanical means.
ME = KE + PE
W is done on an object only if it moves in the direction of the force (or parallel to the force).
If the force and displacement are in opposite directions, the work done is negative.
Only the component of the force in the direction of the motion does work.

8. Work = Force x displacement (d)
Work Formulas
Work = Force x displacement (d)
W = Fd
The unit for Work is ________.
The unit for Force is ________.
The unit for Displacement/Dist. is ____.
1 joule (J) = 1 Newton * meter (N * m) F d

9. W=FD
Work = F x d
Calculate: If a man pushes a concrete block 10 meters with a force of 20 N, how much work has he done?

10. W=Fd
Calculate: If a man pushes a concrete block 10 meters with a force of 20 N, how much work has he done? 200 joules
(W = 20N x 10m)

11. Power Power is the rate at which work is done. Power = Work*/Time
P = W / t FYI – W = Fd
P = Fd / t
The unit of power is the watt.
Watt (W) 1 joule of energy transferred in 1 second.
Kilowatt ( kW) = ___________ watts

12. √ YR Understanding
1. Two physics students, Ben and Bonnie, are in the weightlifting room. Bonnie lifts the 50 kg barbell over her head (approximately .60m)
10 times in 1 minute; Ben lifts the 50 kg barbell the same distance over his head 10 times in 10 seconds.
a. Which student does the most work?
b. Which student delivers the most power?
Explain your answers.

13. Lets review taking the formulas into account.
Ben and Bonnie do the same amount of work; they apply the same force to lift the same barbell the same distance above their heads.
Yet, Ben is the most powerful since he does the same work in less time.
Power and time are inversely proportional.

14. Another power question
2. A man pushes a cart with a force of 10 N a distance of 5 meters in 2 seconds. How much power was involved with this action?
Find the work: W=Fd
W= 10 N x 5 meters = 50 J
Divide by the time to find power
P= W/t = 50 J / 2 seconds
P= 25 Watts

15. History of Work
Before engines and motors were invented, people had to do things like lifting or pushing heavy loads by hand. Using an animal could help, but what they really needed were some clever ways to either make work easier or faster.

16. 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.

17. Simple Machines
Lever
Wheel and Axle
Pulley
Inclined Plane
Wedge
Screw

18. 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.

19. Ideal Machines- write this info below Mech. Adv
Ideal Machines- write this info below Mech. Adv. Notes - back of 1st page, at the bottom.
Ideal machines exist only in a frictionless, and neglect air world.
No energy or work is lost to the system through outside forces
Formula for ideal machines: Work input = Work out put Findin = Fout dout

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

21. However, some output force is lost due to friction.
Efficiency
Efficiency % = Useful Work Out x Work In
However, some output force is lost due to friction.
The comparison of work input to work output is called efficiency.
No machine has 100 percent efficiency due to friction.

22. Work-Energy Theorem Work = Δ KE F*d = KEf – KEi
According to the Work-Energy theorem,
the work done on an obj., by the Net Force acting on it, is = to the Δ KE of the body
Work = Δ KE
F*d = KEf – KEi
F*d = ½ m vf 2 – ½ m vi2
F*d = ½ m (vf2 – vi2)

23. Work Energy Theorem cont.
According to the Work-Energy theorem, the work done on an obj., by the Net Force acting on it, is = to the Δ GPE of the body

24. Wk-Energy Theorem example
The KE of a falling apple changes from
40 J to J. During this time, how much work did the force of gravity transfer to the apple?
Work = Δ KE
Solve by finding the difference bet.
KE final and KE initial. ( Subtract )
100 J – 40 J = 60 J

25. Refresher: If the mass is tripled in the KE formula
The KE is ____________ .
So we say it increases by ____.
________________________________
If the velocity is tripled in the KE formula
The KE is exponentially increased.
Therefore, we say it increases by ( )2
which = ____
So if the m and v are tripled in the KE formula
We say m = ___ x v = ___, which results in a factor of _____.