1. Work and Energy

2. Work Work? Yuk! Which is more work?… Studying 3 hrs for an exam OR
Bowling?
BOWLING! (physically speaking)

3. What is work? Chair demo Two identical chairs/stools It seems that…
Work deals with what two variables…
Distance
Force
Work is applying some amount of force over some distance

4. Formula and Units of Work=
force X
distance
units
Newtons (N)
Meters (m)
Work =
N m “Newton meter”
Joule!
A Newton meter is also called a…
Thus Work will be measured in Joules.

5. A quick example How exactly is bowling work?
Your hand pushes the bowling ball over a short distance to get it going.
Let’s say while bowling you exert a 20 Newton force over a .5 meter distance. How much work have you done?
W=Fd
W= (20 N) (.5m)
W= 10 J (Nm)
You study reviewing your notes for 3 hrs. How much work have you done?
NONE!

6. Work is a Vector! (direction matters)
A boy pulls a crate along the ground using a rope.
Warning!
Be sure the force and distance are in the same direction!
Use cosine!
52 Newton pull
25 Newton box
520 angle
22 meters
How much work is done on the box? (direction!)
Cal= 704
R= 7.0 x 102 J

7. Remember…
The force and distance must be IN THE SAME DIRECTION in order to calculate work in that direction.

8. How Fast Can You do Work? POWER
Power is the rate at which work is done.
Work (J) P =
Time (s)
Power = J/s
A J/s is also called a Watt
Units?
Thus a Watt is how much work is done each second that passes

9. A quick example A girl lifts a box off the floor.
(78N)= F= weight (F=ma a=9.80m/s2)
8.0 kg
The force while lifting lasts 2.0 seconds
P= 78N(1.5m)
Moves 1.5 meters upward
2.0sec
Just how powerful was this girl?
Cal= 58.5 or 60.

10. 59 Watts or 60.!

11. Changing Work?
Is it possible to lessen the amount of work it takes to accomplish a given task?
Ask your neighbor what they think
Have you considered using a simple machine (lever, incline plane, pulley, etc.) in your discussion?
Would using one make a difference?

12. The answer: In a word… NO!
Any given task takes a certain amount of work to accomplish it…
This amount of work cannot be lessened
A simple machine can help accomplish doing this amount of work, but the work needed will be the same!

13. Simple machine Machine- a device that helps you do work
Simple machine- a machine that does work using one movement
The machines we will study will all be run by HUMAN power- not motors.
We will study these 4
Levers, Incline planes, wheel and axles, and pulleys

14. Simple machines!
A screw is easier to put into a board with a screwdriver- Magic!
I can lift a car as long as I use a large enough lever- Magic!
With the aid of an incline plane I can lift objects that otherwise I could not- Magic!
Using a pulley, I can lift a piano to a third story building- Magic!

15. Levers Levers consist of two parts:
A board or bar that is free to rotate
A fulcrum (point on which the bar pivots)
LET’S USE A LEVER TO SEE HOW THEY WORK

16. How levers (and all machines) accomplish “magic”
Assume it takes 20 J to accomplish a task (W=FD)
If I use 10 N of force to do the work…
20 J = 10N (?d)- what distance must I go?
2 m! (10N x 2 m= 20 J)
What if I am only strong enough to put in 4 N of force? Can I still accomplish the task?
Yes- (4 N x 5m) = 20 J
If less Force is used, more distance must be covered!
This is true for all simple machines… there is a tradeoff between force and distance!

17. IMA de dr Effort distance IMA Or = Resistance distance
Ideal Mechanical Advantage
The number of times the machine should multiply the force you put into it (ignore friction involved).
If you move 16 m and in return the machine moves an object 4m, what is the IMA?
4! (note: IMA has no units)
Effort distance de
IMA Or =
Resistance distance dr
Effort distance= the distance you (the human) moves to accomplish a task
Resistance distance= the distance the machine moves in return to accomplish a task

18. Lets try one… IMA of lever = 3.0! (no units) IMA= de Ribbit 4.0 N dr
A frog lifts a crate of flies using a lever.
IMA= de
Ribbit
4.0 N dr
6.0 m frog
Caution:
flies
IMA=
2.0 m machine
2.0 m
2.0 N
6.0 m
What is the IMA of this machine?

19. It’s 3 times easier with the machine!
Conclusion?
What does an IMA of 3 mean in reality?
The machine is multiplying what the frog puts in by 3!
It’s 3 times easier with the machine!
Caution:
flies
One froggy NightOn.vob

20. AMA Fr Fe Resistance Force AMA Or = Effort Force
Actual Mechanical advantage
The number of times the machine actually multiplies the work you put into it (friction stops it from being “ideal”).
If you push with 4 N and the machine actually gives back 12 N what is the AMA?
3! (still no units)
Resistance Force Fr
AMA Or =
Effort Force Fe
Effort Force= the force you (the human) exert to accomplish a task.
Resistance Force= the force the machine exerts to accomplish a task

21. Revisit the frog AMA of lever = 2! (no units) AMA= Fr Ribbit 4.0 N Fe
A frog lifts a crate of flies using a lever.
AMA= Fr
Ribbit
4.0 N Fe
4.0 N machine
Caution:
flies
AMA=
2.0 N frog
2.0 m
2.0 N
6.0 m
What is the AMA of this machine?

22. Conclusion?
What does an AMA of 2 (not 3 like IMA) mean?
The machine is actually multiplying what the frog puts in by 2!
Friction at work here!
2 not 3!
Caution:
flies

23. Efficiency of a machine
Conceptually…
Is a measure of how close to “ideal” a machine is performing
Two ways to find it.
X 100
AMA
Efficiency =
IMA
Work Out (machine puts out) OR
X 100
Work in (you put in)

24. Efficiency? 4.0 N flies 2.0 m 2.0 N 6.0 m W= fd AMA Using Eff=
Using Eff= Wout
IMA 2
Win
X 100 3 =
.67 or 67%
(4.0 N)
(2.0 m)
X 100
4.0 N
(2.0 N)
(6.0 m)
Caution:
flies
.67 or 67%
2.0 m
2.0 N
6.0 m

25. ENERGY
Energy- non-matter property that is capable of causing change/doing work.
We will primarily focus on only 2 types of energy
Potential Energy
Kinetic Energy

26. Kinetic Energy Anything that is moving has
kinetic energy (KE)- the energy of something moving.
Logically thinking, what factors of an object or how it is moving might affect how much energy it has?
Its velocity
Its mass

27. ½ KE’s formula and units m v 2 KE = ½mv2 KE x Kg x (m/s)2 = J (Joules)
How do you think m and v are related to KE?
Think of a real life example…
Do you think that mass or velocity has more effect when it come to how much KE it has?
Take a look at the formula m v 2
KE = ½mv2 KE x =
= Kgm2/s2
Kg x (m/s)2
= J (Joules)
Units?:

28. Try a quick example A dog launches a shot-put. 7.26 Kg 22.0 kg
Vi = 7.50 m/s
7.26 Kg
22.0 kg
Needed?
How much KE does the shot have as he releases it?
Don’t forget to square velocity!
KE= ½ mv2 2
R= 204 J!
½ (7.26 kg)
(7.50 m/s)
Cal =
KE =

29. Potential Energy
Potential energy (PE): Stored energy of an object, often due to its location.
Picture a rock on a ledge…
It has no kinetic energy…
but it has the potential to fall because its height.
It currently has only Potential Energy due to its height.
What 3 variables are affecting this rock’s potential energy?
mass
Gravity a=-9.80 m/s2= g
Distance/height

30. PE’s formula and units = J PE = mgh h= m Units? Mass= kg g = m/s2
m= mass g= m/s2 h = relative height
h= m
Units?
Mass= kg
g = m/s2
PE= kg m2/s2
= J

31. A quick example A pole vaulter vaults over a bar using a pole.
…an aptly named sport.
m= 72.0 Kg
h= 4.50 m
What PE does he have at the top of his vault?
Cal=
R= 3180 J

32. Work & Energy are linked!
Mathematically: They are connected by the unit of Joules
Conceptually: It takes work to give an object potential energy. The work you put into it is equal to the E it has.

33. The situation Potential Energy
You carry a rock from ground level to the top of the football stadium- then drop it.
Potential Energy
Has a certain amount of PE at the top (J).
Kinetic Energy
As it hits the ground it should have the same J of Kinetic energy. WHY?
10m
5 kg

34. The Math to prove the concept
You carry a rock from ground level to the top of the football stadium- then drop it.
Work
W=fd
W= (49 N) (10.m)
Potential Energy
PE= mgh
PE= (5kg) (9.80 m/s2) 10m
Kinetic Energy
KE= ½ mv2
KE = ½ (5 kg) (14 m/s)2
49N= 5 kg
10.m
49N
490 J
490 J
490 J

35. The Conservation of Energy
Energy in a system may change forms but is not created or destroyed

36. Bellwork: Date:
PE= 353 J
PE at top=KE at bottom
A 3.00 Kg ball is dropped from a 12.0 meter high building. How fast will it be moving as it hits the ground? (do not use acceleration formulas!)
V= 15.3 m/s as it hits the ground