1. (1) Work & Power
Unit 4

3. Energy ENERGY THERMAL The ability to cause change. MECHANICAL NUCLEAR
internal motion of particles
ENERGY
MECHANICAL
NUCLEAR
motion of objects
changes in the nucleus
ELECTRICAL
joules (J)
CHEMICAL
motion of electric charges
bonding of atoms

4. Energy Kinetic Energy (KE) energy in the form of motion
depends on mass and velocity
Which has the most KE?
Which has the least KE?
80 km/h
50 km/h
80 km/h truck
50 km/h motorcycle

5. Energy Potential Energy (PE) stored energy
depends on position or configuration of an object
Which boulder has greater gravitational PE?

6. https://www.youtube.com/watch?v=g7u6pIfUV y4

8. Work = Force x Distance W= F x d A transfer of energy!
Work is a force exerted across a distance!

9. Work Work is the transfer of energy through motion!!!
In order for work to be done, 2 things must happen:
Object must move. (displacement).
Applied force must be in the same direction of the displacement!!!

10. https://www.youtube.com/watch?v=u6y2RPQw 7E0

11. A teacher applies a force to a wall and becomes exhausted.
Work????
A teacher applies a force to a wall and becomes exhausted.
This is not an example of work. The wall is not displaced. A force must cause a displacement in order for work to be done.

12. A book falls off a table and free falls to the ground.
Work????
A book falls off a table and free falls to the ground.
Yes.
This is an example of work. There is a force (gravity) which acts on the book which causes it to be displaced in a downward direction (i.e., "fall").

13. Work????
A waiter carries a tray full of meals above his head by one arm straight across the room at constant speed.
No.
This is not an example of work. There is a force (the waiter pushes up on the tray) and there is a displacement (the tray is moved horizontally across the room). The applied force and the displacement are not in the same direction!!

14. Is any work done in lifting an object?
Is a force applied?
YES
Does object move?
Does object move in the direction of the applied force?

15. W = Fd W: work (J) F: force (N) d: distance (m) 1 J = 1 N·m Work
transfer of energy through motion
force exerted through a distance
W = Fd
W: work (J)
F: force (N)
d: distance (m)
1 J = 1 N·m
Distance must be in direction of force!

16. Additionally…
The force and the movement must be
in the same direction.

17. Work examples The SI unit for work is Joules W=Fd
Force is measured in Newtons
Distance is measured in meters
So work is Newtons x meters
This equals a Joule (J)
This is the amount of work used to lift a quarter- pound cheeseburger 1 meter.

18. NO! The weight-lifter holds the weight over his head. Is he doing
work?
NO!
There is no change in distance!

19. W= F x d
F = Newtons (N)
d = meters (m)
W = N·m or Joule (J)

20. W= F x d How much work is done by a crane that raises 1000 N of
material a distance of 150 m?
W = F x d
= 1000 N x 150 m
= 150,000 J

21. A force of 10,000 N is applied to a stationary wall. How
much work is performed?
W = F x d
= 10,000 N x 0 m!
= 0 J!
No work is done because there is no movement!

22. Brett’s backpack weighs 30 N
Brett’s backpack weighs 30 N. How much work is done on the backpack when he lifts it 1.5 m from the floor to his back?
GIVEN:
F = 30 N
d = 1.5 m
W = ?
WORK:
W = F·d
W = (30 N)(1.5 m)
W = 45 J

23. d W F GIVEN: m = 40 kg dd = 1.4 m - during da = 2.2 m - after W = ?
A dancer lifts a 40 kg ballerina 1.4 m in the air and walks forward 2.2 m. How much work is done on the ballerina during and after the lift?
GIVEN:
m = 40 kg
dd = 1.4 m - during
da = 2.2 m - after
W = ?
WORK:
W = F·d F = m·a
F =(40kg)(9.8m/s2)=392 N
W = (392 N)(1.4 m)
W = 549 J during lift
No work after lift. “da” is not in the direction of the force. F W d

24. No Work was done!! The car did not move!!
Ex. On a recent Rodgers’ Family vacation, the engine stalled out and would not restart. Mr. Rodgers made his family(including his wife) get out of the car and try to push the car up the road. The kids(along with the wife) pushed with a combined force of 3500 N, but the car didn’t move. How much work was done?
No Work was done!!
The car did not move!!

25. Ex. A force of 525 N is applied to push a box across the room
Ex. A force of 525 N is applied to push a box across the room. If 4900 J of work is done in pushing the box, how far was the box pushed?
Given:
F = 525 N
W = 4900 J
d = ???

26. Need more help? Watch this video:

27. We can compare the work we put into a machine and the work we get out of it to determine a machine’s efficiency

28. GIVEN: WORK: Eff.= Wout W = F·d Win Win =(200N)(1.5m) Win =300J
A person applies a force of 200 N over 1.5 m to a jack. The jack exerts a 1000-N force on a car a distance of 0.02 m. What is the efficiency rating of the jack?
GIVEN:
Fin = 200 N
din = 1.5 m
Win = ?
Fout = 1000 N
dout = 0.02 m
Wout = ?
WORK: Eff.= Wout
W = F·d Win
Win =(200N)(1.5m)
Win =300J
Wout =(1000N)(0.02m)
Wout =20J
x100
20N
300N
Eff.= 6.6%
x100

29. A student uses a manual air pump to inflate his basketball
A student uses a manual air pump to inflate his basketball. He applies 235 J of work to the pump. The pump releases air by doing 170 J of work. How efficient is the air pump?

30. Can a machine ever be more than 100 % efficient?

32. Power is the rate of work
Work = N·m or J
time = s
Power = Watt (W)

33. Power is the time rate of doing work!!!
It is how fast you are at doing work!!
If you consider the definition of work and now the definition of Power, then Power is how fast you transfer energy through motion!!

34. Power P = W/t What is the unit for work? What is the unit for time?
Joules
What is the unit for time?
Seconds
One joule per second is one watt (W).
So a 40-watt light bulb ?
Power is measured in Watts!

35. How can we increase power?
Increase work
…Increase force, increase distance…
Decrease time

36. I am just as strong as you. We are lifting the same amount!!
But I am more Powerful, because I do the work faster!!

38. Sometimes, you may have to find work using W= F d
Power Equation
Sometimes, you may have to find work using W= F d
Where:
P = Power in Watts(W)
W = work in Joules(J)
t = time in seconds(s)

39. Word problems can be confusing; but w/ some practice they’re not that bad. Here are a few hints to make them easier
1. Be sure you remember the “Need-to-Know” formulas
V =d/t ; A = Vf – Vi ; F = MA ; W = F x D
Time
In the word problem be sure you know the units for each of the variables in the particular formula being discussed.
Distance – Meter; Force – Newton; Volume - cm3 or Liter
2. In the word problem, all but one of the variables is told to you in one way or another. Identify what variable is being asked to solve, then plug in the remaining variables to the formula
Solve it!! Make sure you also keep track of the units

41. A 750-N person climbs to the top of a 4-meter ladder
in 15 seconds. What is the person’s power?

42. Ex. An elevator motor does 360,000 J of work lifting a load of people
Ex. An elevator motor does 360,000 J of work lifting a load of people. If the elevator ride lasts for 9 s, how much power did the motor consume?
Given:
W = 360,000 J
t = 9s
P =

43. How much work is performed if you apply 118 newtons of force on a car that is stuck in the mud and doesn’t move?:
W = F x D
W = 118N x 0m = 0 Nm
0J =0Nm You might be tired from pushing but no work was done!!

44. Ex. You do 900 J of work pushing a fridge. It took 5 seconds
Ex. You do 900 J of work pushing a fridge. It took 5 seconds. What was your power?

45. Ex. When doing a chin-up, a physics student lifts her 42
Ex. When doing a chin-up, a physics student lifts her 42.0-kg body a distance of 0.25 meters in 2 seconds. What is the power delivered by the student's biceps?

46. A tired ferret (mass of approximately 1 kg) does push-ups by applying a force to elevate its center-of-mass by 5 cm in order to do a mere 0.50 Joule of work. If the tired ferret does all this work in 2 seconds, then determine its power.