Transcript Work and Power - reynardearthsci

Work and Power
Work:
• In physics we say that work is done on an
object if a force is applied to it and that force
causes it to move a certain distance.
Work = W = Fd
WORK IS ENERGY!!!
N
m
N•m = J = Joule
What is a Joule?
N•m
kg m • m
s2
kg m2
s2
The energy required to lift a small apple one meter straight up.
Here’s the important thing about work…work is only done if
the force has a component in the same direction as the
displacement.
F = 25N
m = 10 kg
d = 2m
Is this guy doing work on the box?
Yes. The force vector is in the same direction as the displacement.
How much work is being done?
W = Fd
W =25N(2m)
W = 50 Nm = 50J
Is work done when pulling this dog?
Yes. The force vector has some component in the same direction as the displacement.
How much work is being done?
F
Fy
30°
Fx
= 70N
30°
d = 10m
Fx = cosΘ = A
H
Fx = FcosΘ
Fx = 70Ncos30°
Fx = 61N
W = Fd
W =61N(10m)
W =610J
Is work being done by this waiter?
No. He could carry around that tray all day and according to
physics he wouldn’t be doing any 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). Yet the force does not cause the displacement. To
cause a displacement, there must be a component of force
in the direction of the displacement.
• How much work is needed to lift at a
constant speed a 15kg book 3m?
W = Fd
W = mgd
W = (15kg)(10m/s2)(3m)
W = 450 J
Which path (incline vs. ladder) requires more
work to get the box to the top?
mbox = 10 kg
10 m
5m
30°
W = Fd
W = Fd
W = (50N)(10m)
W = (100N)(5m)
W = 500 J
W = 500 J
Same amount of work!
A particular task may require a certain amount of
work but it might be done over different lengths of
time…
• This is known as Power (P). It measures the
rate at which work is done.
P=W
t
P = Fd
t
P = Fv
P = Fd
t
J = watt = W
s
d=v
t
Who has more power?
• Dan Parker and Brad Bowen are in the weightlifting
room. Dan lifts the 50 kg barbell over his head 10 times
in one minute; Brad lifts the 50 kg barbell over his head
10 times in 10 seconds. Which student does the most
work? Which student delivers the most power?
Brad is more "power-full" since he does the
same work in less time. Power and time are
inversely proportional.
Try this…
• A crane lifts a load with a mass of 1000kg a vertical
distance of 25m in 9s at a constant velocity. How
powerful is the crane?
P = W = Fd
t
t
= Fgd
t
= mgh
t
= (1000kg)(10m/s2)(25m)
9s
= 27000 W
Try this…
• A 45 kg bicyclist climbs a hill at a constant speed
of 3 m/s by applying an average force of 80 N.
How much power does the bicyclist develop?
P = Fv
P = (80 N)(3 m/s)
P = 240 W
• Power is a rate (ENERGY PER SECOND).
• Your electric bill (power bill) is based on your
rate of energy use.
• A lightbulb with a 60 Watt power rating means
that the bulb uses 60 joules of energy per
second.
W = ΔET
Work = a change in total energy