Download Energy and Work

Energy and Work
Part 2
What is Work
Work is the transfer of energy through
motion
– When force acts over a distance in the
direction of an object’s motion
W=FxD
W = Work F = Force
D = Distance
Measured in Joules (J) J = N x m
Example
A dancer lifts a 400 N ballerina overhead a
distance of 1.4 m and holds her there for
several seconds. How much work is done
during the lift?
W=FxD
F = 400 N
D = 1.4 m
W = 400N x 1.4m
W= 560 J
or 560 N x m
How much work was being done while the
ballerina was overhead?
Why was no work being done while
the ballerina was overhead?
Work – transfer of energy through motion
If the ballerina is not moving what type of
energy does she represent
– Potential
What if she was dropped. What type of
energy would she represent?
– Kinetic
Conservation of Energy
Law of Conservation of Energy
– Energy may change form but it cannot
be created nor destroyed under ordinary
conditions
Calculating Kinetic Energy
Kinetic Energy - KE =1/2 x M x V2
– Example: Determine the kinetic evergy
of a 25 kg roller coaster car that is
moving with a speed of 18.3 m/s
KE = ½ x 25 kg x 18.32
KE = 4186 Joules
Calculating Potential Energy
Potential Energy – PEgrav=mgh
– m = mass
– g = gravitational acceleration
– h =height
– A 4 kg book is placed on a shelf 3
meters above the ground. What is the
potential energy of this book?
PEgrav= 4kg x 9.8 m/s2 x 3 m
PEgrav = 117.6 Joules
Mechanical Energy
Mechanical Energy – is the total
amount of kinetic and potential
energy in a system
Mechanical Energy
E
A
B
D
C
At what points do you have the greatest potential energy?
Answer: A and E
At what point do you have increasing/decreasing potential energy?
Answer: B (decreasing) and D (increasing)
At what point do you have the greatest kinetic energy?
Answer: C
At what point do you have increasing/decreasing kinetic energy?
Answer: B (increasing) and D (decreasing)
Power
The rate at which work is done.
Measure of the amount of work done
in a certain amount of time.
– Calculating Power:
power = Work/Time
p = W/t
Calculating Power
The units for power are Watts
– Watts = 1 joule per second
Work = Joule
Time = second
–Hence Watts = Joule/Second
– Example: A figure skater lifts his
partner, who weighs 450 N, 1.0 m in 3.0
s. How much power is required?
Solution
What is known?
F = 450 N
D = 1.0 m
T = 3.0 s
What is not known?
W=?
W = f x d W = 450 N x 1.0 m
W = 450 J
Now you can calculate power:
p = 450 J/3.0 s
p = 150 W