Work, Power and Energy

1. Work, Power & Energy

2. Work
The product of force and the amount of
displacement along the line of action of
that force.
Units: Newton•meter (Joule)
ft . lbs (horsepower)
nt
displaceme
Force
Work 


3. Work = F x d
To calculate work done on an object, we
need:
The Force
The average magnitude of the force
The direction of the force
The Displacement
The magnitude of the change of position
The direction of the change of position

4. Calculate Work
During the ascent phase of a rep of the
bench press, the lifter exerts an
average vertical force of 1000 N
against a barbell while the barbell
moves 0.8 m upward
How much work did the lifter do to the
barbell?

5. Calculate Work
Table of Variables:
Force = 1000 N
Displacement = 0.8 m
Force is positive due to pushing upward
Displacement is positive due to moving
upward

6. Calculate Work
Table of Variables:
Force = 1000 N
Displacement = 0.8 m
Select the equation and solve:
   
J
Joule
Nm
Work
m
N
Work
nt
displaceme
Force
Work
800
800
800
8
.
0
1000










7. - & + Work
Positive work is performed when
the direction of the force and
the direction of motion are the
same
ascent phase of the bench press
Throwing a ball
push off (upward) phase of a jump

8. Calculate Work
During the descent phase of a rep of
the bench press, the lifter exerts an
average vertical force of 1000 N
against a barbell while the barbell
moves 0.8 m downward

9. Calculate Work
Table of Variables
Force = +1000 N
Displacement = -0.8 m
Force is positive due to pushing upward
Displacement is negative due to movement
downward

10. Calculate Work
Table of Variables
Force = +1000 N
Displacement = -0.8 m
Select the equation and solve:
   
J
Joule
Nm
Work
m
N
Work
nt
displaceme
Force
Work
800
800
800
8
.
0
1000













11. - & + Work
Positive work
Negative work is performed
when the direction of the force
and the direction of motion are
the opposite
descent phase of the bench press
catching
landing phase of a jump

12. Work performed climbing
stairs
 Work = Fd
 Force
 Subject weight
 From mass, ie 65 kg
 Displacement
 Height of each step
 Typical 8 inches (20cm)
 Work per step
 650N x 0.2 m = 130.0 Nm
 Multiply by the number of steps

13. Power
The rate of doing work
Work = Fd
Units: Fd/s = J/s = watt
velocity
Force
Power
t
Fd
Power
time
Work
Power




/
/

14. Calculate & compare power
During the ascent phase of a rep of the
bench press, two lifters each exert an
average vertical force of 1000 N
against a barbell while the barbell
moves 0.8 m upward
Lifter A: 0.50 seconds
Lifter B: 0.75 seconds

15. Calculate & compare power
Lifter A
Table of Variables
F = 1000 N
d = 0.8 m
t = 0.50 s
Lifter B’s time would
be .75 sec instead
of .5 sec
w
s
J
Power
s
m
N
Power
t
Fd
Power
1600
50
.
0
800
50
.
0
8
.
0
1000






16. Energy
 Energy (E) is defined as the capacity to do
work (scalar)
 Many forms
No more created, only converted
 chemical, sound, heat, nuclear, mechanical
 Kinetic Energy (KE):
 energy due to motion
 Potential Energy (PE):
 energy due to position or deformation

17. Kinetic Energy
Energy due to motion reflects
the mass
the velocity of the object
KE = 1/2 mv2

18. Calculate Kinetic Energy
How much KE in a 5
ounce baseball (145 g)
thrown at 80 miles/hr
(35.8 m/s)?

19. Calculate Kinetic Energy
Table of Variables
Mass = 145 g  0.145 kg
Velocity = 35.8 m/s

20. Calculate Kinetic Energy
Table of Variables
Mass = 145 g  0.145 kg
Velocity = 35.8 m/s
Select the equation and solve:
KE = ½ m v2
KE = ½ (0.145 kg)(35.8 m/s)2
KE = ½ (0.145 kg)(1281.54 m/s/s)
KE = ½ (185.8 kg m/s/s)
KE = 92.9 kg m/s/s, or 92.9 Nm, or 92.9J

21. Calculate Kinetic Energy
How much KE possessed by
a 68.1 kg female volleyball
player moving downward at
3.2 m/s after a block?

22. Calculate Kinetic Energy
Table of Variables
 68.18 kg of mass
 -3.2 m/s
Select the equation and solve:
KE = ½ m v2
 KE = ½ (68.18 kg)(-3.2 m/s)2
 KE = ½ (68.18 kg)(10.24 m/s/s)
 KE = ½ (698.16 kg m/s/s)
 KE = 349.08 Nm or J

23. Calculate Kinetic Energy
Compare KE possessed by:
 a 220 pound (100 kg) running back
moving forward at 4.0 m/s
 a 385 pound (175 kg) lineman moving
forward at 3.75 m/s

24. Calculate Kinetic Energy
Table of Variables
m = 100 Kg
v = 4.0 m/s
Select the equation
and solve:
KE = ½ m v2
KE = ½ (100 kg)(4.0
m/s)2
KE = 800 J
Table of Variables
m = 175 kg
v = 3.75 m/s
Select the equation
and solve:
KE = ½ m v2
KE = ½ (175)(3.75)2
KE = 1230 J

25. Potential Energy
Two forms of PE:
Gravitational PE:
energy due to an object’s position
relative to the earth
Strain PE:
due to the deformation of an
object

26. Gravitational PE
Affected by the object’s
 weight
mg
elevation (height) above reference point
 ground or some other surface
h
GPE = mgh

27. Calculate GPE
How much gravitational potential energy
in a 45 kg gymnast when she is 4m
above the mat of the trampoline?
Trampoline mat is 1.25 m
above the ground

28. Calculate GPE
GPE relative to mat
Table of Variables
m = 45 kg
g = 10 m/s/s
h = 4 m
PE = mgh
PE = 45kg * 9.81 m/s/s
* 4 m
PE = 1764 J
GPE relative to ground
Table of Variables
m = 45 kg
g = 10m/s/s
h = 5.25 m
PE = mgh
PE = 45kg * 9.81 m/s/s
* 5.25 m
PE = 2317.6 J