2. the time rate of doing work; or the time rate transfer of energy.
Power,
by definition, is
the time rate of doing work;
or the time rate transfer of energy.
P = W / t
Power is a scalar quantity.

3. 1.0 J of energy is transferred
The SI unit of power
is the Watt,
named in honor of
James Watt.
One Watt, W, of power
is the power achieved
when 1.0 J of work is done or
1.0 J of energy is transferred
in a time of 1.0 s.

4. Power example
An 80 kg human walks up a flight of stairs in .22 s that have an altitude gain of 3.75 m. What is the power of the person?

5. Power Example First find the work= force*distance. F=mg 80*9.8=784N
W=Fd
784N*3.75m=2940J
P=W/t=2940/.22= W

6. W = F·d Work, Work is a scalar quantity.
by definition, is
the product of the force exerted on
an object and the distance the object
moves in the direction of the force.
W = F·d
Work is a scalar quantity.

7. The SI unit of work is the Joule, named in honor of
James Prescott Joule.
One Joule, J, of work
is the work done when
1.0 N of force is applied
through a distance of 1.0 m.

8. Work Example
An intern pushes a 75 kg patient on a 15 kg gurney, producing an accleration of .06m/s2. How much work does the intern do by pushing the patient and gurney through a distance of 2.5 m? Assume there is no friction.

9. Find the F= ma =90 kg* .60 m/s2
F= ma 90*.6= 54N
W=Fd= 54*2.5=135J

10. “Force vs. Displacement” graph.
Graphically, work is
the area under a
“Force vs. Displacement” graph.
displacement, m

11. work = Fd(cosq), W = Fd(cos q) = (40N)(3.0 m)(cos 35) = 98 J
If the force and displacement are not
in the exact same direction, then
work = Fd(cosq),
where q is the angle between the force
direction and displacement direction.
F =40 N
d = 3.0 m
Example 2:The work done in moving the block 3.0 m
to the right by the 40 N force at an angle
of 35 to the horizontal is ...
W = Fd(cos q) = (40N)(3.0 m)(cos 35) = 98 J

12. Law of Conservation of Energy
“Energy can be neither created nor destroyed.
It may only change forms.”
S all types of energy before the event
= S all types of energy after the event
Examples:
A dropped object loses gravitational PE as it gains KE.
A block slides across the floor and comes to a stop.
A compressed spring shoots a ball into the air.

13. Energy comes in many forms: mechanical, electrical , magnetic, solar,
the ability (capacity) to do work
Energy comes in many forms:
mechanical, electrical , magnetic, solar,
thermal, chemical, etc...
The SI unit of energy is the Joule.
Energy, like work, is a scalar.

14. KE = 1/2 mv2 Kinetic Energy energy of motion
All moving objects that
have mass have kinetic energy.
KE = 1/2 mv2
m - mass of the object in kg
v - speed of the object in m/s
KE - the kinetic energy in J

15. Energy Example
A 50 kg boy and his 100 kg father went jogging. Both ran at a rate of 5 m/s. Who had more kinetic energy? Show your work and explain.

16. Example Problem - answer
KE = ½mv2
Boy…
KE = ½(50 kg)(5 m/s)2
KE = 625 J
Dad…
KE = ½(100 kg)(5 m/s)2
KE = 1250 J
Dad had more Kinetic energy because his mass was greater.

17. Work-Energy Theorem the net work done on an object is
equal to its change in kinetic energy

18. Potential Energy
Energy stored in a motionless object, giving it the potential to cause change

19. Potential Energy energy of position or condition
Chemical Potential Energy - energy stored in chemical bonds between atoms (Snickers bar, food, even gasoline)

20. PEg = mgh Potential Energy energy of position or condition
gravitational potential energy
PEg = mgh
m - mass of object in kg
g - acceleration of gravity in m/s2
h - height of object, in m,
from some arbitrary reference point
PE – gravitational potential energy in J

21. Example Problem
What is the potential energy of a 10 N book that is placed on a shelf that is 2.5 meters high?

22. Example Problem - answer
GPE = mgh
GPE = (10 N) (2.5m)
GPE = 25 J
Remember that weight = mg and that the force provided is weight.
NOTE: you may want to change your variable for weight to Fg.

23. PEe = ½ kx2 Potential Energy energy of position or condition
elastic potential energy
PEe = ½ kx2
k – elastic constant in N/m
x - elongation or compression in m
PEe – elastic potential energy in J
Click here to investigate elastic constants.

24. Example
A spring with a spring constant of 120 N/m is compressed a distance of 2.3 cm. How much potential energy is stored in the spring?

25. Example U= ½ kx2 U= ½ (120)*(.025)2 Remember we work in meters!
U= .032J