Transcript Energy, Work and Power

Energy, Work and
Power
Energy and Work

Energy is the ability to do work
 Ex.
Kinetic, Thermal, Gravitational, Elastic,
Chemical, Electric, Magnetic, Radiant…

Work is the transfer of energy
James Prescott Joule (1818 – 1889):
- English physicist
- Discovered relationship btw. heat & mechanical work (energy)
- Conservation of Energy Theorem
Pg 218
Power

Power is the rate at which work is done
W
P
t
[N.m] ,[J] Joule
[W] watt
James Watt (1736 – 1819):
- Scottish inventor/mechanical engineer.
- Improved the efficiency of steam engines.
- Introduced horse power (750 watts).
[s] second

If a hair dryer does 3000 J of work to heat
the air every two seconds, what is its
power?

A 613.0 kg mass is placed on a forklift that
can generate 950 W of power. What is the
constant speed of the forklift while lifting
this load?

A HD TV consumes 5.0 kW∙h of energy in
30 minutes.

What is its power?

How much does it cost to watch TV for 2
hours if the cost of electricity during peak
time is $0.12/ kW∙h
Mechanical Energy
Mechanical Energy
is a combination of two fundamental types of
energy:


Mechanical Energy
is a combination of two fundamental types of
energy:

Kinetic energy (the energy of motion)

Potential energy (energy that is stored)
- Gravitational Potential energy
Kinetic Energy


The kinetic energy of an object of mass m,
in kg, and speed v, in m/s:
Kinetic Energy
The work done by the net force causes a
change in speed
 The kinetic energy of an object of mass m,
in kg, and speed v, in m/s:

1 2
Ek  mv
2
Gravitational Potential Energy

Ex: A mass m is lifted from a height h1 to a
height h2 at a constant speed.
W  F  d
W  Fg h
W  mg h
W  mg (h2  h1 )
W  mgh2  mgh1
Eg 2
Eg1
Gravitational Potential Energy

“stored energy” in an object at a particular height
w.r.t. a reference point.
Ex: A mass m is lifted from a height h1 to a
height h2 at a constant speed.
W  F  d
W  Fg h
W  mg h
W  mg (h2  h1 )
W  mgh2  mgh1
Eg 2
Eg1
Gravitational Potential Energy

The gravitational potential energy Eg of an object
of mass m, in kg, that is at height h, in m, above
the reference point is:

g = 9.8 m/s2
Gravitational Potential Energy

The gravitational potential energy Eg of an object
of mass m, in kg, that is at height h, in m, above
the reference point is:
Eg  mgh

g = 9.8 m/s2
W.E.T. (Work-Energy Theorem)

The total work done on an object equals the change in
the object’s kinetic energy OR gravitational potential
energy, but NOT BOTH.
Wtotal  EK
Wtotal  EK 2  EK 1
1 2 1 2
F  d  mv2  mv1
2
2
Wtotal  Eg
Wtotal  Eg 2  Eg1
F  d  mgh2  mgh1
Ex 1: What happens to the kinetic energy of an
object when work is done on it?
Ex 2: Calculate the work done to speed up a
1500 kg Honda Civic:
a) From rest to 20 km/h
b) From 80 km/h to 100 km/h
Ex 3: What role does gravitational potential energy
play in the production of electricity by Ontario Power
Generation?
Ex 4: Ontario Power Generation at Niagara Falls
operates under a normal head of about 55 m (height
from which water falls). If about 4.54x108 kg of water
falls every minute, how many mega-joules of energy
are created by the falling water in a hour?