Work, Energy
and
Power
Work (a scalar)
• In physics, work is done when a force is
applied through a distance. The force must be
in the direction of movement!
• W = F . d =magnitude of force . Distance
moved.
• Units of measure:
N • m or Joule in the metric system.
Ft • Lb or calorie in the English system.
•If you pick up a 3N rock a distance of 2 meters, how
much work have you done?
•If you push a wall with a force of 100N how much
work have you done?
Work
• Is there actually work being done on the
left?
Yes. But not by the 15N force used to hold it up. If you knew
the coeff. of friction between the box and hands then you
could calculate the force used to overcome friction that is in
the direction of the motion.
Power
• Power is the rate at which work is done.
• P = W/t
• So the faster you do work, the more power is needed.
• SI unit of measure is the Watt.
Thomas Newcomen
invented the first steam
engine in 1712 to pump
water out of coal and tin
mines.
It underwent a number
of improvements but it
was James Watt in the
mid eighteenth century
who spectacularly
improved its efficiency.
Power
• P = W/t
• So, if you pick up that same 3N rock a
distance of 2 meters, and do it in 6
seconds how much power is needed?
Calculate the Power
• A set of pulleys is used to lift a piano
weighing 1000N. It is lifted 6 meters in 30
seconds. How much work was put into it
and how mush power was used?
Energy:
Everything has a quantity called
energy associated with it.
Energy is the ability to do work.
The SI unit of measurement for
energy is also the Joule (J).
Energy has many forms
Energy
Forms
Kinetic
Heat
Chemical
Electromagnetic
Nuclear
Gravitational
Energy
Energy
Energy
Energy
Energy
Energy
Falling objects,
waterfalls
Wind.
Winding a clock.
Falling .
Generated from
the internal motion
of particles
of matter.
Energy that
bonds atoms
together.
Can be released.
Moving electric
charges have
electromagnetic
energy.
When a nucleus is
split, it releases
heat energy and
light energy.
Sound
Fuels: wood, petroleum, food
Light
(Solar)
Machines change energy from one form
to another.
Potential and Kinetic Energy
• In all these types of energies,
energy can exist as either
Potential Energy (_______),
Kinetic Energy (_________),
or both.
Potential Energy (PE)
• Can be thought of as “stored”
energy.
• It is the energy possessed by
an object due to it’s position
or condition.
Name the machine or transducer
Input
Electricity
Out put
Light
Light
Electricity
Sound
Electricity
Electricity
Sound
Name the machine or transducer
Input
Electricity
Output
Heat
Heat
Kinetic
Energy
Kinetic
Energy
Electricity
Electricity
Kinetic
Energy
Name the machine or transducer
Input
Out put
Chemical
energy
Light &
Heat & KE
Chemical
energy
Electricity
Unstable
Nuclei
Light &
Heat
Electricity
Sound
Name the machine or transducer
Input
Electricity
Out put
Magnetism
Electricity
Gravitational
PE
Chemical
Energy
Light &
Heat
KE
Sound
Gravitational Potential Energy
• The potential energy “stored
energy” in an object due to it’s
height off the ground.
• Work was done against the force of
gravity (weight), through a
distance (height) to lift an object.
• The work done (W) is equivalent
to the Gravitational Potential
Energy (GPE) acquired by the
object.
Gravitational Potential Energy
.
Fd
ΔPE due
to gravity
W=
Weight is the
force due to
W = ΔP.E = F.d
gravity
.g
.
w=m
ΔPE = F d
PEG = weight . Δheight
PEG = m
.g .
h
Distance is
just the height
off the ground
Kinetic Energy (KE)
• Is the energy due to motion.
• A moving object has the
ability to do work on another
object.
• It is a scalar quantity
Don’t use trig or vectors to add energy. Adding energy is path
independent.
4J of work against
gravity to lift
3 J of work against
friction to slide.
Kinetic Energy (KE)
Using W=F.d and Newton’s 2nd Law, F=m.a
The work done by a
force to accelerate a
mass is equivalent to
the KE gain.
W= F .d
KE=m.a.d
KE = m.v/t.v.t
KE = m.v/t.v/2.t
v
t
a=v/t assuming vi =0
d=avg.velocity
multiplied by time
Avg. velocity=v/2
KE=
.
2
mv
Assuming vi =0
E total = KE + PE + Q