Transcript Work, Energy, & Power

Work is only done by a force on an
object if the force causes the object
to move in the direction of the force.
Objects that are at
rest may have many
forces acting on them,
but no work is done if
there is no movement.
Work,
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 that
may be positive or negative.
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.
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
35
d = 3.0 m
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
Energy
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, and
may be positive or negative.
Kinetic Energy
energy of motion
All moving objects that
have mass have kinetic energy.
KE = 1/2
2
mv
m – mass of the object in kg
v – speed of the object in m/s
KE – the kinetic energy in J
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
Potential Energy
energy of position or condition
elastic potential energy
PEe =
2
½kx
k – elastic constant in N/m
x – elongation or compression in m
PEe – elastic potential energy in J
Click here to investigate elastic constants.
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.
Work-Energy Theorem
the net work done on an object is
equal to its change in kinetic energy
Wnet KE
A net force causes
an object to change its KE because
a net force causes an object to accelerate,
and acceleration means a change in velocity,
and if velocity changes, KE changes.
Learn more about the
Work-Energy Theorem
here and here.
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.
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.
Click here to explore a computer simulation
illustrating the conservation of mechanical energy.
http://www.physicsclassroom.com/Physics-Interactives/Work-and-Energy/Roller-Coaster-Model/Roller-Coaster-Model-Interactive
Click here to explore energy and
its conservation in more detail.
Energy (and work) may also be specified in
units of calories. This unit is commonly used
when referring to thermal (heat) energy.
By definition, a calorie is the amount of energy
needed to raise the temperature of 1.0 gram of
water (= 1.0 mL) at standard atmospheric
pressure 1° C.
The food calorie, which is commonly
capitalized (Calorie) is equal to 1000 calories.
1 cal = 4.184 J
1 Cal = 4184 J
http://science.howstuffworks.com/life/human-biology/calorie.htm