Energy can change from one
form to another without a net
loss or gain.
Energy Notes
•
Definition of energy:
•
Energy can be stored, transferred and used.
•
Definition of work:
•
Work is done when a force causes something to move, and, as a result, energy moves from
place to place or changes from one form to another.
•
SI Unit for Work and Energy:
•
Difference between kinetic energy and potential energy
Kinetic energy
The energy of motion
The ability to cause change. Every change involves energy.
W = Fd (Work equals force times distance)
Joules (J)
Potential energy
The energy of position or stored energy
Energy Notes
•
What is the equation for KE?
•
How does KE increase or decrease?
KE = 1/2mv2 Kinetic energy = ½ mass x velocity2
Increase or decrease the velocity of the object (change speed).
Double your speed, Quadruple the KE
Prove it: Calculate the KE of a 2500 kg car traveling at 20 m/s and at 40 m/s
•
•
KE at 20 m/s
(500,000 J)
KE at 40 m/s
(2,000,000 J)
More mass, same speed, more KE.
Double the mass, double the KE
Prove it: Calculate the KE of a 100 kg cart and a 200 kg cart, each traveling at 15 m/s
•
•
100 kg cart at 15 m/s
200 kg cart at 15 m/s
•
(11,250 J)
(22,500 J)
Definitions
•
•
•
•
Energy (the ability to cause change)
can be transferred, stored or used
Work (net force x distance moved)
method of energy use, transfer and
storage
• Power (work per unit time)
• the rate at which work is done or energy
is transferred
Chapter 9.1
Work
9.1 Work
Energy is the ability to cause change.
Work is done when a net force acts on an
object and the object moves in the direction of
the net force (force causes displacement).
Work involves a transfer of energy between
something and its surroundings.
9.1 Work
Two categories of work:
1) Work done against another force
a) Transfer of energy to change position or store
energy—work against another force
Ex: Pulling back the string on a bow stores
energy.
b) Transfer of energy (one form to another)
Ex: Rubbing your hands together transfers one
form (kinetic) of energy to another (thermal).
2)
Work done to change the speed of an object
Ex: Bringing an automobile up to speed increases
its (kinetic) energy.
9.1 Work
= force × distance
Did the weightlifter do work on the barbell
and weights?
•Yes, when he first lifted them above his
head.
Is the weightlifter currently doing work on
the barbell and weights?
No, the barbell and weights are not moving.
•Explain two ways that the work done by
the weightlifter be increased.
1) Increase the weight on the ends
of the barbell
2) Increase the distance over
which the weightlifter pushes
the barbell and weights.
9.1 Work
Work has the same units as energy
Joules
Newton x meter
J
Nxm
•One joule (J) of work is done when a
force of 1 N is exerted over a distance
of 1 m (lifting an apple over your
head).
9.1 Work
If we lift two loads, we do twice as much work as lifting
one load the same distance, because the force needed is
twice as great.
If we lift one load twice as far, we do twice as much work
because the distance is twice as great.
9.1 Work
While the weight lifter is holding a
barbell over his head, he may get
really tired, but he does no work on
the barbell.
Work may be done on the muscles
by stretching and squeezing them,
but this work is not done on the
barbell.
When the weight lifter raises the
barbell, he is doing work on it.
9.1 Work
think!
Suppose that you apply a 60-N horizontal force to a 32-kg
package, which pushes it 4 meters across a mailroom floor.
How much work do you do on the package?
9.1 Work
think!
Suppose that you apply a 60-N horizontal force to a 32-kg
package, which pushes it 4 meters across a mailroom floor.
How much work do you do on the package?
Answer:
W = Fd = 60 N × 4 m = 240 J
9.1 Work
When is work done on an object?
When is work not done on an object?