Transcript Chap. 8 Work, Power, Energy

CH 8: Energy
Last chapter:

How long a force acts makes a
difference in an object’s motion.

F x t = Impulse
This chapter:

How long (how far) a force acts, which
is known as work.

1. Which takes more work lifting 1
book from the floor to your waist or
lifting 10 books from the floor to your
waist?

Why?

2. Which takes more work, lifting 1
book from the floor to your waist or
lifting 1 book from the floor to over your
head?

Why?

3. Which requires more work holding
100 pounds stationary over your head
or holding 300 pounds stationary over
your head?

Why?
Work


A. product of a force acting in the
direction of motion and the distance
moved.
B.
Work = force x distance


W
=
F x
d
C. Units for work

Joules (J) = N
m
Practice problem:

How much work is done on a 50 N
skater that is lifted by her partner 1.5 m
into the air?
Another Problem:


Who does more work, a person who
lifts 100 pounds 2 m over their head
very quickly or a person who struggles
and takes a long time to lift the 100
pounds 2 m over their head?
Why?
Power


A. rate at which work is done
B. Power = work done



time interval
P = W/ t
C. Units for power

watts (w) = joules / t


horsepower (hp)
1 hp = 747.7 w
Practice problem:

How much power is required to lift
Bertha, (who weighs 70 N) 20 m in an
elevator in 6 s?
Energy

A. ability to do work

B. Units = joules

C. Mechanical energy
1. Potential Energy



a. stored energy
b. energy due to position
c. examples
rubber band
 spring
 fuel - gas, batteries, food
 a book held high

1. Potential Energy


d. potential energy is obtained by work
being done.
e. PE = mgh
2. Kinetic energy




a.
b.
c.
d.
energy of motion
energy due to movement
depends on mass and speed
KE = 1/2 mv2
Conservation of Energy


Energy can not be created or
destroyed, only transformed from one
form to another.
Total amount of energy never changes.
Conservation of Energy

3. Energy transforms from one form to
another
Sunlight




Heat
Chemical
Mechanical
Electrical



Plants
Warm air rises
Solar cells
IV. Machines



A. device used to multiply forces or
change the direction of forces
B. basic idea is same as conservation
of energy
C. work in = work out
work in = work out
. d)
F . d


(F
in
= (F
10 N x 1 m
=
=
. d)
f.d
out
1 N x 10 m
D. lever
F . d
=
f
.d
E. Mechanical Advantage



MA = output force
input force
pg. 112
80 N = 8
10 N
MA = input distance
output distance

F. Pulley (bonus question on the test)
G. Efficiency


1. ideal machines have 100%
efficiency
Does not happen in the real world
(some energy is ALWAYS lost as heat)
G. Efficiency

Eff =

OR

Eff =
Work (out)
Work (in) X 100%
useful work output X 100%
total work input
AMA
TMA
X 100%