Transcript Chapter 14.1

Work and Power
The word “WORK”
• What does it mean in
everyday life?
• “Work” has a specific
definition in science.
st
Remember: Newton’s 1 Law
• An object only begins
moving when an
unbalanced force
acts on it.
• Net force > 0
Work
• Work is done when a force
acts on an object in the
direction the object moves.
• Work = force x distance
• Work requires motion
• If there is no movement, no
work is done.
• Force must act in SAME
DIRECTION as motion
• Any part of a force that does
not act in the direction of
motion does no work on an
object.
• Examples
Which ones require work?
• Ronnie pushes on a wall until he is
exhausted.
• Timara knocks a book off the table
and it falls to the ground.
• A rocket accelerates through space.
• Doneisha carries her books to class.
Question:
• You carry two heavy grocery bags
upstairs to your kitchen.
• Will you do more work on the bags if
you carry them up one at a time?
• Explain.
Units of Work
• Remember: units of force?
– Newton
• Remember: units of distance?
– meter
Work = Force x Distance
Work = N x m
N x m = 1 Joule
JOULE (J) is the
unit for Work
Calculating Work
A weightlifter lifts a 1800-N barbell
over his head. He has to lift it for 2.0
m. What amount of work is done?
F = 1800 N
W = 1800 N x 2.0 m
D = 2.0 m
W = 3600 Nm = 3600 J
W=FxD
Practice
How much work does a 25Newton force do to lift a
potted plant from the floor to
a shelf 1.5 meters high?
Practice
A factory machine uses 2600Newtons of force to move a
robotic arm 3.5 meters. How
much work does the machine
perform?
Power
• What does it mean in
everyday life?
• “Power” also has a
specific meaning in
science.
Power
• The rate of doing work.
• How FAST can work get done?
• To get work done faster, more
power is required.
Power =
Work
Time
Which has more power:
• A person with a snow shovel?
• A snow blower?
• A truck with a snow plow?
Predicting
• Two cars have the same weight, but
Car A has an engine that provides
twice the power of Car B.
• Which car can make it to the top of a
mountain pass first?
• Which car does more work to reach
the top?
Units of Power
• Remember: units of Work
– Joule
• Remember: units of Time
– Seconds
Power =
Work
Time
Power =
Joule
second
= 1 Watt
Watt (W) is the
unit for Power.
Example:
• 40-Watt light bulb
–Requires 40 joules of work for
every second that it is lit
• Same power as lifting your
textbook a height of 1 meter in half
a second.
Calculating Power
You exert a vertical force of 72
Newtons to lift a box to a height of
1.0 meter in a time of 2.0 seconds.
How much power is used to lift the
box?
F = 72 N
Power =
Force x Distance
=
Time
D = 1.0 m
T = 2.0 s
Work
Power =
72 N x 1.0 m
Time
=
2.0 s
Power = 36 J/s = 36 Watts
72 J
2.0 s
Practice
1. Deshawna ice skates 70 m in 60
seconds with a force of 10 N. How
much work did she do? How much
Power did she have?
2. Darane runs up the NLCP stairs in 3
seconds. She uses a force of 20 N
and the stairs are 4 meters tall. How
much work did she do? How much
Power did she have?