Transcript Calculating Work and Power

Calculating Work and
Power
0011 0010 1010 1101 0001 0100 1011
1
2
4
What is work?
• Work is done on an object when an
object moves in the same direction in
which a force is applied.
• Force and motion must be in the same
direction
• No work without motion!
0011 0010 1010 1101 0001 0100 1011
1
2
4
Applying force to do work.
• Work is only done in direction of
applied force and when object is
moving.
0011 0010 1010 1101 0001 0100 1011
1
2
– Lifting books off of desk = work by
arms
– Carrying books to class = not
work by arms but by legs
4
Work and Energy
• When you do work on an object, because
it has moved, you increase its potential or
kinetic energy.
0011 0010 1010 1101 0001 0100 1011
– Potential Energy = energy of position/stored
energy
• Height above earth
• Stretched rubber band
1
2
4
– Kinetic Energy = energy of motion
• All moving objects have kinetic energy
• Work = objects in motion
Examples of Work
• Work
0011 0010 1010 1101 0001 0100 1011
–Is done when a force causes an
object to move.
• Carrying books
• Pushing grocery cart
• Lifting weights
• Stacking blocks
1
2
4
Calculating Work
•
•
•
•
•
Work = force x distance
W=Fxd
Force is measured in Newtons
Distance is measured in Meters
Work is measured in Newton-meters
(N·m) or Joules (J)
0011 0010 1010 1101 0001 0100 1011
1
2
4
Calculating Work Practice
• Work is measured in Joules (J). Because
work is the transfer of energy to an
object, the Joule (J) is also the unit used
to measure energy!
• Example: You use 42 N of force to move
a couch 4m along the floor. How much
work has been done?
0011 0010 1010 1101 0001 0100 1011
1
–W=Fxd
W = 42 x 4
W = 168 J
2
4
Calculating Work
• What is the total amount of work
done when a football player carries
the ball with a force of 25 N for 50
meters?
0011 0010 1010 1101 0001 0100 1011
1
2
4
Solution
0011 0010 1010 1101 0001 0100 1011
•
•
•
•
W=Fxd
W = 25 N x 50 m
W = 1250 N x m
W = 1250 J
1
2
4
Calculation Work
• A man applies a force of 500 N to push
0011 0010 1010 1101 0001 0100 1011
a truck 100 m down the street. How
much work does he do?
• In which situation do you do more
work?
1
2
4
– You lift a 75 N bowling ball 2 m off the
floor.
– You lift two 50 N bowling balls 1 m off
the floor.
Power-How fast work is done
• Power is the rate at which work is done
0011 0010
1101 0001
1011
or 1010
energy
is 0100
transferred.
• Power = work ÷ time
• Power = (distance x force)/time
• Power is expressed in Joules per
seconds, J/s or watt (W)
• Power is increased when more work is
done in a given amount of time.
• Power is also increased when less time is
needed to do a certain amount of work
1
2
4
Power
• Who is more powerful?
0011 0010 1010 1101 0001 0100 1011
– A man who can lift 50 N of weight in
10 seconds or a woman who can lift 50
N of weight in 8 seconds?
– Woman. Same weight and work
accomplished, less time.
1
2
4
Calculating Power
0011 0010 1010 1101 0001 0100 1011
• Power = work/time
• P = w/t
• Watts = Joules/second
1
2
4
– What is your power if you do 200
Joules of work in 20 seconds?
0011 0010 1010 1101 0001 0100 1011
•
•
•
•
P = W/t
P = 200 Joules/20 seconds
P = 10 Joules/sec.
P = 10 Watts
1
2
4
Calculating Power
1. Motor exerts a force of 12,000 N to lift
0011 0010 1010 1101 0001 0100 1011
an elevator 8 m in 6 seconds. What is
the power produced by the motor?
2. A crane lifts an 8,000 N beam 75 m to
the top of a building in 30 seconds.
How much power does the crane use?
1
2
4
Answers
1. 16,000 W or 16 kW (kilowatt)
2. 20,000 W or 20 kW (kilowatt)
0011 0010 1010 1101 0001 0100 1011
1
2
4
Equations Summary
• Work = Force x Distance
0011 0010 1010 1101 0001 0100 1011
– Joules (J) = Newtons x meters
• Power = Work ÷ Time
– Watts (W) = Joules ÷ seconds
1
2
4
Your Turn
0011 0010 1010 1101 0001 0100 1011
• A force of 75 N is exerted on a 45 kg couch and
the couch is moved 5 m. How much work is
done in moving the couch?
• 375 J
• A lawn mower is pushed with a force of 80 N. If
12,000 J of work are done in mowing a lawn,
what is the total distance the lawn mower was
pushed?
• 150 m
1
2
4
Your Turn
• The brakes on a car do 240,000 J of work in
stopping the car. If the car travels a
distance of 50 m while the brakes are being
applied, what is the total force the brakes
exert on the car?
• W = f x d 240,000/50 m = 4,800 N
• To lift a baby from a crib 50 J of work are
done. How much power is needed if the
baby is lifted in 0.5 s?
• P = W/t 50/0.5 = 100W
0011 0010 1010 1101 0001 0100 1011
1
2
4
Your Turn
• If a runner’s power is 130 W, how
0011 0010 1010 1101 0001 0100 1011
much work is done by the runner in 10
minutes?
• 10 min(60 s) = 600 s P=W/t W = Pt
130 W (600 s) = 78,000 J
• The power produced by an electric
motor is 500 W. How long will it take
the motor to do 10,000 J of work?
• P = W/t t = W/P 10,000/500 = 20s
1
2
4
Work and Power Problems
1. You use 75 N force to move your book across the
table, which is a distance of 2 m. How much work
0011 0010
1010
1101 done?
0001 0100 1011
has
been
2. If it takes you 10 sec to do 150 J of work on a box
to move it up a ramp, what is your power output?
1
2
3. A light bulb is on for 12 sec, and during that time
it uses 1,200 J of electrical energy. What is the
wattage (power) of the light bulb?
4
4. A tow truck tows a car a distance of 40 m using a
force of 500 N. How much work has been done?
5. How much work must you do to move 100, 10 N
rocks a distance of 50 meters?
Work and Power Problems
6. How far did you push the grocery cart if it required
500 Joules of work and needed 25 N of force?
0011 0010 1010 1101 0001 0100 1011
7. Who has more power? Mrs. Presutti lifts 15 pieces
of cake (1 piece = 1 J of work) to her mouth in 60
seconds and Mrs. Witt lifts 21 pieces of cake to her
big mouth in 80 seconds.
8. What kind of energy does the cake have as it is being
swallowed to the depths of Mrs. Witt’s belly?
9. What kind of energy did the cake have when it was
being held high above her mouth ready to plummet
to its dark grave?
1
2
4
Discovery
1. Use a loop of string to attach a spring scale to a
book.
2. Slowly pull the book across a table by the spring
scale. Use a stopwatch to determine the time.
Record the force and time.
3. With a ruler measure the distance you pulled the
book.
4. Now, quickly pull the book across the same
distance. Record the time and force.
5. Calculate work and power for both trials.
6. How were the amounts of work and power affected
by your pulling the book faster?
0011 0010 1010 1101 0001 0100 1011
1
2
4
What did you discover?
0011 0010 1010 1101 0001 0100 1011
• The amount of work done in both trials was
the same.
• Because the work was done faster in the
second trial, the power was greater.
1
2
4
Work and Power Interactive Notes
What is work?
• Example: You use 42 N of force to move a couch 4m along the
floor. How much work has been done?
W = F x d _____________________
0011 0010 1010 1101 0001 0100 1011
• Work is done on an object when an object ______________ in
Calculating Work
the same direction in which a ______________ is applied.
• Force and motion must be in the ______________ direction
• No work without ______________!
What is the total amount of work done when a football player carries
the ball with a force of 25 N for 50 meters?
Solution
Applying force to do work
W=Fxd
______________
• Work is only done in direction of applied ______________
and when object is moving.
Calculation Work
• Lifting books off of desk = work by ______________
A man applies a force of 500 N to push a truck 100 m down the
street. How much work does he do?
• Carrying books to class = not work by arms but by
– In which situation do you do more work?
______________
W = ______________
W = ______________
W=
• You lift a 75 N bowling ball 2 m off the floor.
Work and Energy
• You lift two 50 N bowling balls 1 m off the floor.
When you do work on an object, because it has moved, you
increase its potential or kinetic energy.
Power – How fast work is done
• Potential Energy = energy of ______________/stored energy
– ______________above earth
– ______________rubber band
• Kinetic Energy = energy of ______________
– All ______________objects have kinetic energy
– Work = objects in ______________
• Power is the ______________at which work is done or
______________is transferred.
• Power = work ÷ time
• Power = (distance x force)/time
• Power is expressed in ______________per seconds, J/s or
______________ (W)
• Power is increased when ______________ ______________is done
in a given amount of time.
Examples of Work
Carrying books
______________ - How fast work is done
______________grocery cart
______________weights
Stacking blocks
• Power is also increased when ____________________________is
needed to do a certain amount of work
Calculating Work
Power
• Work = force x distance
• Who is more powerful?
•W=Fxd
– A man who can lift 50 N of weight in 10 seconds or a woman
who can lift 50 N of weight in 8 seconds?
• Force is measured in ______________
• Distance is measured in ______________
• Work is measured in Newton-meters (N·m) or ______________ (J)
Calculating Work Practice
• Work is measured in Joules (J). Because work is the transfer of
energy to an object, the Joule (J) is also the unit used to measure
energy!
– ______________. Same weight and work accomplished, less
time.
Calculating Power
• Power = work/time
• Watts = Joules/second
• What is your power if you do 200 Joules of work in 20
seconds?
1
2
4