Download Work Power Energy Notes

Physics
A.
Unit 09
Work, Power
Work, Power, & Energy WPE
B.
Name______________Per__
Energy
Work:
W = FNET  d
Example 1:
You discover a box of rocks in middle of your room. They have a mass of 20kg. You apply an
effort force of 65N to push them 2 meters so they are against the wall. How much work
have you done?
You lift the rocks 1.5 meters. How much work have you done?
What is the total work done on the rocks?
Example 2:
Artie pulls a 500-kg boulder a distance of 4m at a constant velocity along a horizontal
surface by applying a 250-N force along a rope that is parallel with the horizontal. He then
stoops down and lifts the boulder above his head to a height of 2 m. Artie carries the
boulder at a constant velocity for a distance of 5 m. How much work does he do to the
boulder?
Example 3:
Challenge: A reindeer pulls a sled a distance of 3.0 m across a frictionless surface. He
applies a force of 50.0 N with the horizontal. Calculate the work done on the sled. How
would the work change if the force was applied at an angle? A reindeer pulls a sled a
distance of 3.0 m across a frictionless surface. He applies a force of 50.0 N at an angle of
30º with the horizontal. Calculate the work done on the sled.
Sauder
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Physics
Unit 09
Work, Power, & Energy WPE
Name______________Per__
Power:
How does power differ from work?
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What exactly is a watt?
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Example 4:
1 hp = 746 watts
One watt is one joule of energy transferred in one second. This is very small: calculate the
power when lifting a 5N plate of apples 0.25 m in 1 second.
Example 5:
Who has the highest power rating?
A. Person A pushes a 20 kg box with a net force of 60N. She moves it 30 m across the
floor in 5 seconds.
B. Person B pushes a 20 kg box with a net force of 72N. He moves it 25 m across the
floor in 4 seconds.
Example 6:
Challenge: A winch lifts a load of 600 kg at a constant velocity for a distance of 4 meters
during a time period of 2 seconds. What power in watts did this require? What would be the
horsepower? What power in kilowatts did this require?
1 HP = 746 watts
1 kW = 1000 watts
Energy:
What is energy?
potential energy:
kinetic energy:
PE = Fd = mgh
Example 7:
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KE = ½mv2
A panda lifts a barbell with a mass of 85 kg above his head. The barbell is 243 cm above
the ground. How much gravitational potential energy does the barbell have with respect
to the ground? How much work did the Panda do to lift the barbell?
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Physics
Unit 09
Example 8:
An object with a mass of 2.0 kg is moving horizontally with a velocity of
0.5 m/sec. Calculate the kinetic energy possessed by the object.
Example 9:
A 1400-kg car has a net forward force of 4500 N applied to it. The car starts from rest
and travels down a horizontal highway. What is its kinetic energy and speed after it has
traveled 124 m?
Example 10:
Work, Power, & Energy WPE
Name______________Per__
A kitchen pot has a weight of 180 N. (about 40 pounds!!!)
A. How much work is required to lift it the pot to a height of 35 cm above the ground with
nothing but the hair on your face?
B. What was the increase in GPE of the kitchen pot at this height?
C. What is the final velocity if the pot falls?
D. What is the kinetic energy upon impact?
Work Energy Theorem
Example 11:
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W = ΔE
A man pushes a 12.00 kg cart horizontally across the floor starting at 0.625 m/s and
accelerating to 3.70 m/s. How much work is done on the cart?
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Physics
Unit 09
Work, Power, & Energy WPE
Name______________Per__
Law of Conservation of Energy
Example 12:
A 60-kg skier is at the top of a ski slope. At this highest point the skier is 10 m
vertically above the chalet.
What is the skier’s gravitational potential energy at the peak?
What is the skier’s gravitational potential energy at the chalet?
What is the skier’s gravitational potential energy at a point directly between the peak and
the chalet?
What is the skier’s kinetic energy at this very point? (directly between the peak and the
chalet?
Example 13:
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A rocket with a mass of 7.8 kg is launched vertically with a kinetic energy of
1420 J. How high does it rise?
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Physics
Unit 09
Work, Power, & Energy WPE
Name______________Per__
“Extreme” Law of Conservation of Energy
According the conservation of mechanical energy…
PEi + KEi = PEf + KEf
mghi + ½mvi2 = mghf + ½mvf2
Example 14: A rocket is launched vertically with a velocity of 29 m/s. How high does it rise?
Example 15: A dog is riding a sled. Together weigh 844 N. They move down a frictionless hill
through a vertical distance of 12.8 m. Use the conservation of mechanical energy to find
the speed of the dog and sled system at the bottom of the hill, assuming they are pushed off
with an initial speed of 5.29 m/s.
References:
Trinklein, F. E. Modern Physics (TE). Austin: Holt, Rinehart and Winston, 1992.
Chapters 10 & 11 of Zitzewitz, P. W. Glencoe Physics: Principles and Problems. New York:
Glencoe/McGraw-Hill, 2002.
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