Transcript Energy Problems

Energy Problems
Review for Potential energy, Kinetic
energy, Total Energy work, power.
Conservation of energy
• Energy cannot be created or destroyed. It
can be transformed from one form to
another, but the total amount of energy
never changes.
Example:
• When a roller coaster goes down a hill some
energy is “lost through friction.” Not really,
it simply transforms to heat and sound.
Potential Energy
PE = mgh
• Where :
– Pe= Energy in Joules
– m = mass (in kilograms)
– g = gravitational acceleration of the earth (9.8 m/sec2)
– h = height above earth's surface (in meters)
A cart is loaded with a brick and pulled at constant speed
along an inclined plane to the height of a seat-top. If the
mass of the loaded cart is 3.0 kg and the height of the
seat top is 0.45 meters, then what is the potential energy
of the loaded cart at the height of the seat-top?
Answer: PE = aprox 13.5 Joules
Kinetic Energy
KE = (1/2)mv2
• Where:
– KE = Energy (in Joules)
– m = mass (in kilograms)
– v = velocity (in meters/sec)
Determine the kinetic energy of a 1000-kg roller
coaster car that is moving with a speed of 20.0
m/s.
Answer: KE = 200,000 Joules
Kinetic to Potential
• KE=1000 PE=500
• KE=500 PE=1000
• KE=0 PE= 1500
Total Energy
Te=Ke+Pe
• Where:
– Te = total energy
– Ke = kinetic energy
– Pe = potential energy
A roller coaster starts falling from a height of 50 m.
When the roller coaster hits the the 1st drop it is still
20 meters above the ground. What would the KE
and PE energy be at this point? And what is the
velocity?
Roller coaster: 200 kg
Work
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Work
Two things enter into into every case where work is done
The application of force
And the movement of something
W=FD
F= force D= Distance
Work is measured in Joules
Force in Newtons
Distance in meters
Examples…
Carrying something up stairs, lifting anything,
etc.
• How much work is a person using if he
pushes a block with a force of 6N for 10
meters?
F= (6)(10)
F=60 Joules
Power
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The rate at which work is done
Measured in Watts
Power = Work done/ time interval
Examples- lifting weights, climbing stairs, etc.
Question: if a forklift is replaced with a new
forklift that has twice the power, how much
greater a load can it lift in the same amount of
time?
Quiz Questions
• How much work is done on a 100 N boulder
you carry horizontally across a 10 meter
room?
• No work . “its carried horizontally”
• How much work is done if the boulder is
lifted 1 meter?
• (100N)(1m) = 100 J
Quiz Questions
• What's the power expended when a 500
Newton barbell Is lifted 2.2 m in 2 sec?
• P=work/time interval
W= (2.2m)(500N) = 1100 J
P= 1100J/ 2 sec= 550 Watts
Questions
• What is the KE of 3kg toy that moves at
4m/s ?
• KE= .5mv squared
• KE= (.5) 3kg (4m/s) squared
• KE= 24 J
Questions
• a 5kg toy is dropped from the top of a
building. Just before the toy hits the ground
its KE is 7000 J’s . How tall is the building?
• SO….
• PE= 7000 M= 5kg H= ? G= 9.8
• 7000= 5kg (9.8) (?)
• H= 142.85 m