Work, Power, and Energy...Power The rate at which work is done The rate of energy transfer Power...
Transcript of Work, Power, and Energy...Power The rate at which work is done The rate of energy transfer Power...
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Work, Power, and Energy
Honors Physics
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Work
Work changes an objectβs energy
This is a dot product, meaningβ¦
W= πΉππππ (if there is an angle between F and d)
Units: Joules
1 Nm = 1 Joule
π = ΰ΄±πΉ β ΰ΄±π
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Example
A 50 kg crate is dragged along a floor for 5 meters with a force of 60 N. The rope makes an angle of 30 above the horizontal. How much work was done on it by the applied force?
a) 0 N
b) 250 N
c) 260 N
d) 300 N
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Power
The rate at which work is done
The rate of energy transfer
Power measures how quickly
work is done
Units: Watts
1 Watt = 1 Joule per second
π =ππππ
π‘
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Example
A crane lifts a 900 kg car vertically upward at a constant speed to a height of 15 meters in 45 seconds.
a) How much work is done on the car?
b) Calculate the power output of the motor.
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Example
A 40-kilogram student runs up a staircase to a floor that is 5.0 meters higher than her starting point in 7.0 seconds. The studentβs power output is
a) 29 W
b) 280 W
c) 1.4 Γ 103 W
d) 1.4 Γ 104 W
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Potential Energy
ππΈ = ππβ
Generally, potential energy is the energy an object has by virtue of its position in a system
Gravitational potential energy is a result of an objectβs height above Earth
A result of the work done to lift an object against gravity
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Kinetic Energy
The energy an object has by virtue of its motion and its velocity
Units: Joules πΎπΈ =1
2ππ£2
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Example
Calculate the gravitational potential energy a 2500 kg roller coaster has at the top of a 9 meter hill.
Calculate the kinetic energy a 0.050 kg bullet has when traveling 380 m/s.
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Example
The gravitational potential energy, with respect to Earth, that is possessed by an object is dependent on the objectβs
a) acceleration
b) momentum
c) position
d) speed
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Example
As a ball falls freely toward the ground, its potential energy
a) decreases
b) increases
c) remains the same
Justification???
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Example
As a ball falls freely toward the ground, its kinetic energy
a) decreases
b) increases
c) remains the same
Justification???
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Springs
Hookeβs law
k = spring constant (in N/m) ; this is a measure of how βstiffβ or βslinkyβ the spring is
x = the distance the spring is stretched or compressed
πΉ = ππ₯
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Springs
Elastic potential energy is the stored energy in a stretched or compressed spring
Example: energy stored in spring loaded nerf gun
Units: Joules
ππΈ =1
2ππ₯2
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Example
A 2.0 kg mass is placed on the end of a spring and the spring stretches a distance of 0.8 meters.
What is the spring constant of the spring?
If the same spring were stretched to 1.5 meters, how much energy would be stored in the spring?