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Conservation of Energy
Coin A is thrown up in the air at a speed v from an height of h. Coin B is thrown down at the same speed from the same height. Which coin hits the ground at the highest speed, neglecting air resistance?
A. Coin A B. Coin BC. They hit the ground at the same speed. D. It cannot be determined.
Conservation of Energy
Coin A is thrown up in the air at a speed v from an height of h. Coin B is thrown down at the same speed from the same height. Which coin hits the ground at the highest speed, neglecting air resistance?
A. Coin A B. Coin BC. They hit the ground at the same speed. D. It cannot be determined.
Conservation of Energy
Coin A is thrown up in the air at a speed v from an height of h. Coin B is thrown down at the same speed from the same height. Which coin hits the ground at the highest speed, allowing for air resistance?
A. Coin A B. Coin BC. They hit the ground at the same speed. D. It cannot be determined.
Conservation of Energy
Coin A is thrown up in the air at a speed v from an height of h. Coin B is thrown down at the same speed from the same height. Which coin hits the ground at the highest speed, allowing for air resistance?
A. Coin A B. Coin BC. They hit the ground at the same speed. D. It cannot be determined.
Efficiency and Energy Loss
3U Physics
Efficiency
Efficiency is the ratio of useful energy or work output to the total energy or work input:
Efficiency
Efficiency is the ratio of useful energy or work output to the total energy or work input (written as a percent):
%100energyinputtotal
energyoutputusefulEfficiency
Efficiency Example 1
A model rocket engine contains explosives storing 3500 J of chemical potential energy. Calculate how efficiently the rocket transforms stored chemical energy into gravitational potential energy if the 0.50 kg rocket is propelled to a height of 1.0 x 102 m.
Efficiency Example 1
A model rocket engine contains explosives storing 3500 J of chemical potential energy. Calculate how efficiently the rocket transforms stored chemical energy into gravitational potential energy if the 0.50 kg rocket is propelled to a height of 1.0 x 102 m.
JmkgmghEE
JE
sm
goutput
input
490100.18.950.0
35002
2
Efficiency Example 1
A model rocket engine contains explosives storing 3500 J of chemical potential energy. Calculate how efficiently the rocket transforms stored chemical energy into gravitational potential energy if the 0.50 kg rocket is propelled to a height of 1.0 x 102 m.
JmkgmghEE
JE
sm
goutput
input
490100.18.950.0
35002
2
%1414.03500
490or
J
J
E
EEfficiency
input
output
Efficiency Example 2
A 120-W motor accelerates a 5.0-kg mass from rest to a speed of 4.0 m/s in 2.0 s. Calculate the motor’s efficiency.
Efficiency Example 2
A 120-W motor accelerates a 5.0-kg mass from rest to a speed of 4.0 m/s in 2.0 s. Calculate the motor’s efficiency.
JkgmvEE
JsWtPEt
EP
sm
koutput
inputinputinput
input
400.40.5
2400.2120
212
21
Efficiency Example 2
A 120-W motor accelerates a 5.0-kg mass from rest to a speed of 4.0 m/s in 2.0 s. Calculate the motor’s efficiency.
JkgmvEE
JsWtPEt
EP
sm
koutput
inputinputinput
input
400.40.5
2400.2120
212
21
%1717.0240
40or
J
J
E
EEfficiency
input
output
Efficiency, alternately
Efficiency is also the ratio of useful power output to the power input:
in
out
tEt
E
in
out
P
P
E
EEfficiency
in
out
Efficiency Example 2, alternatelyA 120-W motor accelerates a 5.0-kg mass from rest to a
speed of 4.0 m/s in 2.0 s. Calculate the motor’s efficiency.
W
s
kg
t
mv
t
EP
WP
sm
outputoutput
input
200.2
0.40.5
120
212
21
%1717.0120
20or
W
W
P
PEfficiency
input
output
But what about conservation?Given that energy should be conserved, where
does the energy go?
But what about conservation?Given that energy should be conserved, where
does the energy go?
The input energy is only “lost” in that it has been converted to non-useful forms: heat energy, sound energy, etc. by forces such as friction.
The 3 Laws of Thermodynamics(according to British scientist C.P. Snow)
The 3 Laws of Thermodynamics• You can’t win.
The 3 Laws of Thermodynamics• You can’t win.• You can’t break even.
The 3 Laws of Thermodynamics• You can’t win.• You can’t break even.• You can’t even get out of the game.
You can’t win.
You can’t get something (energy) for nothing; energy is always conserved.
You can’t break even.
You can’t return to the state at which you started because some energy is converted to non-useful forms.
You can’t break even.
You can’t return to the state at which you started because some energy is converted to non-useful forms. The entropy (disorder) of a system always increases.
You can’t get out of the game.Absolute zero is the only state in which no energy
is ever lost – and absolute zero is unattainable.
More Practice
Homework Set 8: Power and Efficiency
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