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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? Coin A B. Coin B C. They hit the ground at the same speed. D. It cannot be determined.

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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? Coin A B. Coin B C. They hit the ground at the same speed. D. It cannot be determined.

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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, allowing for air resistance? Coin A B. Coin B C. They hit the ground at the same speed. D. It cannot be determined.

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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, allowing for air resistance? Coin A B. Coin B C. They hit the ground at the same speed. D. It cannot be determined.

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**Efficiency and Energy Loss**

3U Physics

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Efficiency Efficiency is the ratio of useful energy or work output to the total energy or work input:

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Efficiency Efficiency is the ratio of useful energy or work output to the total energy or work input (written as a percent):

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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.

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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.

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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.

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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.

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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.

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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.

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**Efficiency, alternately**

Efficiency is also the ratio of useful power output to the power input:

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**Efficiency Example 2, alternately**

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.

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**But what about conservation?**

Given that energy should be conserved, where does the energy go?

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**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.

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**The 3 Laws of Thermodynamics**

(according to British scientist C.P. Snow)

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**The 3 Laws of Thermodynamics**

You can’t win.

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**The 3 Laws of Thermodynamics**

You can’t win. You can’t break even.

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**The 3 Laws of Thermodynamics**

You can’t win. You can’t break even. You can’t even get out of the game.

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You can’t win. You can’t get something (energy) for nothing; energy is always conserved.

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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.

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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.

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**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.

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More Practice Homework Set 8: Power and Efficiency

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Aim: How can we explain the Law of Conservation of Energy? Do Now: Homework Review.

Aim: How can we explain the Law of Conservation of Energy? Do Now: Homework Review.

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