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# 7 Physics Problems

1. Consider the energy of a 100 kg rock.
a) What is the gravitational potential energy (PE) of the rock that is on a table 5 m above the ground?
b) The rock falls to the ground. What is its PE on the ground?
c) What is the change in PE of the rock from the table to the ground?
d) From conservation of energy, what is the change in the kinetic energy of the rock?

2. A car is traveling at a constant 60 miles per hour. What is its kinetic energy in Joules if its mass is 800 kg? (Remember to convert mph to m/s)

3. A cubic meter of water has a mass of 1000 kg. What is the change in the potential energy of a cubic meter of water falling 50 m in a hydropower plant? Compare this to your answer to question 2.

4. The sun outputs about 3.9x1026 W of power, a small fraction of which reaches the Earth's surface. The sun thus provides about 100 W/m2 of power to the Earth's surface averaged over time and weather. How much energy is deposited on average by the sun on Washington state in an average day? Washington is 71,302 square miles in area (convert to m2). Give your answer in both Joules and in kilowatt-hours (1 KWH = 3.6x106J).

5. When energy is produced, only a fraction of the energy is released in usable form such as electricity. Typical efficiencies are about 20%-30%. Where does the rest of the energy go? (a one or two word response is sufficient)

6. Describe the fate of the sun.

7. Cell phones operate in a frequency band around 850 MHz, output about 600 mW of transmitted power, and have a battery containing about 9720 J of energy.
a. What is the wavelength corresponding to this frequency?
b. How many hours will the battery last if used continuously?

#### Solution Summary

This posting contains the solution to the given problems.

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