phys.docx.pdf
Question 1 (8 points)
You are going 30 m/s in a car of mass 1000 kg when you reach a red light and stop your car.
a. How much heat energy is released by the brakes of your car? (1 point)
KE = ½mv² = 450
...
phys.docx.pdf
Question 1 (8 points)
You are going 30 m/s in a car of mass 1000 kg when you reach a red light and stop your car.
a. How much heat energy is released by the brakes of your car? (1 point)
KE = ½mv² = 450000 J
b. If your car could recapture 40% of the heat energy released from your brakes, how much energy
would you recapture? (1 point)
You'd recapture 180000 J
c. Imagine that you have an electric car and all of the recaptured energy is put back into the battery that
powers your car. What speed could your car achieve from rest by using only the recaptured energy from
part (b)? (2 points)
180000 J = ½ * 1000kg * v²
v = 19 m/s
d. If you brake from the speed in part (c) and then start again from rest, how fast will you be able to
drive? (2 points)
26.6 m/s
This study source was downloaded by 100000830919685 from CourseHero.com on 06-08-2022 13:08:34 GMT -05:00
https://www.coursehero.com/file/30993233/226-physdocx/e. An advertisement says that if you have regenerative brakes like these, you'll never have to put any
more energy into your car again. Is this true or false? Explain your answer. (2 points)
False because if you’re on the highway and don’t break for a very long time you will need energy. While
you may need it less, you will still need it.
Question 2 (4 points)
Andrea rides her bicycle at a speed of 10 m/s. She and the bike have a combined mass of 50 kg.
a. If she uses her brakes to slow down to 5 m/s, how much energy is converted to heat? (1 point)
Heat energy = ½ * 50 * 10^2 – ½ * 50 * 5^2 = 1875 J
b. Andrea has a particularly old bike, and she has not lubricated the chain or gears. This causes friction
inside the gears as she rides. She begins coasting on flat ground at a speed of 10 m/s, and after a few
seconds is only going 8 m/s. How much energy has been converted to heat? (1 point)
This study source was downloaded by 100000830919685 from CourseHero.com on 06-08-2022 13:08:34 GMT -05:00
https://www.coursehero.com/file/30993233/226-physdocx/c. Andrea is going 10 m/s toward a hill. She coasts up the hill without pedaling. If friction causes 10% of
her energy to be converted to heat inside the gears, how high up the hill will she be able to coast? (2
points)
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