Practice Problems for 1st Semester Exam
Practice Problems for 1st Semester Exam
Practice Problems for 1st Semester Exam
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20. a. How much work is done on a 10 kg backpack that you carry up a flight of stairs that is 3 m high?<br />
m = 10kg d = 3m W = ? W = Fd F = Fg Fg = mg Fg = (10kg)(10m/s 2 ) = 100N<br />
W = Fd W = (100N)(3m) W = 300J<br />
b. What power is expended if you lift the backpack a distance of 3 m in a time of 8 s?<br />
t = 8s P=? P = W/t P = 300J / 8s P = 37.5 watts<br />
c. How much power is expended lifting the same backpack up the stairs in half of the time (twice as fast)?<br />
decrease time … increase power P ~ 1/t ½ time … double power 2(37.5 watts) = 75watts = P<br />
mathematically solved: P = W/t P = 300J/ 4s P = 75 watts<br />
d. What is the PEg of the backpack at the top of the stairs? (hint: you do not have to do any math on paper)<br />
the work (changing backpack’s energy) done on the backpack is = to its Peg at top of stairs<br />
W = 300J … PEg = 300J<br />
mathematically solved: PEg = mgh PEg = (10kg)(10 m/s 2 )(3m) PEg= 300J
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