Quiz A solutions

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Energy quiz:
1)
a) Friction “sucks out” all of the energy. Therefore, the KE at the bottom of the hill is equal to the work of
friction on the flat section.
1
1
mv 20 − W f = mv 2
2
2
1
mv 20 − F f d = 0
2
1
µmgx = mv 20
2
− (0.35)(9.8)(17) = −(0.5)v 2
v = 10.8m / s
b)
1 2
mv
2
1 2
1
kx + mgh = mv 2
2
2
1
1
(1000) x 2 + (3)(9..8)(1.7) = (3)(10.8 2 )
2
2
x = 0.5m
Us +Uk =
2)
a) at x=0, U=0. Therefore, all 7 J of energy is found as kinetic energy. Therefore,
7=(0.5)mv2
but m=1 kg
v=3.74m/s
b) energy at x=0 must equal energy at =2
7=Ke+U
7=KE+-6
KE=13 J
c) No, it does not have enough energy to reach x=-2 m.
3) D
4) C
Ug = K
1 2
mv
2
1
9.8(.5) = v 2
2
v = 3.13m / s 2
mgh =
5) D
Us + K = K
1 2 1
1
kx + mv 2 = mv 2
2
2
2
1
1
1
(80)(.04) 2 + (.5)(.5) 2 = (.5)v 2
2
2
2
v = 0.71m. / s
6) D
7) B
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