Physics 3A: Basic Physics I
Quiz #8c : Solutions
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W
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F
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r
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A
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B
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A
๎๎๎
B
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cos
๎๎๎๎๎
Wnet
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Kf
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Ki
Ug
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mgy
Emech
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K
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U
Ei
๎
Ef
Us
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1
2k x2
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Eint
๎
fk
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x
Ug
๎
๎
G M m
r
Uf
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๎๎๎
xi
xf
Fxdx
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Ui
Fx
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๎
dU
dx
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K
๎
๎
fk
๎
x
๎๎๎
Wother forces
W
๎
๎
xi
xf
Fxdx
K
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1
2mv2
P
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dW
dt
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๎
F
๎๎๎
v
P
๎
W
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t
P
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dE
dt
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K
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๎
U
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๎
Eint
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W
(circle the letter of your answer)
1. (2 pts) You throw a ball straight up with speed v. Define the system as the ball and the
earth. At what point in the ball's flight is the system's total energy the greatest? (ignore air
resistance)
a.) Just has you release the ball.
b.) Just before the ball returns to your hand.
c.) When the ball reaches its highest point
d.) The system's total energy does not change
e.) Not enough information to tell
2. (2 pts) A block slides 1.5 m down a 45o frictionless incline. The block hits a spring, with
spring constant k1, at the bottom and compresses the spring as it comes to a stop. The
potential energy in the spring is then U1. If the same thing is repeated, but with a different
spring at the bottom, k2 = 2 k1. The new elastic potential energy is U2. Then U2 is
a.) equal to U1
b.) twice as large as U1
c.) ยฝ as large as U1
d.) 4 times as large as U1
e.) ยผ as large as U1
3. (6 pts) A potential energy function for a three dimensional force on a particle is of the form
U
๎
2 x4y2
๎
3 xz3
๎
12
. Find the force that acts on the particle at the point (x,y,z). Give
the force in unit-vector (i,j,k) notation. (use back of sheet as well, circle your final answer
and show all work)
Concept is conservative force and potential energy it gives rise to. Use:
Fx
๎
๎๎๎
U
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x, etc.
