Download Quiz 3 - High Point University

Physics 221
Name:
Quiz 3, Form: A
Date:
Here are some useful constants. R is radius, and r is the distance between two objects.
Mearth = 6.0 × 1024 kg
Mmoon = 7.35 × 1022 kg
Msun = 2.0 × 1030 kg
rmoon,earth = 3.84 × 108 m
rsun,earth = 1.5 × 1011 m
Rearth = 6.38 × 106 m
Rmoon = 1.74 × 106 m
Rsun = 7.0 × 108 m
1. State the momentum principle in words.
2. State the momentum principle mathematically.
3. Which is larger, the magnitude of the gravitational force of the moon on the Earth or the magnitude
of the gravitational force of the Sun on the Earth. (Note: full credit will only be given if you calculate
each force and compare them–no guessing allowed!)
4. The initial momentum of a GPS satellite is in the direction indicated below when a thruster fires in the
direction indicated by the vector F~net ∆t which is the net force on the satellite times the time interval
during which the thruster fired. Sketch the vector for the final momentum of the satellite.
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5. The magnitude of the gravitational force of the Sun on the Earth is
gravitational force of the Earth on the Sun.
(a)
greater than
(b)
less than
(c)
equal to
the magnitude of the
6. If the net force on an object is zero for a time interval ∆t, then
(a)
the object’s momentum is constant.
(b)
the object’s momentum is zero.
(c)
the change in an object’s momentum is positive.
(d)
the change in an object’s momentum is negative.
7. Suppose you write a VPython simulation of a tennis ball, like the one you did in lab. Here are a few
lines of code.
ball.m=0.15
ball.p=ball.m*vector(2,5,0)
Fnet=vector(0,-1.5,0)
dt=0.05
while 1:
ball.p = ball.p + Fnet*dt
ball.pos =
The last line that is incomplete should calculate the new position of the ball after a time interval dt.
Complete the line so that the program will run without errors and will correctly simulate the motion
of the ball.
8. You drop a tennis ball from rest, beginning the stopwatch at the moment you release the ball. If you
measure and plot the y-momentum of the ball with respect to time as shown in the graph in Figure
1. What does the slope represent and what is the value of the slope (use appropriate units) for this
graph?
9. Does the previous graph tell you anything about the x-component or z-component of the momentum
of the tennis ball?
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Figure 1: Graph of py vs. t for a falling ball.
10. If a planet of mass 1 × 1027 kg is located at < 3 × 108 , −2 × 108 , 0 > m, and if a star of mass 1 × 1030
kg is located at < 2 × 108 , 1 × 108 , 0 > , what is the force of the star on the planet? Express this as
a vector, and remember how you did this on the homework. There are multiple steps needed in order
to answer this question.
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Answer Key for Exam A
1. The change in momentum of an object is equal to the net force on the object times the time interval
during which the net force acts.
2. ∆~
p = F~net ∆t
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3. Fg = GMm ME /rm,E
= 2 × 1020 N
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Fg = GMS ME /rS,E
= 3.57 × 1022 N
The Sun exerts a larger gravitational force on the Earth than the moon.
4. See key.
5. (c)
6. (a)
7. ball.pos = ball.pos + ball.p/ball.m*dt
8. slope = 2 N
The slope is the y-component of the net force on the ball.
9. no, just the y-component of the momentum; you would need to measure and graph px and py to know
those components of the net force on the ball.
10. ~r = ~rs − ~rp < −1 × 108 , 3 × 108 , −1 × 108 > m
|~r| = 3.16 × 108 m
r̂ = ~r/|~r| =< −0.316, 0.949, 0 >
F~g = GM m/r2 r̂ =< −2.12 × 1029 , 6.37 × 1029 , 0 >
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