PH230 Week 2 Homework
Due: Wednesday 9/7 2005 9:00 AM CDT
Description:
Do Problem 2.1 and the following questions.
Instructions
Before doing this assignment, study the textbook, doing the "stop and think" activities and the in-line
exercises.
9 pts______ 1. Check all of the following that are correct statements. Read each statement
very carefully to make sure that it is exactly correct.


a)
The relationship among position, velocity, and time is r f  ri  vt .
c)


Pf  Pi  Fnet t .



The relationship among position, velocity, and time is r f  ri  v t .
d)
At speeds small compared to the speed of light, momentum is approximately equal to
e)
The momentum principle can be written
b)
f)
g)
h)
i)
The momentum principle can be written


P  Mv .

 
Pf  Pi  Fnet t .

 
The relationship among position, velocity, and time is r f  ri  v t


1
The momentum principle can be written P 
Mv .
2
v
1  
c
 
The momentum principle can be written P  Fnet t .

 
The momentum principle can be written Pf  Pi  Ft .
3pts______2. A ball moves in the direction of the arrow labeled a below. The ball is struck by a
stick which briefly exerts a force on the ball in the direction of the arrow labeled c below. Which

arrow best describes the direction of P , the change in the ball's momentum?
5pts______ 3. At t = 17.0 seconds an object with mass 6 kg was observed to have a velocity of
<12, 25, -9> m/s. At t = 17.2 seconds its velocity was <24, 17, 24> m/s. What was the
average (vector) net force acting on the object?

Fnet = < , , > N
5pts______4. A hockey puck is sliding along the ice with nearly constant momentum <13, 0, 4>
kg · m/s when it is suddenly struck by a hockey stick with a force = <0, 0, 1900> N that
lasts for only 2 milliseconds (2x10-3 s).What is the new momentum of the puck? f = < , , >
kg · m/s
5pts______5. Complete the following VPython statement correctly, so that it will create a red
sphere located at < -5, -1, 9 >m with radius = 3m.
redball = sphere(pos=vector(____ ,____ ,____ ), radius= ____,color=color.red)
Complete the following VPython statement correctly, so that it will create a yellow arrow with its
tail at <14, 17, -17> and its head at <2, 23, -30>.
yellowarrow = arrow(pos=vector(____ ,____ ,____ ), axis=vector( ____,____ ,____ ), color=color.yellow)
Suppose you have already created two spheres in VPython named "pingpongball" and
"basketball". Complete the following VPython statement so it will create a blue arrow with its tail
at the center of sphere "pingpongball" and with its head at the center of sphere "basketball".
ba = arrow(pos=___________, axis=___________, color=color.blue)
10pts______6. The mass of the Sun is 2 x1030 kg, and the mass of the Earth is 6 x1024 kg. The
distance from the Sun to the Earth is 1.5 x 1011 m.
(a) Calculate the magnitude of the gravitational force exerted by the Sun on the Earth.
(b) Calculate the magnitude of the gravitational force exerted by the Earth on the Sun.
20pts______7. (a) Calculate the magnitude of the gravitational force exerted by Venus on a 60 kg
human standing on the surface of Venus. (The mass of Venus is 4.9 x 10 24 kg and its
radius is 6.1x 106 m.)
(b)Calculate the magnitude of the gravitational force exerted by the human on Venus.
(c) For comparison, calculate the approximate magnitude of the gravitational force of this
human on a similar human who is standing 2.5 meters away.
(d) What approximations or simplifying assumptions must you make in these
calculations?
(Note: Some of these choices are false because they are wrong physics!)
Treat Venus as though it were a uniform-density sphere.
Ignore the effects of the Sun, which alters the gravitational force that one object exerts on
another. Use the same gravitational constant in (a) and (b) despite its dependence on the
size of the masses. Treat the humans as though they were points or uniform-density
spheres.

5pts______8. A star exerts a gravitational force of magnitude | F | on a planet. The distance
between the star and the planet is r. If the planet were 4 times farther away (that is, if the
distance between the bodies were 4*r), by what factor would the force on the planet due

to the star change? For example, if the new force would be 0.01*| F | , you should type

0.01 in the box below. Force at distance 4*r =
*| F |
5pts______9. A planet exerts a gravitational force of magnitude 9e22 N on a star. If the planet were
3 times closer to the star (that is, if the distance between the star and the planet were 1/3
what is is now), what would be the magnitude of the force on the star due to the planet?

| F | = _____N
11pts______10. The windshield of a speeding car hits a hovering insect. Compare the magnitude
of the force that the car exerts on the bug to the force that the bug exerts on the car. (Note: the
interatomic forces between bug and windshield are electric forces.) Check all that are correct
a) The two forces are equal.
b) The force that the car exerts on the bug is bigger.
c) The force that the bug exerts on the car is bigger.
d) Compare the magnitude of the change of momentum of the bug to that of the car:
e) The magnitude of change of momentum of the bug is bigger.
f) The magnitude of change of momentum of the car is bigger.
g) The magnitudes of the change of momentum are equal.
h) Compare the magnitude of the change of velocity of the bug to that of the car:
i) The magnitude of change of velocity of the bug is bigger.
j) The magnitude of change of velocity of the car is bigger.
k) The magnitudes of the change of velocity are equal.
35pts______11. A planet of mass 2e24 kg is at location <2e11, -3e11,0 > m. A star of mass 6e30 kg
is at location <-4e11, 4e11,0> m. It will be useful to draw a diagram of the situation,
including the relevant vectors.

a) What is the relative position vector pointing from the planet to the star? r = < , , > m

b) What is the distance between the planet and the star? | r | = m
c) What is the unit vector in the direction of ? r̂ = < , , >

 on planet| = N
e) What is the magnitude of the force exerted on the star by the planet? | F on star| = N

f) What is the force (vector) exerted on the planet by the star? F on planet = < , , > N

g) What is the force (vector) exerted on the star by the planet? F on star = < , , > N
d) What is the magnitude of the force exerted on the planet by the star? | F
5pts______12. A planet of mass 2e24 kg is at location <-7e11, 4e11,0 > m. A star of mass 8e30 kg is
at location <4e11, -4e11,0> m. What is the force exerted on the planet by the star? (It will

probably be helpful to draw a diagram, including the relevant vectors.) F = < , , > N
55pts______13. Suppose you have already created two spheres in VPython, one named Sun and
the other named Earth, to represent the planet. Before the "while" loop, you specified their
masses and initial momenta as Sun.m, Earth.m, Sun.p, and Earth.p, and you've chosen a
value for deltat and initialized the time t to zero. You've also specified G = 6.7e -11. The
following questions will review how to express vector calculations in VPython syntax, to
instruct VPython to carry out repetitive calculations inside a "while" loop to model a planet
orbiting a star. (You may wish to enlarge the text size. In Internet
a) How do you calculate the vector that points from the Sun to the planet?
b) How do you calculate the magnitude of the vector that points from the Sun to the planet?
c) How do you calculate the unit vector that points from the Sun to the planet?
d) How do you calculate the magnitude of the gravitational force exerted by the Sun on the planet?
e) How do you calculate the vector gravitational force exerted by the Sun on the planet?
f) How do you update the momentum of the planet?
g) What is the name of the physics concept you used to update the momentum in the
preceding question?
h) How do you update the position of the planet?
i) How do you update the time?
j) Suppose that instead of calculating the vector that points from the Sun to the planet, you
had calculated the vector that points from the planet to the Sun. Now what would be the
correct statement for calculating the vector gravitational force exerted by the Sun on the
planet?
k) Suppose the planet had enough mass to significantly affect the motion of the Sun, so you
need to update the momentum of the Sun. With F representing the (vector) force acting
on the planet, how would you update the momentum of the Sun?
11pts______14. A review of the basics of circular motion: The radius of a merry-go-round is 11
meters, and it takes 14 seconds to go around once. We want to calculate the speed of an
atom in the outer rim. First, how far does the atom go in 14 s? (This distance is the
circumfernce of the circle.)
1. distance = ______m
2. Therefore, what is the speed of the atom? speed =______ m/s
3. What is the direction of the velocity of this atom?
a) towards the center
b) away from the center
c) tangential