Phys101
Term:121
Online HW-Ch13-Lec01
Q1:
Three particles, two with mass m and one with mass M, might be arranged in any of
the four configurations known below. Rank the configurations according to the
magnitude of the gravitational force on M, least to greatest.
A.
B.
C.
D.
E.
1, 2, 3, 4
2, 1, 4, 3
2, 1, 3, 4
2, 3, 4, 2
2, 3, 2, 4
Ans:
Q2:
𝐂
In the figure, a square of edge length 17.0 cm is formed by four spheres of masses m 1
= 5.00 g, m 2 = 2.80 g, m 3 = 3.30 g, and m 4 = 5.00 g. Find the magnitude of the net
gravitational force (in N) from the spheres at the corners on a central sphere with
mass m 5 = 5.16 g. (Give your answer in three significant figures form)
a
a
a√2
2
a
�F⃗3
a
Ans:
The forces form m 1 and m 4 cancel each other.
The distances between m 5 and m 2 , and m 5 and m 3 are the same:
a√2
17 × √2
=
= 12 cm = 0.12 m
2
2
Fnet on m5 = |F3 − F2 |
r=
Gm3 m5 Gm2 m5
Gm5
|m3 − m2 |
= �
−
�
=
r2
r2
r2
6.67 × 10−11 × 5.16 × 10−3
× |3.3 × 10−3 − 2.8 × 10−3 | = 1.20 × 10−14 N
0.122
KFUPM-Physics Department
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Phys101
Term:121
Online HW-Ch13-Lec01
Q3:
A spaceship of mass = 200 kg is going from Earth (mass = M E ) to the Moon (mass =
M M ) along the line joining their centers. At what distance (in km) from the center of
Earth will the net gravitational force on the spaceship be zero? (Assume that M E = 81
M M and the distance from the center of the Earth to the center of the Moon is 3.8 ×
105 km). (Give your answer in three significant figures form)
Earth
ME
�⃗E
F
�⃗M
F
d
Moon
MM
D
Ans:
m = 200 kg
Distance from Earth Center to spaceship is d.
Distance from Moon Center to spaceship r = D − d
where D = 3.8 × 105 km = 3.8 × 108 m
|FE | = |FM |
GME m
GMM m
=
(D − d)2
d2
⇒
⇒
⇒
G(81MM )m
GMM m
=
(D − d)2
d2
1
81
=
(square root of both sides)
2
(D − d)2
d
9
1
=
⇒ d = 9D − 9d ⇒ 10 d = 9 D
d
D−d
9
9 × 3.8 × 108
D=
= 3.42 × 108 m
⇒d=
10
10
= 3.42 × 105 km
KFUPM-Physics Department
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