Download Q1: In planetary motion the line from the star to the planet

Phys101
Term:121
Online HW-Ch13-Lec03
Q1:
In planetary motion the line from the star to the planet sweeps out equal areas in equal
times. This is a direct consequence of:
A.
B.
C.
D.
E.
the conservation of energy
the conservation of mass
the conservation of momentum
the conservation of angular momentum
none of the above
Ans:
Q2:
𝐃
Each of the four corners of a square with edge d =1 m is occupied by a point mass 1
kg. To remove one mass from the corner to a point far away, the work (in J) that must
be done by an external agent is: (Give your answer in three significant figures form)
Ans:
Wap = ∆U = Uf − Ui
= (potential energy of one the masses due to the other
three masses)
Wap = ∆U = − �−
⇒ Wap =
=
Gmm Gmm Gmm
−
−
�
d
d
d√2
d√2
d
m
Gm2
1
Gm2 2√2 + 1
�1 + 1 + � =
�
�
d
d
√2
√2
6.67 × 10−11 × 12 2√2 + 1
�
� = 1.81 × 10−10 J
1
√2
KFUPM-Physics Department
1
Phys101
Term:121
Online HW-Ch13-Lec03
Q3:
A projectile is fired straight upward from Earth's surface with a speed that is half the
escape speed. If R is the radius of Earth, the highest altitude reached, measured from
the surface, is:
A. R/4
B. R/3
C. R/2
D. R
E. 2R
Ans:
B
1
1 2GM
⇒ Vi = Vescape = �
2
2
R
Conservation of Mechanical energy: Emf = Emi
0
⇒ K f + Uf = K i + Ui
⇒−
⇒−
1
GmM
GmM
= mVi 2 −
2
R
r
GM
r
=
1 1 2GM
�
2 4 R
� −
GM
R
⇒ r = 4R/3; but the altitude h = r − R =
KFUPM-Physics Department
R
4R
−R=
3
3
2