Phys101
Term: 122
Online HW-Ch09-Lec02
Q1:
A particle moves along the x axis. Its momentum is graphed below as a function of
time. Rank the numbered regions according to the magnitude of the force acting on
the particle, greatest to least.
A.
B.
C.
D.
E.
1, 2, 3, 4
2, 3, 4, 1
1, 4, 3, 2
1, 3, 4, 2
2, 4, 3, 1
Ans:
C, because the force represents the slope of momentum v.s. time
Q2:
A 5 kg object can move along the x axis. It is subjected to a force F in the positive x
direction; a graph of F as a function of time t is shown below. Find the change in the
velocity (in m/s) of the object over the time the force is applied. (Give your answer in
three significant figures
Ans:
∆P = area under F between t1 and t 2
⇒ m∆V =
⇒ ∆V =
1
(2) × (4) = 4
2
4
4
= = 0.800 m/s
m
5
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Phys101
Term: 122
Online HW-Ch09-Lec02
Q3:
A stream of gas consists of n molecules. Each molecule has mass m and speed v. The
stream is reflected elastically from a rigid surface as shown. The magnitude of the
change in the total momentum of the stream is:
y
x
A.
B.
C.
D.
E.
mnv sin 30o
mnv cos 30o
2mnv sin 30o
2mnv
Mnv
Ans:
�⃗ occurs along y direction
The change in ∆P
�⃗ = n ∆py ȷ̂
⇒ ∆P
= n�pyf − pyi � ȷ̂
= n�mvyf − mvyi � ȷ̂
where vyf = vsin30 and vyi = −vsin30
�⃗ = n(m(vsin30) − (m)(−vsin30)) ȷ̂
⇒ ∆P
= n(2mvsin30) ȷ̂
�⃗� = 2nmvsin30
∴ �∆P
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