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pasha (sep635) – HW #24 – Antoniewicz – (56445)
1
This printout should have 20 questions.
Multiplechoice questions may continue on
the next column or page – fnd all choices
beFore answering.
001
20.0 points
AtennisballhasamassoF0
.
057 kg. A proFes
sional tennis player hits the ball hard enough
to give it a speed oF 50 m
/
s. The ball hits a
wall and bounces back with almost the same
speed. As indicated in the Following fgure,
highspeed photography shows that the ball
is crushed 2 cm at the instant when its speed
iss momentarily zero, beFore rebounding.
2cm
~
p
i
Making the very rough approximation that
the large Force that the wall exerts on the
ball is approximately constant during contact,
determine the approximate magnitude oF this
Force.
Hint:
Think about the approximate
amount oF time it takes For the ball to come
momentarily to rest. (±or comparison note
that the gravitational Force on the ball is quite
small, only about
(0
.
057 kg)(9
.
8N
/
kg)
≈
0
.
6N
.
AForceoF5Nisapproximatelythesameasa
Force oF one pound.)
Correct answer: 3562
.
5N.
Explanation:
±irst we want to fnd the time interval as
the ball is slowing down during the collision.
~
v
avg
=
~
v
i
+
~
v
f
2
v
avg
,x
=
v
i,x
+
v
f,x
2
=
50 m
/
s+0
2
=25m
/
s
⇒
v
avg
,x
=
Δ
x
Δ
t
⇒
Δ
t
=
Δ
x
v
avg
,x
=
0
.
02 m
25 m
/
s
=0
.
0008 s
.
Now apply the momentum principle. De
fne the system to be the tennis ball. The only
signifcant Force on the ball during the colli
sion is due to the wall. Thus
F
net
,x
=
F
wall
,x
.

F
net
,x

=
±
±
±
±
Δ
p
x
Δ
t
±
±
±
±
±
±
F
wall
,x
±
±
=
±
±
±
±
p
f,x

p
i,x
Δ
t
±
±
±
±
=
±
±
±
±
0

(0
.
057 kg)(50 m
/
s)
0
.
0008 s
±
±
±
±
=
3562
.
5N
.
002 (part 1 of 9) 3.0 points
A0
.
6kgb
locko
Ficei
ss
l
id
ingbyyouona
very slippery ²oor at 2
.
6m
/
s. As it goes by,
you give it a kick perpendicular to its path.
Your Foot is in contact with the ice block For
0
.
003 s.
The block eventually slides at an
angle oF 22
◦
From its original direction. The
overhead view shows in the Following diagram
is approximately to scale.
The red arrow
represents the average Force your toe applies
brie²y to the block oF ice.
II
I
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View Full Documentpasha (sep635) – HW #24 – Antoniewicz – (56445)
2
1.
III
2.
I
3.
II
correct
Explanation:
Image II is correct because the kick is brief,
and after the kick,
~
v
is constant so the block
will move with a constant velocity.
003 (part 2 of 9) 3.0 points
Which components of the block’s momentum
are changed by the impulse applied by your
foot? (List all that apply, separated by com
mas. Remember that the diagram shows a top
view, looking down on the
xz
plane.) Enter
all choices that apply, separated by commas.
A. The
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 Fall '08
 Turner

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