The floor also needs to support the weight of the ball during impact.
w
=
mg
= (0.200 kg)(9.80 m/s
2
) = 1.96 N.
So the total force is
22.0 N + 1.96 N = 24.0 N upward
.
26.
(a) Hitting it back
requires a greater force.
Force is proportional to the change in momentum.
When a
ball changes its direction, the change in momentum is greater. If
∆
p
=
p
1
−
p
2
and
p
1
and
p
2
are
opposite (hitting it back), then
∆
p
=
p
1
– (
−
p
2
) =
p
1
+
p
2
.
Had you caught the ball, then
p
2
= 0 so
∆
p
=
p
1
.
(b) The final velocity is opposite to the initial velocity (“hitting it back”).
F
avg
=
∆
p
∆
t
=
mv
−
mv
o
∆
t
=
(0.45 kg)(
−
7.0 m/s
−
4.0 m/s)
0.040 s
=
−
1.2
×
10
2
N =
1.2
×
10
2
N in direction opposite
v
o
.
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View Full Document27.
F
avg
∆
t
=
mv
−
mv
o
=
−
mv
o
,
F
avg
=
−
mv
o
∆
t
.
So the magnitude is
mv
o
∆
t
.
F
avg1
=
(0.16 kg)(25 m/s)
3.5
×
10
−
3
s
= 1.1
×
10
3
N
;
F
avg2
=
(0.16 kg)(25 m/s)
8.5
×
10
−
3
s
= 4.7
×
10
2
N
.
28.
(a) Impulse = area of the trapezoid
=
1
2
(0.30 s + 0.14 s)(900 N) = 76.5 N = 77 N·s
.
(b)
F
avg
=
impulse
∆
t
=
76.5 N·s
0.14 s
= 5.5
×
10
2
N
.
(c)
F
avg
∆
t
=
mv
−
mv
o
,
so
v
=
v
o
+
F
avg
∆
t
m
= 6.0 m/s +
76.5 N·s
3.0 kg
= 32 m/s
.
35.
40 km/h = 11.1 m/s,
2400 lb = 10 680 N.
The force on the infant is opposite to the velocity.
F
avg
∆
t
=
mv
−
mv
o
,
∆
t
=
mv
−
mv
o
F
avg
=
(55 kg)(0
−
11.1 m/s)
−
10 680 N
= 0.057 s
.
40.
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 Spring '12
 Soohoo
 Kinetic Energy, Mass, Momentum, Special Relativity, Velocity, m/s

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