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Exercises of Chapter 2 Kinetics
Q1 (BF14.7) The 8kg object starts from rest on an inclined surface with friction
coefficient of
μ
k
=0.1 at
t
=0 and is subjected to a constant horizontal force
F
=40N.
(a) How fast is it moving at
t
=3s? (b) What distance has it moved at
t
=3s?
Ans: (a) 0.751 m/s, (b) 1.13 m.
Q2 (BF14.8) The schussing skier is on a 25
0
slope. His mass is 80 kg. The kinetic
coefficient of friction between his skis and the snow is
μ
k
= 0.08. His velocity at the
beginning is 9 m/s.
(a)
What is his acceleration in the direction parallel to the slope?
(b)
What is his velocity when he has gone 20 m down the slope?
Hint: you may need the useful formula of vdv=ads
Ans: 3.43m/s2, 14.77m/s
Q3 (BF14.16). A 2kg cart containing 8 kg of water is initially stationary. The center
of mass of the ‘object’ consisting of the cart and water is at x = 0. The cart is
subjected to the timedependent force shown in Fig. 3b, where F
0
= 5 N and t
0
= 2 s.
Assume that no water spills out of the cart and that the horizontal forces exerted on
the wheels by the floor are negligible.
(a)
Do you know the acceleration of the cart during the period 0 < t < t
0
?
(b)
Do you know the acceleration of the center of mass of the ‘object’ consisting of
the cart and water during the period 0 < t < t
0
?
(c)
What is the
x
coordinate of the center of mass of the ‘object’ when t > 2t
0
?
Figure 3. (a)
and
(b)
Ans: (a) No, (b) 0.5m/s
2
, (c) 2m
Q4 (BF14.32). The masses
m
A
= 15 kg and
m
B
= 30 kg, and the coefficients of friction
between all of the surfaces are
μ
s
= 0.4 and
μ
k
= 0.35. What is the largest force
F
that
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 Spring '11
 wong

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