Unformatted text preview: nt b ? (b) What is the acceleration
at t 0? (c) What is the acceleration when the speed is
32. (a) Estimate the terminal speed of a wooden sphere
(density 0.830 g/cm3) falling through the air if its radius is 8.00 cm. (b) From what height would a freely
falling object reach this speed in the absence of air
33. Calculate the force required to pull a copper ball of radius 2.00 cm upward through a ﬂuid at the constant
speed 9.00 cm/s. Take the drag force to be proportional
to the speed, with proportionality constant 0.950 kg/s.
Ignore the buoyant force.
34. A ﬁre helicopter carries a 620-kg bucket at the end of a
cable 20.0 m long as in Figure P6.34. As the helicopter
ﬂies to a ﬁre at a constant speed of 40.0 m/s, the cable
makes an angle of 40.0° with respect to the vertical. The
bucket presents a cross-sectional area of 3.80 m2 in a
plane perpendicular to the air moving past it. Determine the drag coefﬁcient assuming that the resistive WEB 37. 38. 39. force is proportional to the square of the bucket’s
A small, spherical bead of mass 3.00 g is released from
rest at t 0 in a bottle of liquid shampoo. The terminal
speed is observed to be vt 2.00 cm/s. Find (a) the
value of the constant b in Equation 6.4, (b) the time
the bead takes to reach 0.632vt , and (c) the value of the
resistive force when the bead reaches terminal speed.
The mass of a sports car is 1 200 kg. The shape of the
car is such that the aerodynamic drag coefﬁcient is
0.250 and the frontal area is 2.20 m2. Neglecting all
other sources of friction, calculate the initial acceleration of the car if, after traveling at 100 km/h, it is
shifted into neutral and is allowed to coast.
A motorboat cuts its engine when its speed is 10.0 m/s
and coasts to rest. The equation governing the motion
of the motorboat during this period is v vi e ct, where
v is the speed at time t, vi is the initial speed, and c is a
constant. At t 20.0 s, the speed is 5.00 m/s. (a) Find
the constant c. (b) What is the speed at t 40.0 s?
(c) Differentiate the expression for v (t ) and thus show
that the acceleration of the boat is proportional to the
speed at any time.
Assume that the resistive force acting on a speed skater
kmv 2, where k is a constant and m is the skater ’s
mass. The skater crosses the ﬁnish line of a straight-line
race with speed vf and then slows down by coasting on
his skates. Show that the skater ’s speed at any time t
after crossing the ﬁnish line is v (t ) vf /(1 ktvf ).
You can feel a force of air drag on your hand if you
stretch your arm out of the open window of a speeding
car. (Note: Do not get hurt.) What is the order of magnitude of this force? In your solution, state the quantities
you measure or estimate and their values. (Optional) 6.5 Numerical Modeling in Particle Dynamics
40. A 3.00-g leaf is dropped from a height of 2.00 m above
the ground. Assume the net downward force exerted on
the leaf is F mg bv, where the drag factor is b
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