1' 10 Points

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1. (10 Points) A 4.0-kg object is constrained
to move along the x-axis. At time t 0 it is
has a velocity of 6.0 m/s. After t 0 a force
as shown by the graph acts on the object. What
is the speed of the object at time t 8.0
sec? (a) 4.0 m/s (b) zero
(c) 16 m/s (d) 9.0 m/s
(e) 24 m/s
All areas under the curve cancel except that
shown. The area of the region is 40 Nsec. A
Nsec is a kgm/s when you divide by the mass you
get the change in velocity to be 10 m/s.
Velocity has thus changed from 6 m/s to 16 m/s.
The speed is 16 m/s.
Based on time varying force on HW4
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2. (10 points) A rifle has a muzzle velocity of
700 m/s. A soldier aims directly at the center
of a target and fires. His shot strikes the
target 15.0 cm below the center. How far was he
away from the target? (a) 160 m (b)
280 m (c) 190 m (d)
120 m (e) 100 m
Just like problem 6.11 on HW4
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3. (5 points) A 2.5-kg mass hangs by a string
from the ceiling of an elevator. The elevator is
moving upwards with a speed of 4.0 m/s and is
slowing down at rate of 3.0 m/s2. What is the
tension in the string? (a) 32 N
(b) 14 N (c) 17 N
(d) 21 N (e) 12 N
If the speed of the mass is decreasing the
velocity and acceleration are in opposite
directions. Assuming up to be positive the
acceleration is negative.
I said there would be an elevator problem
4
4. (10 points) An 8.0-kg box is initially at
rest on a rough surface. The coefficients of
static and kinetic friction between the box and
the surface are 0.60 and 0.30 respectively. A
force as shown is then applied to the box. What
now is the force of friction acting on the
box? (a) 41 N (b) 28 N
(c) 20 N (d) 32 N
(e) 8.9 N
Solution Assume the block is at rest and
determine if the friction force needed to keep it
at rest is greater than fs(max)
Since fs lt fs (max) the box is still at rest.
Done in class, Lecture 12, slides 20-21
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5. (10 points) A small plane can fly 150 mph in
still air. The pilot wishes to fly to an airport
that is 100 miles north of his current location.
How long does it take him to make the trip if the
wind is blowing at 40 mph towards the
northeast? (a) 34 min (b) 40 min
(c) 38 min (d) 36 min
(e) 32 min
You could also use Law of Cosines to find v.
Done in class Lecture 14
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6. (10 points) A volleyball rules require that a
volleyball have a circumference of 66 cm. The
terminal speed of a volleyball is 17.5 m/s. What
is the mass of the ball? (a) 340 g
(b) 310 g (c) 220 g
(d) 300 g (e) 270 g
Like Terminal Speed of baseball in HW4
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7. (10 points) A 400-g sphere hangs from a
string on an outdoor porch. A steady wind
blowing parallel to the ground causes the sphere
to hang at an angle of 20 degrees with the
vertical. What is the tension in the string? (a)
3.9 N (b) 4.2 N (c)
11 N (d) 4.4 N (e)
6.1 N
This is a case of static equilibrium. The forces
form a triangle.
PRS question for June 8, 2005
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8. (10 points) An airliner is taking off from an
airport. When it crosses the end of the runway
its altitude is 49.0 m, its airspeed (magnitude
of velocity) is 60.0 m/s, and its rate of climb
(vertical velocity) is 14.7 m/s. At this same
instant a tires falls off the airplane. How far
from the end of the runway does the tire land?
(note use g 9.80 m/s2) (a) 291 m
(b) 116 m (c) 120 m
(d) 300 m (e) 72 m
Assume up is positive and the runway is y0
5 points credit for (d) 300 m
Done in class Lec 14, slide 12-13
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9. (10 points) A 4.0-kg block slides across a
horizontal surface due to an applied force F as
shown. The coefficient of kinetic friction
between the block and the surface is 0.25. What
is the magnitude of F if the block moves with a
constant speed? (a) 31 N (b) 46 N
(c) 23 N (d) 50 N
(e) 41 N
Same concept as pushing a box with horizontal
force on HW 3, but easier
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10. (10 points) A wooden box is placed on the
left end of a 7.00-m long ramp. The coefficients
of static and kinetic friction between the block
and the ramp are 0.50 and 0.25 respectively. The
left end of the ramp is raised until the box
begins to slide. What is the speed of the box
when it reaches the other end of the ramp? (a)
5.5 m/s (b) 4.2 m/s (c)
6.1 m/s (d) 3.2 m/s (e)
2.5 m/s
Extension of Wooden box on wooden ramp on HW4
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11. (5 points) Baggage is loaded onto an
airplane using a conveyer belt that is inclined
30.0? with the horizontal and moves at a constant
speed of 1.50 m/s. A baggage handler drops a
suitcase vertically onto the belt a distance of
10.0 meters from the door of the plane. If the
coefficient of kinetic friction between the belt
and the suitcase is 0.650, how long does it take
the suitcase to reach the door of the plane? (a)
6.67 sec (b) 8.10 sec (c)
7.88 sec (d) 7.40 sec (e)
9.26 sec
The suitcase slides for a time ts until it
reaches a speed of 1.5 m/s as a result of the
force of friction. In this time it slides a
distance of xs and then moves the remaining
distance at 1.5 m/s.
12
After 2.43 sec the suitcase is moving at 1.5 m/s
and has moved 1.823 meters. It moves the
remaining distance at 1.5 m/s.
Based on problem 5.43 on HW4