 # 6 Forces

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Forces
1. The propellers of a small airplane produce a
forward thrust of 6.2 ! 104 N on the plane
which has a mass of 2.8 ! 104 kg. What is the
plane’s forward acceleration?
2. If the plane in Problem 1 is flying
horizontally, what is the lift (upward
force of air) on the plane?
3. A girl in a canoe uses a paddle to push the
canoe at rest from a dock giving it a speed of
0.30 m/s. If the paddle is in contact with the
dock for 0.75 seconds, what is the average
force on the canoe? (The mass of the canoe is
27 kg and that of the girl is 52 kg.)
4. A student attaches a spring scale to a 2.30-kg
book and then releases the book. If the
student maintains a scale reading of 25.0 N,
what is the magnitude and direction of the
book’s acceleration? (Assume upward is the
positive direction.)
5. If the student in problem 4 maintains a
scale reading of 22.0 N, what will be the
displacement of the book in 1.0 s?
6. Two laboratory masses are tied to opposite
ends of a weightless string, which passes over
a frictionless pulley as shown below.
c. A student decides to demonstrate this
problem. After setting up the equipment,
she accurately measures the time it takes
the 0.350-kg mass to reach the floor as
0.92 s. If she attributes the cause of the
difference in times to the frictional force
on the pulley, what is the size of this
force?
7. A horse is pulling two loaded sleds across
the snow. The first sled, which has a mass of
250 kg, is attached to the horse by a harness
that can withstand a maximum tension of
4500 N. The second sled, which has a mass of
150 kg, is attached to the first sled by a rope
that can withstand a maximum tension of
450 N.
a. What is the acceleration of the sleds just
before the rope snaps?
b. What is the tension in the harness just
before the rope snaps?
8. A pendulum with a length of 1.00 m is in a
rocket that is far removed from any
gravitational effects. If the rocket is
accelerating as shown below, what is the
period of the pendulum?
Frictionless pulley
v
0.350 kg
0.250 kg
a " 3.2 m/s2
0.500 m
a. If the masses are released from rest, what
is the acceleration of the 0.250-kg mass?
b. After the masses are released, how long
will it take the 0.350 kg mass to strike the
floor?
Physics: Principles and Problems
Supplemental Problems • Chapter 6
9
6
Forces
9. A 65.0-kg passenger in a car is securely
fastened in a safety belt as shown below.
x
m " 65.0 kg
Safety belt
Calculate the magnitude and determine the
direction of the horizontal force that the
safety belt and seat exert on the passenger for
each of the following conditions. (Assume the
car is moving only horizontally.)
10. In Problem 9, the car is traveling at 20.0 m/s
and the driver suddenly slows down the car at
a rate of 5.00 m/s2.
a. What is the magnitude and direction of
the force that the passenger exerts on the
safety belt and seat?
b. After slowing down, the driver accelerates
the car forward at a rate of 1.50 m/s2.
What is the magnitude and direction of
the force that the passenger exerts on the
safety belt and seat?
a. v0 " #20.0 m/s, a " 0 m/s2
b. v0 " #20.0 m/s, a " #1.50 m/s2
c. v0 " 10.0 m/s, a " \$3.00 m/s2
d. v0 " \$3.0 m/s, a " \$1.20 m/s2
e. v0 " \$10.0 m/s, a " #2.3 m/s2  