Exam Review Answer Key
1) Force of Friction = 50N
a. Explanation: Drawing a force diagram: The force of gravity and
normal force cancel each other out. The applied force is 100N
to the right. What is the force of friction? First you have to find the net force
acting on the object, which you find by using F = MA. Since the mass is 10kg
and the acceleration is 5m/s/s, the net force is 50N. If the net force is 50 N, what must
the force of friction be? 50N in the direction opposite the applied force.
2) Force of Friction = 1000N
a. Explanation: According to Newton’s third law, forces come in pairs. Gravity pushes
down on the object with a force of 1000N, so the surface must push up with an equal
and opposite force since the object is not accelerating.
3) Mass = 11kg
a. Explanation: use the formula: F=MA and solve for M.
4) Net force = 0 N
a. Explanation: if the object is travelling at a constant velocity, it is not accelerating.
Therefore the forces are balanced and the net force = zero.
5) Net force = 50N
a. Explanation: Draw a force diagram: gravity and normal force are balanced and cancel
out. Use F = MA to find the net force acting on the book.
6)
a. False - An object which is moving to the right could have unbalanced forces, but only if
it is accelerating. The presence of unbalanced forces must always be associated with
acceleration, not mere motion. In this case, an object moving to the right could have a
balance of forces if it is moving with a constant velocity.
b. False - An object would never slow to a stop unless the forces acting upon it were
unbalanced. In fact, an object which slows down must have a unbalanced force directed
in the direction opposite their motion.
c. False - An unbalanced force is only required to accelerate an object. A balance of forces
is required to keep an object moving at a constant velocity. For instance, a car moving to
the right at constant velocity encounters as much rightward force as leftward force.
d. False - An object which moves in a circle has a changing direction. As such, there is an
acceleration and this acceleration requires that there be an unbalanced force present on
the object.
e. False - Any object that accelerates has a changing velocity. An object that accelerates at
a constant rate has a velocity that changes by the same amount each second. For
instance, a free-falling object changes its velocity by -9.8 m/s ever second. It is said to
have a constant acceleration of -9.8 m/s2. A free-falling object, or any object with an
acceleration (whether constant or non-constant) must be experiencing an unbalanced
force.
f. False - Consider an object which weighs 1000 N (a 1000 N downward force) which is
being pulled on by two people, each exerting 500 N of upward force. Such an object has
three forces acting upon it and the three forces together balance each other.
Exam Review Answer Key
g.
False - A free-falling object is an object upon which the only force is gravity. As such,
there is an unbalanced force acting upon it; this unbalanced force explains its
acceleration.
h. False - Balanced forces cause stationary objects to stay at rest. However balanced
forces would never cause moving objects to stop; an unbalanced force would be
required to stop a moving object.
i. False - Unbalanced forces do more than cause objects to move; unbalanced forces
cause objects to accelerate. Though one could make a strong argument that an object
that is accelerating must also be moving (albeit with a changing velocity). In this sense,
this statement is true.
7)
a.
True - Absolutely true. Like all true scientific laws, they govern all objects. In the case of
Newton's first law of motion: An object that is nonmoving remains at rest (unless acted
upon by an unbalanced force); and a moving object will continue in its motion at a
constant velocity (unless acted upon by an unbalanced force).
b.
False - A football which is moving upwards and rightwards towards its peak, then it has
both an upward and a rightward velocity; it does not however have an upward and a
rightward force. In fact, if acting as a projectile, it has no horizontal force and maintains
a constant horizontal velocity; similarly, it would have a downward force of gravity and a
slowing down motion as it rises. If the football were not a projectile, then the horizontal
force would be leftward (air resistance opposing its motion) and the vertical force would
be gravity and air resistance, both directed downward.
c. False - An unbalanced force would accelerate an object. If directed against its motion,
then it would actually slow it down rather than keep is motion going. A balance of forces
is all that is required to keep an object going at a constant velocity. An unbalanced force
directed in the direction of motion would be required to keep an object going with an
increasing speed.
d. False - If an object is at rest, then there are no unbalanced forces acting upon it. There is
a force of gravity and at least one other upward force capable of balancing the force of
gravity.
e. False - This is dead wrong. It would take a balance of forces to keep an object in motion
at constant velocity. An unbalanced force would cause some form of acceleration.
f. False - If you answered TRUE, then Galileo and Newton just rolled over in their grave. It
is the natural tendency of all objects to maintain their velocity and to resist changes in
whatever state of motion that they have. This is the law of inertia.
g. False - All objects resist changes in their state of motion. In the absence of unbalanced
forces, they maintain their velocity (whether zero or nonzero). The pendulum changes
its state of motion due to an unbalanced force - the force of air resistance.
h. False - For an object to maintain a constant velocity, 0 Newtons of net force (i.e., a
balance of forces) is required.
Exam Review Answer Key
i.
True - Unbalanced forces cause stationary objects to accelerate from rest. In the
absence of an unbalanced force, a stationary object would remain at rest.
a.
True - This is a great definition of force.
8)
b. False - According to Newton's third law, one cannot push on an object without being
pushed back. The force on Billie is the result of an interaction of Bubba's hands with Billie's
body. That force on Billie might cause Billie to go flying, but the reaction force offers
resistance to the motion of Bubba's hands and slows them down. In general, forces will
always (without exception) come in pairs.
c. False - The force of the quarterback on the football is a contact force which can only exist
during the interaction (i.e., the contact) between the quarterback's hands and the football.
Once thrown, the football continues its horizontal motion due to its own inertia and its
vertical motion is effected by the force of gravity.
d. False - Be careful if you answered true to this one. If you did, perhaps you believe in the
fatal misconception that a rightward force is required to sustain a rightward motion. The
sleds motion to the right can be described as a leftward accelerated motion. Such a leftward
acceleration demands that there is a leftward force (despite its rightward force). This
leftward force slows the rightward-moving sled down. The hill cannot push on the sled
unless the hill is in contact with the sled.
e. False - Forces, if unbalanced, can cause objects to accelerate (one form of moving; the
other form is moving at a constant velocity). But by no means can one say that forces always
cause objects to move. For instance, as you sit in your chair, the chair pushes up on your
body but your body does not move.
f. True - Certainly! As you sit in your chair, the chair pushes up on your body but your body
does not accelerate. This upward force (known as the normal force) is balanced by the
downward force of gravity. Many objects experience a force yet do not accelerate.
g. True - There are two broad categories of forces - contact forces and field forces. Contact
forces, by definition, are those which result from the physical contact of two forces.
h. True (mostly) - A field force is a force which can acts between two objects even when
they are separated by a distance. Field forces have magnitudes which are dependent upon
the distance of separation between the two interacting objects. For instance, the force of
gravity between the Sun and the earth is a field force whose value depends upon the
distance of separation between the center of the Earth and the center of the Sun. In this
sense, the force of gravity is a force which acts when two objects are separated in space
from each other. Yet field forces can also occur when the two objects are touching each
other. In this sense, one can be skeptical of the wording of the statement.
Exam Review Answer Key
i. False - Spring and tension are examples of contact forces. The spring or the
rope/cable/wire are in contact with the object upon which it exerts its push or pull. The field
forces are electric force, magnetic force, and gravity force.
j. True - Forces always have a direction associated with them. As such, force is a vector
quantity - a quantity which is fully described by both a magnitude (size, value) and a
direction.
k. False - Force is measured in Newtons in the metric system and in pounds in the British
system. Kilograms is a unit of mass.
9)
a) None - If an object is on a surface, one can be guaranteed of at least two forces - gravity and
normal force.
b) BDFH - If the forces are balanced, then an object is moving with a constant velocity. This is
represented by a horizontal line on a velocity-time plot.
c) ACEG - If an object has a net force upon it, then it is accelerating. Acceleration is represented
by a sloped line on a velocity-time plot.
d) AE - If the net force is directed to the right, then the acceleration is to the right (in the +
direction). This is represented by a line with a + slope (i.e., upward slope).
e) CG - If the net force is directed to the left, then the acceleration is to the left (in the direction). This is represented by a line with a - slope (i.e., downward slope).
10)
Exam Review Answer Key