6/30/13
Concept of Force and
Newton s Laws of Motion
Concept Questions with Answers
8.01
W02D2
Concept Question: Car-Earth
Interaction
Consider a car at rest. We can conclude that
the downward gravitational pull of Earth on the
car and the upward contact force of Earth on it
are equal and opposite because
1.
2.
3.
4.
the two forces form a third law interaction pair.
the net force on the car is zero.
neither of the above.
unsure
8.01
W02D2
Chapter 7 Newton’s Laws of Motion,
Sections 7.1-7.4
Chapter 8 Applications of Newton’s Second
Law,
Sections 8.1-8.4.1
Concept Question: Car-Earth
Interaction Answer
Answer 2. These two forces cannot be an
interaction pair because they act on the same
object. Because the car is at rest, however, its
momentum is constant (and zero). Because net
force equals the time rate of change in
momentum, the net force on the car must be
zero. This means that the two forces must be
equal and opposite.
1
6/30/13
Concept Question: Normal Force
Consider a person standing in an elevator that
is accelerating upward. The upward normal
force N exerted by the elevator floor on the
person is
1.  larger than
2.  identical to
3.  smaller than
Concept Question: Normal Force
Answer
Answer 1. The normal force on the person is greater than
the gravitational force on the person because the normal
force must also accelerate the person, as well as oppose
the gravitational force. Thus N – mg = ma implies that
N > mg. So the magnitude of upward normal force is larger
than the magnitude of the downward gravitational force.
the downward force of gravity on the person.
Concept Question: Tension
Concept Question: Tension Answer
A cart is placed on a nearly frictionless
surface. A force sensor on the cart is attached
via a string to a hanging weight. The cart is
initially held. When the cart is released and in
motion does the tension in the string
1. increase?
2. stay the same?
3. decrease?
Answer 3. When the cart is at rest, the tension in the
rope is equal in magnitude to the gravitational force on
the hanging object, mg = T. When the cart is released
and the object accelerates downward, mg –T = ma. So
T=mg –ma. Thus the tension is less than the
magnitude to the gravitational force hence it has
decreased.
4. cannot determine. Need more
information.
2