195
Chapter 4
Newton’s Laws
Conceptual Problems
*1
••
Determine the Concept
A reference frame in which the law of inertia holds is called an
inertial reference frame.
If an object with no net force acting on it is at rest or is moving with a constant speed in a
straight line (i.e., with constant velocity) relative to the reference frame, then the
reference frame is an inertial reference frame. Consider sitting at rest in an accelerating
train or plane. The train or plane is not an inertial reference frame even though you are at
rest relative to it. In an inertial frame, a dropped ball lands at your feet. You are in a
noninertial frame when the driver of the car in which you are riding steps on the gas and
you are pushed back into your seat.
2
••
Determine the Concept
A reference frame in which the law of inertia holds is called an
inertial reference frame. A reference frame with acceleration
a
relative to the initial
frame, and with
any
velocity relative to the initial frame, is inertial.
3
•
Determine the Concept
No. If the net force acting on an object is zero, its acceleration is
zero. The only conclusion one can draw is that the
net
force acting on the object is zero.
*
4
•
Determine the Concept
An object accelerates
when a
net
force acts on it. The fact that
an object is accelerating tells us nothing about its velocity other than that it is always
changing.
Yes, the object must have an acceleration relative to the inertial frame of reference.
According to Newton’s 1
st
and 2
nd
laws, an object must accelerate, relative to any inertial
reference frame, in the direction of the net force. If there is
″
only a single nonzero force,
″
then this force is the net force.
Yes, the object’s velocity may be momentarily zero. During the period in which the force
is acting, the object may be momentarily at rest, but its velocity cannot remain zero
because it must continue to accelerate. Thus, its velocity is always changing.
5
•
Determine the Concept
No. Predicting the direction of the subsequent motion correctly
requires knowledge of the initial velocity as well as the acceleration.
While the
acceleration can be obtained from the net force through Newton’s 2
nd
law, the velocity
can only be obtained by integrating the acceleration.
6
•
Determine the Concept
An object in an inertial reference frame accelerates if there is a
net
force acting on it.
Because the object is moving at constant velocity, the net force
acting on it is zero.
correct.
is
)
(
c
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196
7
•
Determine the Concept
The mass of an object is an intrinsic property of the object
whereas the weight of an object
depends directly on the local gravitational field.
Therefore, the mass of the object would not change and
local
grav
mg
w
=
. Note that if the
gravitational field is zero then the gravitational force is also zero.
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 Spring '07
 Licini
 Force, Inertia

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