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Discussion Questions
A. When a car accelerates, why does a bob hanging from the rearview mirror
swing toward the back of the car. Is it because a force throws it backward. If
so, what force. Similarly, describe what happens in the other cases described
above.
B. The following is a question commonly asked by students:
“Why does the force vector always have to point in the same direction as
the acceleration vector. What if you suddenly decide to change your force
on an object, so that your force is no longer pointing the same direction that
the object is accelerating.”
What misunderstanding is demonstrated by this question. Suppose, for
example, a spacecraft is blasting its rear main engines while moving forward,
then suddenly begins firing its sideways maneuvering rocket as well. What
does the student think Newton’s laws are predicting.
velocityaccelerationforce
In this example, the rappeller’s velocity has long periods of gradual
change interspersed with short periods of rapid change. These
correspond to periods of small acceleration and force and periods
of large acceleration and force.
Section 8.2The Acceleration Vector