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Class PH1110A2007
Assignment 11
Due at 5:00pm on Monday, September 24, 2007
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Impulse on a Baseball
Learning Goal:
To understand the relationship between force, impulse, and momentum.
The effect of a net force
acting on an object is related both to the force and to the total time the force acts on the
object. The physical quantity
impulse
is a measure of both these effects. For a constant net force, the impulse is
given by
.
The impulse is a vector pointing in the same direction as the force vector. The units of
are
or
.
Recall that when a net force acts on an object, the object will accelerate, causing a change in its velocity. Hence the
object's momentum (
) will also change. The impulsemomentum theorem describes the effect that an impulse has
on an object's motion:
.
So the change in momentum of an object equals the net impulse, that is, the net force multiplied by the time over
which the force acts. A given change in momentum can result from a large force over a short time or a smaller force
over a longer time.
In Parts A, B, C consider the following situation. In a baseball game the batter swings and gets a good solid hit. His
swing applies a force of 12,000
to the ball for a time of
.
Part A
Assuming that this force is constant, what is the magnitude
of the impulse on the ball?
Enter your answer numerically in newton seconds.
ANSWER:
=
8.40
We often visualize the impulse by drawing a graph of force versus time. For a constant net force such as that used in
the previous part, the graph will look like the one shown in the figure.
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Part B
The net force versus time graph has a rectangular shape. Often in physics geometric properties of graphs have physical
meaning.
ANSWER:
For this graph, the
area
of the rectangle corresponds to the impulse.
The assumption of a constant net force is idealized to make the problem easier to solve. A real force, especially in a
case like the one presented in Parts A and B, where a large force is applied for a short time, is not likely to be
constant.
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 Fall '08
 Kiel
 Force, Mass, Momentum, net force

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