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Knowns:
Unknowns:
m = .150 kg
part a)
∆
p
v
i
= 20 m/s
part b) F
v
f
= 20 m/s
∆
t= .014 s
p
mv
momentum = mass times velocity
=
Solution:
F t
m v so F
=
 6.0
.014s
= 430N
m v
t
kgm
s
∆
∆
∆
∆
=
=
⋅
Note: When this equation is used to solve for force, the
force is rarely constant, so you are solving for the average
force.
Unit 9: Momentum, Impulse and Conservation of Momentum
Part 1: Impulse and momentum
In this chapter we will be looking at further applications of Newton’s 2nd and 3rd Laws.
First a physics definition
of the term momentum.
Early scientists thought that when one object collided with another, the damage it did
was directly proportional to how much mass it had and how fast it was moving.
It turns out they weren’t quite
correct, but we will study that in the next chapter.
Momentum
is defined as the product of an object’s mass and
its velocity.
The equation:
The unit of momentum is
kgm
s
⋅
.
Momentum is a vector quantity in that both its magnitude and direction are
important.
If an object changes its velocity, of course it also changes its momentum.
The equation for change in
momentum is:
Whenever one object exerts an unbalanced force on another, we know the second object will accelerate according
to the equation F = ma.
We can rewrite this as F = m
∆
∆
v
t
.
If both sides are multiplied by
∆
t, we end up with
F t
m v
∆
∆
=
.
This equation is called the impulsemomentum equation.
The left side of the equation is a quantity
we call the impulse.
Impulse
is defined as product of the force exerted on an object and the time interval over
which the force is applied.
The equation for impulse is:
The unit of impulse is the
N·s
.
The most important thing the impulsemomentum equation tells us is that the
impulse on an object and the object’s change in momentum are always equal
to each other.
Sample problems
:
1) A club is used to hit a 45 gram golf ball off a tee.
If the ball is moving at 60.0 m/s after being hit, how much
impulse did the club exert on the ball?
Since we do not know the force exerted on the ball, or the time the force was exerted, we must use the fact
that the impulse on the ball is equal to the change in momentum for the ball.
2) A 150 gram softball is thrown towards home plate at 20.0 m/s and is hit by the batter at 20.0 m/s in the
opposite direction.
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This note was uploaded on 10/15/2009 for the course SC 1515 taught by Professor Prof during the Spring '09 term at CyFair College.
 Spring '09
 Prof

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