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Chapter 7
Momentum and Impulse
Chapter 7
IMPULSE AND MOMENTUM
PREVIEW
The
momentum
of an object is the product of its mass and velocity. If you want to change
the momentum of an object, you must apply an
impulse
, which is the product of force and
the time during which the force acts. If there are no external forces acting on a system of
objects, the momentum is said to be conserved, that is, the total momentum of the system
before some event (like a collision) is equal to the total momentum after that event. In
this chapter, we will discuss examples of both one and twodimensional collisions.
The content contained in all sections of chapter 7 of the textbook is included on the AP
Physics B exam.
QUICK REFERENCE
Important Terms
impulse
The product of the average force acting on an object and the time during which it
acts. Impulse is a vector quantity, and can also be calculated by finding the area
under a force versus time curve.
linear momentum
The product of the mass of an object and its velocity. Momentum is a vector
quantity, and thus the total linear momentum of a system of objects is the vector
sum of the individual momenta of the objects in the system.
internal forces
The forces which act between the objects of a system
external forces
The forces which act on the objects of a system from outside the system, that is by
an agent which is not a part of the system of objects which are being studied.
inelastic collision
A collision between two or more objects in which momentum is conserved but
kinetic energy is not conserved, such as two railroad cars which collide and lock
together.
elastic collision
A collision between two or more objects in which both momentum and kinetic
energy are conserved, such as in the collision between two steel balls.
center of mass
The point at which the total mass of a system of masses can be considered to be
concentrated.
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Momentum and Impulse
Equations and Symbols
p
=
m
v
J
=
F
∆
t =
∆
p =
0
v
v
m
m
f

2
1
2
2
1
1
m
m
x
m
x
m
x
cm
+
+
=
where
p
= momentum
m
= mass
v
= velocity
J
= impulse
F
= force
∆
t
= time interval during which a force
acts
x
cm
= position of the center of mass of a
system of particles
x
1
= position of a mass relative to a
chosen origin
Ten Homework Problems
Chapter 7 Problems 5, 9, 17, 22, 28, 29, 32, 39, 41, 55
DISCUSSION OF SELECTED SECTIONS
7.1 The Impulse – Momentum Theorem
The momentum
p
of an object is the product of the mass
m
of the object and its velocity
v
:
p
=
m
v
The momentum of a moving mass is a vector which has a direction that is the same as
the velocity of the mass. Thus, the momentum of an object can be broken down into its
components:
p = p
x
+ p
y
where
p
x
= pcos
θ
and
p
y
= psin
.
=
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 Summer '09
 Turner
 Force, Impulse And Momentum, Mass, Momentum

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