Download Momentum

Physics
Chapter 9
Momentum and its
Conservation
Momentum and its
Conservation
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Let’s check our knowledge (1-12)
Define momentum.
Momentum
 The quantity of motion
 The property of an object in motion
What two things must
an object have to have
momentum?
Mass (kg)
 Velocity (m/s)

Explain how inertia is
different than
momentum.
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Inertia is a property of any object with mass
It is the tendency of an object to resist a
change in its velocity
Momentum is a property of an object in
motion
A stationary object has no (O) momentum
An object must be moving to have
momentum but not inertia
How do you calculate
momentum?
Multiple mass by velocity
 Momentum = mv

What is the symbol and
unit for momentum?
p (actually rho-)
 kgm/s
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Define impulse.

Force applied to an object over a time
interval to change its momentum
What two things must
an object have to have
impulse?
Force
 Time interval

Explain how impulse is
different than
momentum.
Momentum is the property of an object
in motion.
 Impulse is force applied for a certain
time that causes an object’s
momentum to change.

How do you calculate
impulse?
Multiply force applied by time of
application
 Impulse = Ft
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What is the unit for
impulse?
N•s
 There is no symbol for impulse!
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Are momentum and
impulse vector
quantities?
Yes
 They both have magnitude (size) and
direction

Use Newton’s Second
law of Motion and the
definition of
acceleration to derive
the relationship
between impulse and
change in momentum.
F = ma and a = v/t
 F = mv/t
 Ft = mv (fat mav)

Write down the
Impulse- Momentum
Theorem. Explain it in
words.
Ft = p2 – p1
 FΔt = mΔv
 Impulse (Ft) on an object causes its
initial momentum (p1) to change to its
final momentum (p2).

If a tennis ball is hit by
a racket for 0.15 s with
a force of 23 N find its:
Beginning momentum
 Impulse given it by the racket
 Change in momentum
 Final momentum

If a tennis ball is hit by
a racket for 0.15 s with
a force of 23 N find its:
Beginning momentum: 0 kg*m/s
 Impulse given it by the racket: 3.45 N*s
 Change in momentum: 3.45 kg*m/s
 Final momentum: 3.45 kg*m/s

Explain using the terms
momentum and
impulse why
automobile airbags
save lives.
When a person stops there is the
same change in momentum with or
without the airbag.
 The airbag increases the time for the
force so it is less (Ft = mv)

A 725 kg compact car
is moving at 100 km/h
towards the east. Find
its momentum (kg*m/s)
100 km/h = 27.8 m/s (?)
 p = mv = (725)(27.8) = 20155 kg*m/s

If the driver of the 725
kg car traveling at 27.8
m/s, applies the brakes
for 2 s and the brakes
apply an average force
of 5000N. Find:
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The impulse given to the car
The car’s change in momentum
The car’s final momentum
The car’s final velocity
If the driver of the 725
kg car traveling at 27.8
m/s, applies the brakes
for 2 s and the brakes
apply an average force
of 5000N. Find:
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The impulse given to the car: - 10,000 N*s
The car’s change in momentum: - 10,000 kg*m/s
The car’s final momentum: 10,155 kg*m/s
The car’s final velocity: 14 m/s
State Newton’s Third
Law of Motion

For every action there is an equal and
opposite reaction
State the Law of
Conservation of
Momentum
The momentum of any closed, isolated
system doesn't change
 In a closed, isolated system, the initial
momentum is equal to the final
momentum

What is a closed
system?

A system where mass is not lost or
gained.
What is the difference
between internal
forces and external
forces on a system?
Internal forces are those forces that
act within the system.
 External force are forces acting on the
system from the outside (the
surroundings)
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What is an isolated
system?
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When no external forces act upon the
system.
How is an isolated
system different from a
closed system?
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A closed system can have an outside
force acting upon it but an isolated
system cannot.
For the Law of
Conservation of
Momentum to be true,
must a system be closed
or isolated or both.
Explain.
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An isolated system is also closed, so it must
be isolated (and thus also closed).
If only closed momentum can still be
transferred to it by way of an outside force
If there is no air in outer
space for a rocket to push
off of, explain how when a
rocket fires its engines in
space it can move.
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The escaping gas pushes off the rocket and
the rocket pushes off the gas with the same
force.
This causes and equal and opposite change
in momentum for both
So both move but in opposite directions (and
speeds)