Inertia and Momentum
Review:
What is inertia?
How do we measure inertia?
How does the inertia of a cement
mixer compare to a roller skate?
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What is Momentum?
Moving objects have a quantity
that objects at rest don’t have.
In the past, this was called
impedo.
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What is Momentum?
Objects at rest had no impedo.
A rock rolling down a hill had impedo.
More faster it moved, the more
impedo it had.
The change in impedo depended on
force and how long the force acted.
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What is Momentum?
Momentum = Inertia in motion
We calculate momentum as:
Momentum = mass x velocity
A moving object can have a large
momentum if either its mass or its
velocity are large.
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Defining Momentum Activity
1. What is the product of the mass
and the speed called?
Momentum
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Defining Momentum Activity
2. The greater an object’s
momentum, the more force is
needed to bring it to a stop –
true or false?
True
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Defining Momentum Activity
3. Which object had the greater
momentum, the ball bearing or the
marble? What is your evidence?
Which object had more mass?
Which object had more speed?
(distance/time)
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Defining Momentum Activity
4. Use the results of this experiment to
define momentum two ways:
1) With a mathematical expression
momentum = mass x speed
2)
a description of what momentum
does
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What is Momentum?
We decided that a cement truck will
always have more inertia than a
roller skate.
Will a cement truck will always have
more momentum than a roller
skate?
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CHECK YOUR NEIGHBOR
Which has more momentum?
A wagon at rest or a wagon rolling
down a hill?
A small car or a semi-truck moving at
60 mph?
A 1 kg rock moving at 10 m/s or a 10
kg rock moving at 1 m/s?
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Momentum in Action
Supertankers cut off their power when
they are 25 or so kilometers from
port. Because of their huge
momentum (due mostly to their
huge mass), about 25 kilometers of
water resistance are needed to bring
them to a halt.
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Momentum & Impulse
What gets the rock
moving down the hill?
A force is needed to get the rock
moving, but it is also important how
long the rock is pushed.
So, for changing an object’s
momentum, both force and the time
interval are important
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Momentum & Impulse
Impulse = Change in Momentum
To calculate impulse,
Impulse = Force x Time Interval
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CHECK YOURSELF
1.
Does a moving object have
impulse?
2.
Does a moving object have
momentum?
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CHECK YOURSELF
1.
Does a moving object have
impulse?
No – just like an object doesn’t
“have” a force, an object doesn’t
have an impulse. An impulse
requires an interaction with
another object.
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CHECK YOURSELF
2. Does a moving object have
momentum?
Yes!
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Impulse – Momentum Relationship
Newton’s 2nd Law
acceleration = Force/mass
We also said that
acceleration = Δ(velocity/time)
Force/mass = Δ(velocity/time)
multiply both sides by (mass x time)
(Force)(time) = Δ(mass)(velocity)
Impulse = ΔMomentum
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Increasing Momentum
To increase the momentum of an
object, you need to:
1) increase the mass or
2) increase the velocity or
3) increase the force or
4) increase the time interval or
5) or any combination!
(mass)(velocity) = (Force)(time)
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Increasing Momentum
To increase the momentum of an
object, you need to:
increasing the time interval where
the force is applied causes the canon
ball to farther
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Increasing Momentum
More examples:
Pulling a slingshot or arrow in a bow
all the way back
Driving a golf ball – follow through
with swing!
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Decreasing Momentum
If your car is going to crash, would
you rather crash into a concrete
wall or a pile of hay?
Why?
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Decreasing Momentum
What does having your hand go
backward when catching a ball do?
How about falling on a surface with
give (gym mats) vs. a rigid surface
(contrete)?
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Decreasing Momentum
To decrease the momentum of an
object, assuming you can’t decrease
the mass and velocity,
you will either:
1) decrease the time interval and
increase the force
or
2) decrease the force and increase
the time
(mass)(velocity) = (Force)(time)
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How are you feeling about this
concept?

Reflection
How well do you understand
Momentum?
Impulse?
What can you do to improve your
understanding?
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