Lecture series for Conceptual Physics 8th Ed.

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Lecture series for Conceptual Physics 8th Ed.
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Momentum p 82
momentum
mass x velocity
p
m x v
Impulse p83
Together
impulse
force x time
I
F x t
Impulse change in momentum
F x t m x Dv
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Case 1 Increasing Momentum p85
F t
Increasing the forcehitting it harder.
and / or
F t
Increasing the time of contact.
The force on the golf ball builds up to a huge
amount then decreases.
Well use the average force.
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Case 2 Decreasing Momentum Over a Long Time
p85
Long means gentle, gradual, softly.
A large momentum
can be reduced by a small force
in a long time.
OR
A large momentum can by reduced by a large force
in a short time.
Not so gently.
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m Dv F t
Case 3 Decreasing Momentum Over a Short Time
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Bouncing p87
A flower pot falling on your head is bad enough.
But,
if the pot bounces, itll really hurt!
Then, theres the impulse in the other direction
to throw it back.
First, theres the impulse required to stop the
pot.
And
When something bounces, it hits twice as hard.
Figure 5-8 the Pelton water wheel.
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Conservation of Momentum p 88
The momentum before a collision
equals
the momentum after a collision.
AFTER
BEFORE
m (-v) m v 0
p m v m x 0 0
Momentum is conservedit is the same before and
after.
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Collisions p 90
pbefore pafter
These are elastic collisions
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Before
More on Collisions
Notice that these box cars stick together.
During
After
These are inelastic collisions.
pbefore pafter
(m x 10 m/s)before (2m x v)after
vafter 5 m/s
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More inelastic collisions p91
Fig 5-12 p91
m vA m vB k
Before
m v m (-v) 0
Before
After
m 0 m 0 0
After
v (m m) k
m v m (-v) m 0 m 0
m vA m vB v (m m)
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Whats the velocity of the fish after lunch?
m 1 kg
m 5kg
1 m/s
4 m/s
Mv m(-v) (Mm)v
(5kg)(1m/s) (1kg)(-4m/s) (5kg 1kg)v
v 1/6 0.17 m/s
Well do the more complicated collisions on the
board.
The End