Transcript Chap. 7 Momentum - Coal City Unit District #1

Unit 4: Momentum and
Energy
Chap. 7
Momentum
Which is harder to stop, a
truck traveling at 55 mi/hr or a
small car traveling at 55
mi/hr?
 Why?
Which is harder to stop, a
slow moving large truck or a
very fast moving small car?
 Why?
What is MOMENTUM?
A. Momentum: inertia in motion
B. Momentum = mass x velocity (or speed)
p = m.v
C. Momentum is greater if it has
1. high speed
2. large mass
3. or both
 What could you hypothesize if the mass
of an object remained the same (same
object), but the momentum changed?
 Acceleration occurs (velocity increases)
 What causes accelerations?
 Force
 How do you get a large acceleration?
 large force
or
 apply a smaller force for a longer period
of time
Impulse
 Impulse: force over a period of time
 Impulse = force x time
I = F. t
 The higher the impulse, the greater the
change in momentum
Impulse
 The higher the impulse, the greater the
change in momentum.
I = Dp
F . t = D(m . v)
Would you rather hit a brick wall
or a haystack?
 Why
 It has to do with impulse.
 The impulse would be the same BUT the
force and the time that make up the impulse
are not the same.
 You would be in contact with the haystack
longer than you would be in contact with the
brick wall.
brick wall
t
.
F =
same impulse
I
=
haystack
t
. F
Therefore, the force of the brick wall
would be greater than the force of the
haystack.
If you jump off a lab table, should you hit
the ground with your legs straight or
should you bend your legs?
Why?
Which is better, to drop a ceramic dish on
the sidewalk, or on thick carpet?
Why?
Which would be safer, a padded
dashboard or a metal dashboard?
Why?
HOW IS BOUNCING
DIFFERENT?
 Impulse is greater when something
bounces.
 There are really 2 steps when bouncing:
 1. impulse to bring momentum to a stop
 2. provide additional impulse to bring it back
up
HOW IS BOUNCING
DIFFERENT?
 The impact force will still react the same.
 1. If the impulse is over a long time, then the
impact force is small.
 Example - a circus net
 2. If the impulse is over a short time, then the
impact force is large.
 Example - a body dumped from a 10 story window
What happens to momentum
during collisions?
 Conservation of Momentum
 Total momentum before equals total
momentum after (when there are NO outside
forces acting).
 In the absence of an external force, the
momentum of a system remains unchanged.
How do we know?
 Newton’s 3rd Law: for every action, there
is an equal and opposite reaction.
Types of Collisions
 ELASTIC
 1. Objects hit but don’t stick together
 2. Transfers momentum
 INELASTIC
 1. Objects hit and stick together
 2. Shares momentum