Transcript Momentum and Impulse Notes

Bell Ringer
After reading the article
Does slamming on the
brakes save your brake
pads?
Do you believe this saves
gas?
Thought Question
Would
you feel safer
in a Smart Car or
another small car?
Momentum and Impulse
Momentum
 Momentum:
inertia
 Variable:
The motion of
p
momentum = mass x velocity
p = mv
 Units:
kg.m/s
Momentum
 Objects
which aren’t moving
have no velocity, and
therefore have no momentum
Bell Ringer
What
is the
advantage in having
a padded dashboard?
Bell Ringer
 Two
objects collide. If one
has a heavier mass, what
can we say about the
forces experienced relative
to the smaller mass?
Impulse
 Impulse:
The change in
momentum of an object due
to a force that is applied
during a period of time
 Variable:
J
impulse = force x time
J = Ft
 Units:
N.s
Impact


Impact- The time duration in which a
change in momentum is occurring.
Units- seconds
Impulse-Momentum
Theorem
 Impulse
is equal to a change
in momentum
J = Dp
OR
Ft = D(mv)
Bell Ringer

Using the impulse momentum theorem
answer the following question

If you apply a force but extend the time,
what will happen to the object’s change in
momentum>
Bell Ringer

A racket ball and a ball of clay of the
same mass are thrown at an open door.
Which one will be more effective in
closing the door?
Bouncing

Impulses are greater when the
object bounces
 It
takes additional impulse to stop an
object and send it back in the
opposite direction than it does to
simply stop the object
 This additional impulse is supplied by
the surface on which the object
bounces
Forces of Impact
•Forces of impact are reduced
when time of impact increases
Forces of Impact
 Forces
of impact are also
reduced when:
 the
velocity of impact is reduced
 the mass of the object is
reduced
Impulse Demos
Thought Question

You step off of a skateboard.


What happens to you?
What happens to the skateboard?
Conservation of
Momentum
 Momentum
 It
is a vector quantity
has magnitude and direction
 Momentum vectors, like forces,
can cancel each other out
Conservation of
Momentum
Law of Conservation
of Momentum
In the absence of an
external force, the
momentum of a system
does not change.
Collisions
 In
any collision the net
momentum before the
collision equals the net
momentum after the
collision
 There are two main types
of collisions: elastic and
inelastic
Elastic Collisions
 When
objects collide without
being permanently deformed
and without generating heat
 Perfectly
elastic collisions
almost never occur
 Heat is usually generated with
collisions, so energy is
transformed out of the system
Elastic Collisions
Inelastic Collisions
 When
colliding objects
stick together and travel
off as one object
 For
two objects in an
inelastic collision:
momentum1 + momentum2 = combined momentum1&2
m1v1 + m2v2 = (m1 + m2)vf
Inelastic Collisions
Bell Ringer 10/18
One pool ball traveling with a velocity of 5
m/s hits another ball of the same mass,
which is stationary. The collision is head
on, as momentum is conserved, and they
bounce off each other.
What type of collision is this?
What are the final velocities of both
bodies?
Thought Question

When is momentum not conserved?
Rain vs. Hail



Which has a greater change in
momentum?
Which has a greater impulse?
Which causes more damage?
Topic Review
 1.)
Why are padded
dashboards used in cars?
 2.)
Bell Ringer 3/28

A 0.150-kg baseball moving at a speed of
45.0 m/s crosses the plate and strikes
the 0.250-kg catcher's mitt (originally at
rest). The catcher's mitt immediately
recoils backwards (at the same speed as
the ball) before the catcher applies an
external force to stop its momentum.
Determine the post-collision velocity of
the mitt and ball.
Bad Momentum Videos
Bell Ringer

Two meatballs are speeding directly
toward each other. One is a 4.0-kg
meatball moving with a speed of 6.0
m/s, and the other has a mass of 2.0
kg and a speed of 4 m/s. If they
collide inelastically, what will be the
speed of the resulting 6.0 kg meatball
immediately after the collision?
Review Question

A bullet with a mass of 0.020 kg collides
inelastically with a wooden block of mass
2.5 kg, initially at rest. After the collision,
the bullet + block has a speed of 1.2
m/s. What was the initial speed of the
bullet?