After completing this section
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:
The motion of
inertia
Variable:
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, Imp or just the units
impulse = force x time
= 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
Impulse = 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
Thought Question
 You
step off of a
skateboard.
What
What
happens to you?
happens to the
skateboard?
Conservation of
Momentum
Momentum
is a vector
quantity
It
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?
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.
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?
Summary

A moving object has momentum (p) described as
p=m.v kg.m.s-1

Impulse is described by the change in momentum.
Δp kg.m.s-1 Using the change in velocity (v-u)

The change of momentum is not instant and the
time taken lets us calculate the average force
using Newtons second law of motion F=ma where
a=Δv/ Δt m.s-2

So Δp=m(v-u) kg.m.s-1 Impulse
and FΔt= Δp N.s-1 Average
force