Chapter 18 Section 3
Collisions
Mass
Mass is the amount of matter in an object
The mass of an object affects how easy it
is to changes its motion.
Inertia
Inertia is the tendency of an object to
resist a change in its motion.
The amount of resistance to a change in
its motion increases as an object’s mass
increases.
Momentum
The momentum of an object is a measure
of how hard it is to stop the object.
Increasing the speed or velocity of an
object makes it harder to stop.
Momentum has direction that is the same
as the direction of the velocity.
Momentum Equation
Momentum( kg x m/s) = mass (kg) x velocity (m/s)
p=momentum m=mass
P
M
V
v=velocity
Solve the Equation 1
Calculate the momentum of a 14kg bicycle
traveling north at 2 m/s.
Answer 1
(14kg) (2 m/s north) = 28 kg·m/s north
Solve the Equation 2
A 10,000 kg train is traveling east at
15 m/s. Calculate the momentum of the
train.
Answer 2
10,000 kg x 15 m/s east = 150,000 kg·m/s east
Solve the Equation 3
What is the momentum of a car with a
mass of 900 kg traveling north at 27 m/s?
Answer 3
900 kg x 27 m/s north = 24,300 kg · m/s north
The Law of Conservation of
Momentum
In any collision, momentum is transferred
from one object to another.
The momentum lost by one object is equal
to the momentum gained by the other
object.
The law states that the total momentum of
a group of objects remains constant
unless outside forces act on the group.
Example of the Law
In a game of billiards (pool), when the cue
ball hits the other billiard balls, it slows
down because it transfers some of its
momentum to the other billiard balls.
What would happen to the speed of the
cue ball if all its momentum were
transferred to the other billiard balls?
An Example of an Outside Force
An outside force, like friction, can change
the total momentum of the group of
objects.
Types of Collisions
Bounce off of each other, like bowling ball
and pins (elastic)
Collide and stick to each other, like one
football player tackles another. (inelastic)
Example of Momentum
Conservation
Suppose two balls approach each other at
1 m/s from opposite directions. Their total
momentum is zero. After collision, they
both zoom off at 1 m/s in opposite
directions. What is their momentum? What
do you know about the mass of the two
balls?
Answer
If the total momentum is zero and the two
balls have the same speed in the opposite
direction, the balls must have the same
mass.
Predictions
Then law of momentum conservation can
be used to predict the velocity of objects
after they collide.
Suppose a 2 kg backpack initially has a
velocity 5 m/s east. Your mass is 48 kg,
and initially you’re at rest then the total
initial momentum is2 kg x 5 m/s + 48 kg x 0 m/s = 10 kg · m/s
east
P= mv
You can now use the equation for
momentum to find the final velocity.
10 kg · m/s/east = 2 kg + 48 kg x velocity
10 kg · m/s/east = 50 kg x velocity
0.2 m/s east = velocity
The ending velocity is smaller (0.2 m/s east)
is smaller than the initial velocity (5 m/s
east)
Visualize what will happen.
If small car is stopped at a red light is hit
from behind by a truck, what will happen?
Answer
Both vehicles will move forward with the
car moving faster than the truck.
Visualize what will happen.
What will happen if two marbles of the
same mass, traveling towards each other,
collide?
Answer
The marbles will collide and reverse
direction, moving away from each other at
the same speed as before the collision.
Conclusion
Momentum equals the mass of an object
times its velocity
Momentum is transferred from one object
to another in a collision.
The total amount of momentum of a group
does not change unless acted upon by an
outside force/s.