Transcript Momentum

Chapter 6
Momentum and Collisions
Chapter Objectives
Define linear momentum
Compare the momentum of different objects
Describe impulse
Conservation of momentum
Contrast types of collisions
Linear Momentum
Momentum is a vector quantity defined as the product of
an object’s mass and velocity.
Up until this point, we have been able to define several
characteristics of motion: how far (displacement), how fast
(velocity), how quick (acceleration), how long did it take
(time), but nothing about how much?
That is momentum, it is a measure of the quantity of
motion itself.
p = mv
lowercase p means momentum
The units on momentum are
just like the formula tells you
kg m
Things to Think about Dealing with
Momentum is based on both mass and velocity.
That means that an object of larger mass does not always
have the greater momentum.
If an object is not moving, it has no momentum.
Momentum can be positive or negative based on the
direction of the velocity.
Momentum must also include a angle direction based on
the velocity.
According to Newton’s First Law, an object wants to stay
in motion unless acted upon by
an external force
And the longer we apply that net external force, the more
we will change the object’s motion.
Changing motion is changing momentum!
Impulse is the product of the net external force and the
time over which it acts on the object.
More simply put, impulse is equal to the change in
Impulse = FΔt = Δp = mvf - mvi
Things to Think about Dealing
with Impulse
Impulse has the same units as momentum.
Stopping distances depend on our formula for impulse as it
relates to change in momentum.
That is we must know the force that the brakes can apply
and how fast the vehicle is traveling to tell us a time it will
take to stop. Then we can find the distance using one of
our linear kinematic equations.
A change in momentum over a longer time requires less
Conservation of Momentum
Newton’s Third Law, for every action there is an equal and
opposite reaction, is responsible for the conservation of
The total momentum of all objects interacting with one
another remains constant regardless of the nature of the
forces between the two objects.
So momentum is conserved in all collisions.
Also, momentum is conserved for objects pushing away
from each other; such as people on roller skates,
explosions and splitting of atoms.
pi = pf
m1v1i + m2v2i = m1v1f + m2v2f
Types of Collision
An inelastic collision is when
two objects collide and continue
in the same direction after the
This would be like a bowling
ball striking a bowling pin.
A perfectly inelastic collision is
when two objects stick together
and move with a common
velocity after colliding.
This is best described as a car
You start with two objects that
stick together to form one
object after colliding.
An elastic collision is when
two objects collide and change
direction of both objects.
This is like when two billiard
balls collide.
Kinetic energy is conserved is
elastic collisions.
Solving Collision Problems
What Happens
m1v1i + m2v2i = m1v1f + m2v2f
Kinetic Energy
Objects become
deformed in some
way and move in
same direction.
m1v1i + m2v2i = m1v1f + m2v2f
Two objects stick
together to form
one object with
constant velocity.
m1v1i + m2v2i = (m1 + m2)vf
Two objects
bounce and move
separate in