Honors Physics
Ch. 7 Linear Momentum
Sections 7.2, 7.5, & 7.6
Name __________________________________ Date ____________________ Period ______
Conservation of Momentum – The total momentum of any closed, isolated system does not
change. This law allows you to make connections between objects before and after they collide.
po = p
m1v1o + m2v2o = m1v1 + m2v2
Isolated system – A closed system that is free from the influence of a net external force that can
change the momentum of the system; a closed system does not gain or lose mass (objects neither
enter nor leave it).
Types of collisions –
(1) Inelastic collision – Momentum is conserved but not kinetic energy. The kinetic energy,
which is lost, is turned into heat, sound, etc.
Use the conservation equation to solve these collision problems…
m1v1o + m2v2o = m1v1 + m2v2
(2) Elastic collision – Both momentum and kinetic energy are conserved; takes place at the
sub-atomic level. The objects have a “head-on” collision and bounce off each other.
Use the following equations to solve these collision problems…
 m m 
 2m

 2m
 m m 
2
v1 =  1 2  v1o + 
 v2o
 m1  m2 
 m1  m2 

1
2
1
v2 = 
 v1o + 
 v2o
 m1  m2 
 m1  m2 
(3) Perfectly (Completely) Inelastic collision – Two objects collide and stick together so
that their final velocities are the same; only momentum is conserved.
Use the following equation to solve these collision problems…
m1v1o + m2v2o = (m1+ m2) v
(4) Explosion – The two objects are together and then separate; recoil. It is like a backwards
perfectly (completely) inelastic collision. The collision starts from rest (vo = 0).
Use the following equation to solve these types of collision problems…
vo(m1+ m2) = m1v1 + m2v2