- Ch. 9 p.p. 228-249
Momentum = p = (mass)(velocity)= mv [units: kg m/s]
Impulse = (Force) ( time interval) = F Δ t [units: N s]
Impulse = Change in Momentum F Δ t = m Δ v
 If you know the mass and velocity of an object you can calculate its momentum.
 An object’s momentum will change when due to an impulse, an external force acting on an object for some period of time.
 Like acceleration, force and velocity, momentum and impulse have direction as well as magnitude (amount or numerical value). The momentum of an object is in the same direction the object is moving. The impulse is in the direction of the force on the object.
 You can calculate the total momentum of a group of objects by adding up the momentum of each individual object in the group. The group of objects together is called the “system.” You have to take into account which way the objects are moving when finding the total momentum of the system.
 The total momentum of a system stays the same as long as no net external force acts on the system no mass is added or subtracted from the system. This is the Law of Conservation of Momentum.
 A neat thought: If you draw a circle big enough to enclose every object in the Universe, then you would have a system with no unbalanced forces
(there’s nothing outside of the circle to supply a force). This means that the total momentum of the whole Universe must stay the same--momentum just changes hands from object to object.
Collision Problem Solving Strategies
 Always start by drawing “Before” and “After” pictures of the situation described in the problem.
 Show all known masses and velocities (speed and direction of motion) beside each object in your diagrams.
 If no net force acts on the system, then the sum of all the momentum in the system is conserved.
Momentum Before
=
Momentum After m
1 v
1
+ m
2 v
2
=
m
1 v
1
’ + m
2 v
2
’