Chapter 6 - Momentum and Collisions

Section 6-1

Linear momentum

· the product of an object’s mass and velocity

· a vector quantity

p = mv unit: kg m/s

Impulse

· a force applied over a time interval

· changes the momentum of an object

· equivalent to momentum

J = F Dt unit: N s = kg m/s

same as momentum

Impulse-Momentum Theorem

Dp = m Dv = FDt

Section 6-2

Internal force – a force that one part of a system exerts on another part

External force – a force exerted on a part of the system by some agency outside the system

Isolated system – a system with no external forces

Conservation of momentum

The total momentum of an isolated system is constant, or conserved.

*usually disregard friction in momentum problems

*consider forces to be constant over time (real forces vary over time and integration is required)

Section 6-3 Collisions

Three types of collisions

1. Perfectly inelastic collision

· two objects stick together and move with a common velocity after colliding

· Momentum is conserved

· KE is NOT conserved

· Example: two clay balls collide

· Example: an arrow hits a target

2. Inelastic Collision

· two or more objects that collide and are deformed during the collision

· the objects do not stick together

· momentum is conserved

· KE is Not conserved (but ME is)

· Example: tennis ball and racquet

3. Elastic Collisions

· two objects collide and move off separately with no deformation

· Momentum is conserved

· KE is conserved

· Example: two billiard balls colliding

· Example: two marbles colliding

*Our problems will deal with either perfectly inelastic or elastic in this text.