Momentum

Momentum is a vector quantity since velocity is a vector.

ImpulseDefined as a force exerted during a time period

Impulse is the cause of a change in momentum.

Elastic Cord vs a Steel Wire for same block on frictionless surface

Compare the impulse provided by both cord & wire from the time they reach their relaxed length to when the blocks stop.

Elastic

Steel

Jumping from table, mats, packaging

Both the elastic cord & steel wire are equal in length relaxed.

The blocks are given a quick, equal push, sending them sliding towards the right.

Example

During an automobile crash test, a 1000kg car is sent towards a cement wall at a speed of 14m/s and is brought to a stop in 0.08 seconds.  Determine force car exerted on the wall.

Example2

A ball of mass 0.25kg is dropped from a height of 0.75m. The ball bounces back upwards from the floor with a speed of 1.5m/s.

a) Determine the speed the ball strikes the floor prior to bouncing back upwards.

b) Determine the impulse the ball receives from the floor.

Bus hits bug!Who receives the greater impulse, bug or bus?

Who receives the greater change in momentum, bug or bus?

Who undergoes the greater acceleration, bug or bus?

a) Find the magnitude of the impulse delivered to the object. 

b) Determine the change in the object's momentum. 

c) If the object's original velocity was -10m/s, calculate its final velocity. 

d) What was the magnitude of the "average force" acting on the object during the time graphed? 

Force-time graph

A varying, one-dimensional force acts on a 50kg mass as shown in the graph.

Stick vs Bounce + demo

Law of Conservation of Momentum

Total momentum before collision equals total momentum after collision assuming no external forces acting during collision.

3 types of collisions3 types of collisions

• ELASTIC

• INELASTIC

• Perfectly Inelastic

‘p’ conservation equations

KE for collisions

Example• Running at 2.0m/s, a 45-kg quarterback, collides with

a 90-kg tackle, who is traveling towards him at 7.0m/s. Upon colliding, the tackle continues to travel forward towards the quaterback at 1.0 m/s.  Find velocity of quarterback. 

ExampleDuring target practice, a man shoots a 7-gram bullet with a horizontal velocity of 150 m/s directly at a 2-kg pumpkin sitting on a post. If the bullet embeds in the pumpkin, how fast will the pumpkin be knocked off the post? 

Example An 88kg fullback moving east with a speed

of 5.0m/s is tackled by a 97kg opponent running west at 3.0m/s. Assume the collision is inelastic.

a) Find the velocity of the two immediately after the collision.

b) Find the % of kinetic energy lost during the collision.

Example • Jack and Leon are fishing in their boat when they

decide to jump into the water. Jack, 45-kg, jumps off the front of the boat with a speed of 2m/s. While at the exact same moment, Leon, 90-kg, jumps out of the back of the boat at a speed of 4m/s. 

If the boat has a mass of 100 kg and was at rest prior to the two boys jumping off, what will be its velocity just after both boys have abandoned ship? 

Jumping upwards

What happens to the earth, technically speaking, when you jump upwards with glee cuz you can’t wait for physics class? Why?

The recoil momentum of a gun that kicks is ____ the momentum of the bullet it fires.