Car Crash Video http://www.iihs.org/iihs/videos

Momentum, Impulse, and Collisions https://www.youtube.com/watch?v=2UHS883_P60&index=1&list=PLZHDrmpjw4My5KJXCT2inbz7pOv1lqsRk https://www.youtube.com/watch?v=2htckwDkugI&list=PLZHDrmpjw4My5KJXCT2inbz7pOv1lqsRk&index=4&t=0s

Momentum (p) expresses the resistance of a body to slowing down. related to inertia p = mv m = mass (kg) v = velocity (m/s) Unit is kg m/s

Example A toy car has a mass of 5 kg. If it is moving at a velocity of 4 m/s, what is its momentum? 20 kg•m/s Given Unknown Equation Substitute Solve  

Your Turn What is the momentum of a 25 kg boy moving at a speed of 1.2 m/s? 30 kg•m/s

Impulse = ∆ρ = m∆v = F(∆t) Impulse (I) measures a change in momentum To calculate multiply the average force acting on a body by the length of time the force acts. Impulse = ∆ρ = m∆v = F(∆t) F = force (N or Newtons) ∆t = time (s) Unit is N s See SLO equation sheet.

Putting it all together… Newton’s 2nd Law F = ma a = Δv/t F=mΔv / t F(t) = m(vf-vi) Notice impulse, on the left side, is equal to the change in momentum of an object.

Impulse

Example An 8 N force acts upon an object for 5 seconds. What impulse is given this object? 40 N•s Given Unknown Equation Substitute Solve  

(Hint: use Impulse=F(t)) Your Turn A ball changes velocity from 20m/s to 30m/s. If it has a mass of 5kg, what impulse was necessary to cause this change? 50N·s How long would it take a 5N force to change the velocity of the ball? (Hint: use Impulse=F(t)) 10s

The Law of Conservation of Momentum In an isolated system, the total momentum does not change. Momentum is conserved in all closed systems!

There are 2 types of collisions Elastic m1v1i + m2v2i = m1v1f + m2v2f Where 1 stands for object 1, 2 stands for object 2 Where i stands for initial & f stands for final Use + and – velocities to show direction.

2kg(2m/s)+3kg(-1m/s) = 2kg(v1f)+3kg(1.33m/s) Example… A 2 kg ball strikes another ball head on at an initial velocity of +2 m/s. If the second ball had a mass of 3 kg and was initially moving at -1 m/s, what is the final velocity of the 2 kg ball if the vf of the 3 kg ball is now +1.33m/s? Define variables. Then: m1v1i + m2v2i = m1v1f + m2v2f 2kg(2m/s)+3kg(-1m/s) = 2kg(v1f)+3kg(1.33m/s) v1f = -1.5m/s

There are 2 types of collisions Inelastic m1v1i + m2v2i = (m1 + m2)v1&2f Where v1&2f is the velocity of the two objects stuck together after the collision Use + and – velocities to show direction.

Example… A 500 kg car is driving at a velocity of 10 m/s. Another car (700 kg) hits it from behind at a velocity of 25 m/s and the two cars interlock bumpers. What is the resulting velocity of the two cars? Define variables. Then: m1v1i + m2v2i = (m1 + m2)v1&2f 500kg(10m/s)+700kg(25m/s) = (500kg+700kg)(v1&2f) v1&2f = 18.75 m/s