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 (ρ) expresses the resistance of a body to slowing down or speeding up related to an object’s 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 Givens 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 (J) Definition: measures a change in momentum J = ∆p = pf - pi ∆p = m∆v = m(vf - vi)

Impulse (J) = ∆p = m∆v = F(∆t) To calculate multiply the average force (F) acting on a body by the length of time (∆t) the force acts. J = F(∆t) F = force (N or Newtons) ∆t = time (s) Unit is N s Impulse (J) = ∆p = m∆v = F(∆t)

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 Givens Unknown Equation Substitute Solve

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

One more… A ball changes velocity from 20 m/s to 30 m/s. If it has a mass of 5 kg, what impulse was necessary to cause this change? 50 N·s Givens Unknown Equation Substitute Solve

(Hint: use Impulse=F(t)) Your Turn How long would it take a 5 N force to change the velocity of the ball? (Hint: use Impulse=F(t)) 10 s Givens Unknown Equation Substitute Solve

Classwork Work momentum and impulse problems

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

There are 2 types of collisions Elastic: Objects bounce off each other Kinetic energy is conserved m1v1i + m2v2i = m1v1f + m2v2f Where m1 is mass of object 1, and m2 is mass of object 2 Where i stands for initial and f stands for final Use + and – velocities to show direction!

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 +1.4 m/s? Givens Unknown Equation & Substitute Solve

There are 2 types of collisions Inelastic: Objects stick together Kinetic energy is NOT conserved m1v1i + m2v2i = (m1 + m2)vf Where vf is the velocity of 2 objects stuck together after the collision Remember + 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? Givens Unknown Equation & Substitute Solve