Momentum Review A Review

A Tale of Momentum and Inertia

Conservation of momentum in 2D Oblique Impacts

In some collisions, the objects move off at an angle to each other. For example head on collisions between snooker balls.

Example A 1500 kg car travelling East with a speed of 25 ms -1 collides at a junction with a 2500 kg van travelling North at 20 ms -1. Find the direction and magnitude of the velocity of the wreckage after the collision, assuming the vehicles undergo a perfectly inelastic collision, that is they stick together.

Step 1 Draw a diagram Car 1500kg 25 m/s Van 2500 kg 20 m/s VfVf

Step 2 Before the collision the total momentum is the x direction is p 12x This is just the momentum of the car: 1500 kg x 25 m s-1 = kg m s-1 (E) In the y direction, we similarly we have P 12y= kg m s-1 (N ) After the collision, since momentum is conserved along each plane, the vehicles have a resultant momentum that is the vector sum of these momenta.

Step 3 Use Pythagoras to find the resultant p 12f 

Step 4 Now find the angle using tan . Remember velocity is a vector. Answer 16 m s-1 53⁰ North of East