Collision Lab and conservation of momentum

This “conservation” law is one of the biggest ideas in physics and motion

Collision Lab

When colliding cars together and making them stop….the less massive car had to move at higher velocity (proportionally)

Collision Lab

When the cars “pushed off” each other, the same idea seemed to work, the less massive car would have a higher velocity.

Collision Lab

It seems that we are balancing the cars mass and velocity, if one goes up the other goes down.

The product of mass and velocity is called momentum

P = momentum = m * v

Conservation of Momentum

The Big Idea is that when two objects collide or push off each other that the total momentum is “conserved” (none is lost).

The total momentum before they collide (or push off) must = the total momentum after they collide ( or push off).

We say that momentum is not lost during a collision between 2 objects.

Collision Lab and conservation of momentum Numerically you write….. Momentum before = momentum after (m1v1)before + (m2v2)before = (m1v1)after + (m2v2)after

It is very helpful to draw a picture of “before” and “after” so you understand what’s happening.

Conservation of momentum

Example: A train car (m = 5000kg) rolling to the right at 20 m/s collides with a “Big Foot” (sasquatch) (m = 1000kg) running to the left at 10m/s. They stick together. How fast are they moving? (momentum has direction so one velocity will be positive and one will be negative)

(mt*vt)b + (mbf*vbf)b = (mt*vt)a + (mbf*vbf)a = (mt + mbf) vfinal

5000kg*20m/s + 1000kg* (-10m/s) = (5000+1000) vfinal

100000 – 10000 = 6000 vfinal Vfinal = 15 m/s

Conservation of momentum

Example: you (mass = 50 kg) are standing on frictionless ice at rest. You throw a cat (m = 10 kg) at 7 m/s to the right. How fast and in what direction do you go?

(my*vy)b + (mc*vc)b = (my*vt)a + (mc*vc)y

0 + 0 = 50kg * v + 10kg* 7 m/s V = - 70/50 = - 1.4 m/s (minus sign means you move in

opposite direction)