Momentum

We previously talked about conservation of energy - an idea that is a very powerful way of understanding the world. However, actually auditing every form of energy and trying to find out how much goes to what form can be very difficult. Sometimes it helps to apply another, similar rule: Conservation of momentum.

Momentum is a thing’s mass x velocity

Like energy, momentum remains constant unless acted on my some extended outside force. Let’s look at a famously bad example from Lethal Weapon. When Riggs, the protagonist, starts to get too close to the truth, one of the villains drives by and shoots him. In the movie, the blast propels Riggs off his feet, into the air and through a window. Conservation of momentum gives us a simple way to see how plausible this is. Let’s compare things right before and after impact. Before, you have Riggs standing still and the cluster of shotgun pellets flying toward him. After, you have Riggs with the cluster of pellets embedded in his bullet-proof vest moving at some speed we’d like to figure out. 

Before:
Riggs’ momentum = (150 lbs) x (0 mph) = 0 lb-mph
Shot’s momentum = (0.05 lbs) x (820 mph) = 41 lb-mph

After:
41 lb-mph = (150.05 lbs) x V
V = 0.3 mph

That’s pretty close to a giant tortoise pace; ten times slower than average human walking pace. He may stagger, but he definitely will not go flying through the air.