Conservation of Momentum

Any object in motion has momentum. Momentum is defined as mass x velocity according to Momentum from Wikipedia the free encyclopedia. If the mass of the object is increased, then the momentum is increased. If the velocity of the object is increased, then the momentum is increased. A walking person has more momentum than a stopped truck. The stopped trucked has no momentum, because its velocity is 0.

Momentum is always conserved. The momentum before an impact is always equal to the momentum after an impact. For instance, if a 200 kilogram person runs at 2.5 meters/second into 100 kilogram person who is not moving, then the total momentum before the impact is simply the momentum of the 200 kilogram person + the momentum of the 100 kilogram person. The momentum of the 200 kilogram person is his mass x his velocity, which is 200 kilograms x 2.5 meters/second = 500 kilograms meters/second. The momentum of the 100 kilogram person is 100 kilograms x 0 meters/second = 0. The 100 kilogram person has no momentum, since he is not moving. So, the total momentum before impact is 500 kilograms meters/second.

Now let's say that after the 200 kilogram person strikes the 100 kilogram person they fall forward together. What is their forward speed? Well, since we know the momentum before the impact, we also know the momentum after impact. According to the conservation of momentum principle, the momentum before an impact is equal to the momentum after an impact. So, the total momentum after the impact is 500 kilograms meters/second. This is the same value as the momentum before impact. Again, this must be the case, because momentum is always conserved. Momentum after the impact = mass of both people (remember they are falling together) x velocity. So, 500 kilograms meters/second= mass of both people x velocity. Plugging in their masses into the equation gives us: 500 kilograms meters/second = (200+100) kilogramsXvelocity. Adding the masses in the equation gives us: 500 kilograms meters/second = 300 kilograms X velocity. Dividing each side of the equation by 300 will give you a value for velocity. So, the velocity of both people falling forward is 500/300 meters/second. Dividing 500/300 meters/second gives 1.67 meters/second. So, the velocity of the 200 kilogram and 100 kilogram people falling forward is 1.67 meters/second.

The conservation of momentum principle can be applied to any moving object whether it is a breaking glass or a basketball moving through the air. The principle always holds true.

Momentum from Wikipedia the free encyclopedia, Wikipedia