Conservation of Momentum

and Projectiles

BHS -> Mr. Stanbrough ->Physics 1 -> Mechanics -> Momentum -> this page

Wait a minute! Doesn't a projectile have a net force on it - its weight - so what does the Law of Conservation of Momentum have to do with a projectile?

Consider a projectile near the surface of the Earth, with no air resistance. Remember that the weight of the projectile is a force pulling the projectile vertically down (we're talking about projectiles near the surface of the Earth, here...) and that the projectile's weight is the only force acting on it. Also remember that two-dimensional motions can be broken up into 2 independent 1-dimensional motions, or components. A projectile accelerates in the vertical direction due to the action of the net force on it (its weight), but it has a constant velocity in the horizontal direction, since no net force acts on it. The projectile's vertical motion is free fall, and it moves at constant velocity in the horizontal direction.

Since momentum is a vector quantity, the horizontal and vertical components of momentum can be considered independently. Since no net force acts on the projectile in the horizontal direction, its momentum is conserved in the horizontal direction. Since the Earth's gravitational force exerts an impulse on the projectile in the vertical direction, the projectile's momentum is not conserved in the vertical direction.

The important point here is that conservation of momentum is not exactly an "all or nothing" deal - momentum may be conserved in some directions but not others.

last update December 26, 2005 by JL Stanbrough