For those who follow or play lacrosse, hockey, and even soccer know of bar down goals. A bar down goal is one of the coolest goals a person in one of these sports can score, it's where the ball hits the crossbar on the shot and goes straight down or back into the net. It can get a team hyped up in a matter of seconds, but how does it happen?
To start, why doesn't the goal come flying up with a powerful enough shot? Well, knowing the laws of momentum and motion, a lacrosse ball, or hockey puck, hitting the crossbar of an iron goal at 80 miles per hour won't move the goal much, as much momentum as the object may have. The equation p=mv proves that a lacrosse ball or hockey puck 1/100th the mass of a lacrosse or hockey goal won't do much damage and move the goals, and is the primary reason a bar down shot looks so good. The ball accelerates downward off of the crossbar at 9.81 m/s^2, and since the weight of the goal is so great in comparison to the puck or ball, the crossbar actually provides a force for the rubber ball or puck to accelerate off of.
Next time you watch a lacrosse or hockey game and see a bar down goal, remember the physics of it that makes it so cool.