Q & A: Forces in a Collision

Q:
How can I show the forces at work in a collision. I must have this for a science fair. thanks.
- Cameron Turney (age 10)
Round Rock, TX
A:
Cameron,
Great question. The forces involved in a collision can actualy be very large, even if the things colliding are not very big. A good example is the collision between a baseball and a bat. The force exerted by the bat on a well hit fastball can be several thousand pounds. But how can this be? Would the ball not go for miles if this were the case? NO! The reason the force is so big is that the bat is in contact with the ball for such a short period of time, about 1/1000 of a second or so. Even though the force is huge, it only acts for a short time and so the ball only goes a few hundred feet.

The important concept here is summarized by Newtons second law which states that the force required to change the momentum of some object (like a baseball) is proportional to how much the momentum changes and inversely proportional to the time it takes to change it. In other words, if we change the momentum of something very quickly, then the force required is large. If we change the momentum slowly, the force is smaller. (If you have not studied momentum yet just think of it as "movement"...changing the momentum of something means that we are changing how it is moving).

SO, whats a good demonstration for this? One of my favorites is to drop eggs. If you drop an egg from a height of 1 meter onto the floor, it will break (you can put down some paper towels so you dont make a big mess). If you drop another egg from the same height onto a pillow it wont break (better get a grownup to help with this).

The change in momentum of the egg is the same in both cases, but the time taken is not. The egg dropped on the floor stops very quickly, which means the force on the egg is large, so it breaks. The egg dropped on the pillow slows down and stops more slowly since the pillow is squishy, therefore the force on the egg is smaller and the egg does not break. (The pillow also spreads the force over a larger part of the surface of the egg, which also helps save it).

Hope this helps,
MS

(published on 10/22/2007)