Re: Why does a volleyball that strikes a beam in the gym seem to come back with

Excellent question Andrew.

I imaging the situation that your question addresses as a gym class volleyball game in which either the ball hits a column or hits a roof support beam from an errant hit. This being the case let's try to figure out what is going on.

It is not that the ball comes back with greater velocity (speed really). It is that it seems to do so -- it is an effect of your expectation. When a volleyball hits a very solid object it will lose a small amount of energy and recoil with a most of its original energy.

Remember that

Four effects usually lead to the "unexpected" return:

To get an idea of the possibilities we will have to make some approximations.

I suspect in your case the ball hit a low beam or a post. If this is true than the volleyball had a large speed which leads to a large momentum direction change when it hit the beam. When the ball hits a high beam in the gym it is moving slower and the observed effect on the flight of ball will be softer. When the ball hits a low beam it is moving pretty quickly and will appear to change direction more abruptly. The collision will apear more "elastic" to those observing the ball because the contact with the beam is more efficient at changing momentum/direction of the ball than say a collision with a person. The efficiency of a collision with an inflated ball is highly dependent on the deformation/restoration properties of the ball. If the collision happens too hard (great initial speed) over to small a surface area the ball can bust, but if the collision is hard and the deformation is spread properly very little energy is lost by the ball colliding with the object.

When the volleyball hits a beam it the deformation of the ball around the beam will tend to create a restoring force more closely aligned with the angle of impact than when the ball hits a wall. The ball will "wrap around" the beam and the ball could bounce right back in the direction it hit the beam. When a ball hits the wall or floor it will bounce away closer to an angle that obeys the law of reflection. If one is not expecting the ball to bounce right back the ball sure will seem to be moving faster.

I would supsect that a volley ball can lose anywhere from 20% to 90% of its energy in a common collision. Even a volleyball struck by a player performing a spike will likely lose about 40% of its initial energy at the collision of the ball with the arm. The player may add a fair amount of energy to the outgoing ball giving it the appearance of an "elastic" collision. A ball hitting a beam probably losses about 20% of its kinetic energy. A ball hitting an unsuspecting person probably losses closer to 50%. A ball hitting a soft net probably loses somewhere on the order of 80-90% of its initial kinetic energy even with some recoil.

The usual change in spin is probably not involved in this case, but I will discus it briefly for completeness.
An good example of unexpected spin is a baseball skipping on astroturf versus bouncing on grass or dirt. When a baseball strikes astroturf its spin and kinetic energy is less affected than when it hits grass/dirt. The baseball with a lower angle of impact will skip off the astroturf whereas a baseball on dirt/grass with lose a lot of enrgy from friction and will change spin because of the grab of the dirt/grass resulting in a slower bounce. If you are used to the bounce on dirt/grass the astroturf skip will seem a lot faster. If the friction of the astroturf is lowered even more by water -- watch out.

Therefore, for the casual observer it would seem that the ball comes of a low beam much faster than expected because it is more efficient than a spike or a high off a high beam and it comes off at a different direction than a hit off the floor or wall.

Sincerely,

Tom "Bruised from the Bad Hop" Cull