Re: Why does a cold basketball not bounce as well as a basketball at room temp?

Hi Rob.

What makes your basketball bounce is the air pressure inside the ball.  You 
inflate it up to a "pressure" above the atmosphere's pressure.  I visualize 
pressure as a "molecular" force...that is the push that gaseous molecules 
exert upon their surroundings.  This push depends upon temperature.  
Temperature is just energy (also called "thermal energy").  The hotter 
something is, the more energy it has.  This energy is transferred to the 
gas because the gas molecules move very quickly (hundreds of meters per 
second at room temperature) and collisions between the gas and it's 
surroundings cause energy to be transferred until they are all at the same 
"temperature".

Hotter gases move faster and therefore bounce off their surroundings more 
often (leads to higher pressures).  Colder gases don't move as fast and 
don't exert as much pressure on their surroundings.

the Law which governs this behavior is called the "Ideal Gas Law" and 
states:

PV=nRT 
where P is the pressure a gas exerts on its surroundings, V is the volume 
that contains the gas, n is the amount of gas (number of particles...very 
large!), R is a proportionality constant called the "ideal gas constant", 
and T is the temperature of the system.

Hence, as the temperature drops, the pressure drops.  When you bounce the 
basketball, and it hits the floor, there is a force exerted.  If the 
pressure inside is high, you'll get a lot of force exerted back, and the 
basketball will bounce.  If the pressure inside the ball is low, the 
basketball goes "thud".

Another source of the "bounce" is from the material that the ball is made 
of.  Certain rubbers, for example, are very resilient ("springy").  Can you 
imagine how a basketball made of steel would bounce?  A materials 
resilience is dependent upon temperature.  As materials cool, they have 
less energy available, and can become more ordered (this is what happens as 
you freeze water to make ice).  More ordered structures are less resilient, 
so at lower temperatures, your basketball is less "springy" as well.


I hope this answered your question.
Please feel free to email me if you have further questions at:
weibel@chemistry.chem.utah.edu

Best Regards,
Mike