



Just like space and time, 


mass is something many people deal with all the time 


without really understanding. 





In physics, mass has two primary characteristics. 


It resists being moved, 


and it has a gravitational field surrounding it that will attract other mass.






Thanks to Einstein and others, 


we also know that the concepts of mass and energy 


are really just two ways to think about the same thing. 





Modern physics has seriously called into question, 


what
it means to say that something is "solid". 





There are two levels on which we will deal with mass in these pages. 





The first is mass as Newton understood it. 





The second is mass as understood by Einstein,
Heisenberg, and Feynman. 





While he is further removed from us in time, 


Newton's understanding of mass is closer to that of the average person today. 





In concepts from Newton, 


the resistance of mass to being moved is often expressed as: 





F = ma






That is, the force required to move something
(F), 


is equal to the mass of the thing
(m) 


times the acceleration that
the thing experiences (a). 


Acceleration is change in speed or velocity. 





Also in concepts from Newton, 


the force of attraction between two things is
often expressed as: 











That is, 


the force is directly proportional to the total mass of the two objects
(m_{1} + m_{2}) 


and inversely proportional to the square of the distance 


between the two
masses (d ^{2}). 





Depending on the units we use 


to express the mass, acceleration, distance and force, 


we may need to multiply one side of the equation 


to make the units work out right. 





Constants
make the equations a bit more complicated, 


but
do not change the relationship between the players in the equation 





copyright 2005 Bruce Kirkpatrick 
