What is the difference between MASS and WEIGHT? 
The amount of substance in something is its MASS. 
The measure of heaviness that mass has is due to the force of Gravity, which determines its WEIGHT.
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It is not exactly news that if we drop something, it falls.
Simply put, Gravity causes this.

Gravity has been called many things, including a natural phenomenon and an influence. Broadly speaking, gravity is a force that causes an object to move downward. Some scientists believe that Gravity should be classified as a force. 
Others believe that Gravity also may be placed in the energy family, due to the effect known as “gravitational pull” that it has on objects. Gravity also means importance and seriousness — as in the gravity of a situation.
Gravity is the Main Mover and Shaper of the Universe.
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For our purposes, we will use the word weight when working with the instrument, the bow, and performing string playing motions.
Skilled string players practice to master the art of directing Weight into Motion.
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You may be asking yourself what do science topics have to do with learning music. 
The answer is quite a lot!  As you progress, it will become apparent. In addition, when learning rhythm, basic mathematical skills come into play. 
You will also be pleased to know, that words in many languages are used in music notation. And, then there is tons of history too!  My teaching assistants promise to join us and pitch in later on.
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