# Q & A: Mass vs. Weight

Q:
Are mass and weight are same things?
- vinod (age 16)
A:
Hello, Vinod. Good question! There is little distinction between mass and weight in day-to-day life, but they have very different physical meanings. Mass is an absolute measurement of how much of something there is. If two objects of different masses are brought together in a collision, momentum (the product of an object's mass and its velocity) is conserved the object with the larger mass will have more influence on the resulting motion. This is most apparent in fully-inelastic collisions, where the final velocity of two masses moving together is just the mass-weighted average of their initial velocities. Visit for a demonstration. Try manipulating the masses of the wagons and observing the result!

Weight, on the other hand, depends on where you are -- specifically, it's a measurement of the gravitational force an object experiences at any given moment. The relationship between weight and mass is given by F = mg, where F is the gravitational force an object encounters (i.e. its weight), m is the mass of the object and g is a gravitational constant that varies in space. The Moon has a much smaller gravitational constant than the Earth, approximately 1.6 meters/second2 to the Earth's 9.8 meters/second2. That being said, you would experience a more substantial gravitational force (weigh more) on Earth than you would on the Moon, though your mass would be unchanged.

Hope that clears things up!

- Becca

(published on 05/16/2013)

## Follow-Up #1: absence of gravity?

Q:
Also you can have weightlessness such as in the first few seconds of a free fall--before reaction from air equals your weight. But you cannot have gravity-less-ness. Gravity is always there to the end of universe. In fact in a free-fall you have gravity (that pulls you down) and lack of a reactive platform (such as ground) that makes you feel weightless.
- Mehran (age 60)
Miami, FL
A:
Mehran- In a Newtonian framework, the gravitational field is well-defined. However, we do not live in a Newtonian universe. In General Relativity, a better model of our universe, one can always pick a valid free-fall coordinate frame to make the local gravitational field in those coordinates zero.

Mike W.

(published on 02/15/2011)