Why mass is constant whereas weight can change?
mass is a measure of the amount of material in an object; weight is a measure of how much force an object exerts under the influence of a gravitational field. an object – say a bowling ball – will have the same amount of mass most places in the universe (barring relativistic effects), so if you go bowling on the moon the ball will hit and scatter the pins just like it does on earth. however, it will hurt much more if you drop the bowling ball on your foot on the earth than it would on the moon; lunar gravity is far weaker, meaning that the ball has only a fraction of the weight it would have on earth.
