Your mass does not depend on which planet you visit, but your weight does. Weight is the gravitational force acting on your mass. The equation is weight = mass × gravitational field strength. Different planets have different field strengths because their masses and sizes differ. On Mars you would weigh much less than on Earth, although your body would contain the same amount of matter. On a planet with a stronger gravitational field, your weight would be greater. This distinction between mass in kilograms and weight in newtons is important throughout mechanics.

The useful part of this physics is that it helps people predict and design real things. Engineers often begin with the same simplified model used in school, test where it works, then add more detail when measurements show that the real situation is more complicated.

Gravity is an attractive force between masses. Near a planet, weight is given by W = mg, so mass stays the same while weight changes with gravitational field strength. Orbits are not places where gravity disappears: an orbiting object is continually accelerating towards the body it circles, but its sideways velocity means it keeps falling around it rather than straight into it.