1). Weight and mass are different.
. . . . . Mass is a property of the person or thing and doesn't depend
on anything else. Weight depends on what else is in the neighborhood.
. . . . . Mass never changes, no matter where the person or thing goes.
Weight changes, depending on what else is in the neighborhood.
Weight and mass are related:
. . . . . Weight is the mass of the person or thing multiplied by the gravity
in the place where the mass is. So as long as you stay in the same place,
weight and mass are directly connected. If you know either the mass or the
weight, AND you know the gravity in the place where the mass is, you can
always calculate exactly what the other number is.
2). According to Newton’s first law, if no force acts on an object, it will
just continue moving. But we NEVER see this in our daily life on Earth.
In everything that we see, if no force acts on a moving object, it stops !
For example, a car engine must exert a force between the car and the road
to keep the car moving at a constant speed on a straight and level road.
The reason is that whenever we see anything moving, there is ALWAYS
a force on it that's trying to make it stop moving. Always ! That force is
the result of friction.
-- A rolling ball stops rolling. Either there's friction between the ball
and the dirt, or between the ball and the concrete, or the ball has to
keep pushing blades of grass out of the way so it can roll through.
-- A bullet shot from a gun slows down and stops. It has to keep pushing
air molecules out of the way so it can sail through.
-- Even if the road is straight and level, a car slows down and stops if you
don't give it some gas to keep it going.
There is friction between the tires and the road that slows the car down.
AND the car also has to keep pushing air molecules out of the way so that
it can sail through.
On Earth in our daily life, we NEVER see anything moving that DOESN't
have forces acting on it trying to make it stop. If we could see such a
thing, then the object would just keep going and never stop. This can
happen in outer space, where there's no concrete, no blades of grass,
and no air molecules.
