Mass vs Weight - difference and how do you show it?
Date: Around 1993
What is the difference between weight and mass? How can I
demonstrate this in my fourth grade classroom?
Here is a simple start. Masses are attracted to each other by
the force of gravity. The amount of attraction on an object like you and me
at the surface of a planet is what we call weight. It depends on what planet
we are on and or it can depend on whether we are sitting still at the surface
of the planet or are accelerating toward or away from it. If we are
accelerating toward it we will weigh less (this means the force we feel on our
butts against our chair will be less, and even could be zero. we will still
be our old fat selves. We will not have lost mass, we will have simple lost
the feeling of the chair seat pushing up against us. In one sense weight is
about feeling the force of gravity pushing us against the surface of our
chairs and floors. Mass is just the number of particles that make us up.
Demonstrating this is another thing. One idea is to have kids feel their
backs against the chair of the car seat when the car is speeding up or turning
(it has to be accelerating)
A good thought experiment is this: An astronaut is taking a
space walk and an asteroid the size of a car is floating next to him. The
asteroid has no weight because no gravity is acting on it. But it has plenty
of mass (stuff or molecules). If the astronaut gently pushes on the asteroid
he can lift or move it because it has no weight, something he could not do on
Earth. But if he tries to quickly punch it, he would break his hand (if it
was not protected) because the asteroid still has mass. Or if he somehow got
caught between the asteroid and his spaceship he would be crushed, for the
same reason. (This all ignores friction, of course) This may not be exactly
correct, someone else might want to edit it, but the mere fact that the
asteroid is floating means it is weightless.
Click here to return to the Physics Archives
Update: June 2012