What did the above experiment demonstrate? It shows that the scale
on the left was measuring the force of gravity (weight) not mass. On earth the
spring was standardized to read 100g at sea level. A true balance beam (like a
triple beam balance you use at school) measures mass by balancing the scale against
a known (standardized) mass. On the moon the mass on the left side of the balance
may 'exert less force', but then less force will be needed to balance it.
what is really mass and weight if they are not the same thing?
is defined as the amount of matter an object has. One of the qualities of mass
is that it has inertia As an example of inertia, imagine an ice puck resting on
a frozen pond. It takes a certain amount of force to set the puck in motion. The
greater the mass the more force will be needed to move the puck. The same is true
if the puck were sliding along the ice. It would continue to slide until a force
is applied to stop the puck. The more massive the puck is, the more force will
be needed to stop the motion of the puck. Mass is a measure of how much inertia
an object shows.
The weight of an object on earth depends
on the force of attraction (gravity) between the object object and earth. We can
express that force as an equation:
F = G[M m/r2]
where F is the force of attraction, M is the mass of the earth, m is
the mass of the object, and r is the distance between the center of mass of the
two objects (G is called the Gravitational Constant)
this equation show? What will cause the force of attraction to increase or decrease?
If either mass increases the force of attraction increases proportionally. Since
the moon has 1/6 the mass of earth, it would exert a force on an object that is
1/6 that on earth.
Why is the 1/r 2 factor
so important? This is an inverse square relationship which seems to show up a
lot in physics. How does it affect the force?
What is 1/r
2 when r=1, 2, 5, 10? What is the decimal equivalent? Notice that
when r=1 the value 1/r 2 is 1.0, but at r=10 it deceases
to 1/100. That means gravity gets weak 'quick' as we move away from the earth.
get a real feel for the inverse square relationship, see if you can get two magnets.
Move the poles closer and closer slowly, what do you notice when r (the distance
between the poles) is very small?