Mass, Weight, and Weightlessness
- Mass is an object's resistance to acceleration as defined by
Newton's second law, F = ma
- Massive objects are hard to accelerate
- An object's mass is an intrinsic property of the object and thus
does not depend on its environment.
- Weight is a measure of the downward force gravity exerts on an object.
- An object's weight depends on the strength of gravity at the
location of an object.
- Since the gravitational attraction of the Moon at its surface
is 1/6th the gravitational attraction of the Earth at its surface the
same object will weigh 1/6th as much on the Moon as it does on the
- Weightlessness is the absence of the sensation of weight.
- Weightlessness can be achieved in one of two ways.
- Go to a place distant from any object so that the force of
gravity is nearly zero -- This is true weightlessness. It is unachievable with current technology as you would have to travel millions of light years to even approximate this condition.
- Orbit a planet so that both you, the spacecraft, and everything else around you are falling
together at the same rate.
- If you are falling freely you feel no
weight even though you technically do have a weight since you
are experiencing the pull of gravity.
- This apparent weightlessness is
the type of weightlessness we associate with space flight.
- Seeing astronauts weightless leads to a commonly-held
gravity stops acting beyond the Earth's atmosphere.
- In reality, astronauts in orbit experience
almost the same gravitational force as someone
standing on the Earth's surface.
Updated September 9, 2005