### 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 Earth.

• 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 misconception that 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