APPARENT MASS, APPARENT WEIGHT, WEIGHTLESSNESS

First question - how is it you feel your weight?

If you can answer this - you understand all the rest.

We are animals accustomed to living on Earth. Our inheritance tells us instinctively that jumping from trees can lead to broken limbs which is not good for our survival. Jumping from trees therefore is dangerous.

Now what is it that is dangerous ? the jump? the fall? - or the impact? In the C19th, it was believed by many that the speed gained in falling from a cliff would kill - they would be dead before hitting the ground. Bungy jumping belies that quaint belief.

Clearly it is the impact which is dangerous.

What is happening in the impact? The ground stops the falling person in VERY little time. The Ground pushes UP very hard - it provides a LARGE upward (Normal) Force - much larger than usual changing the downwards motion to stationary.

It is the UPWARDS FORCE that we feel ON and IN our BODIES - not gravity!!!

This upwards force is what we interpret as our APPARENT WEIGHT. It is this quantity that you are often asked to calculate in problems.

Whatever force is pushing upwards through our feet or bottom we interpret as our "weight"- not our true weight - that is the force due to gravity "mg". You feel "heavy" when on a trampoline during the bounce - not when up in the air. Your apparent weight is larger at this time during the upward push.

Why do we misinterpret the forces? - because our body is being crushed together more than is usual when standing still. The larger the force crushing us - the larger we feel that gravity is acting on us - but it isn't gravity at all, it is the ground or whatever.

To calculate Apparent Mass from this upward force - we must recognise that most commercial bathroom scales etc used on Earth are usually FORCE measurers, not mass measurers. WE NEED TO DIVIDE THE FORCE BY 9.8 to get the value such scales would give.

Gravity acts producing the same acceleration on all mass in our bodies. SO, in falling freely, all parts of our bodies accelerate at the same rate - so no stresses are set up between different parts of our bodies - we don't feel gravity acting on our body ALONE! THIS CONDITION IS CALLED WEIGHTLESSNESS. Your apparent weight is zero.

Any person in such a condition is literally FREELY FALLING. Another term given to this situation is FREEFALL. All persons or things who are ONLY subjected to gravity are freely falling and feel weightless.

It follows that all such falling gives conditions of weightlessness. Orbiting around the Earth is one such situation as is falling between planets such as happens to space probes like Galileo and Voyager.

howstuffworks

NASA