Gravity causes Weight
An objects weight is how hard gravity is pulling on it.
Our normal experience is that this doesn't change ... because we all live on the surface of the planet Earth!
Gravity is a basic force of nature where all objects with mass attract each other.
The Earth, with a mass of 5,973,600,000,000,000,000,000,000 kg (that is 5.9736×1024 kg in Scientific Notation) pulls objects towards it with quite a large force!
It is funny to think that the 100 kg mass actually pulls the Earth towards it, too! But not by very much at all.
But Scales Measure "Apparent Weight"
|Scales can measure the downwards force ... but they can be fooled, because they measure any "downwards force" and don't know if it is gravity or some other force!|
Just jump up and down (gently!) on your scales at home to see your apparent weight change, while your weight (and mass) stays the same.
Your weight is the same (because the force of gravity hasn't changed), but your apparent weight changes.Here is another example:
Put the object in an elevator as it moves
... the scales show less!
But it still has a weight of 100 kg.
Accelerating (moving faster and faster) or decelerating (moving slower and slower) needs force, and the scales show this.
Question: If the elevator was accelerating UP, what would happen to the apparent weight?
If something is pulled with a continuous force (and there is nothing dragging it back) it will continually speed up! So changing speed and force are connected.
Have you noticed how you feel lighter when an elevator first starts moving down, and you feel heavier when it slows again?
That is because the speed needs to be changing to affect the apparent weight!
If the elevator is moving at a constant speed there is no difference in the apparent weight compared to when it is sitting still.
Why? It takes force to make something move faster (or slower). If something moves at the same speed you will not feel any extra force.
That is why you can sit in a speeding car or train, and everything seems normal (unless the driver speeds up or puts the brakes on).