We know that the weight of an object is actually the force with which the earth attracts it. When you stand up, you are conscious of your own weight. According to Newton’s third law, every action has an equal and opposite reaction.
A person standing up exerts a force on the surface of the earth equal to his or her weight in the downward direction and the surface of the earth applies an equal amount of force on the person, but in upwards direction. These two forces are balanced and so there is no net movement in any direction.
The same is true in the case of measuring weight of an object with the help of a spring balance. You must have played with a spring at some point in your life. When you hold both ends of the spring in your hand and stretch it you can feel it trying to regain its former, compact shape. This force can again be justified with the help of the Newton’s third law. The more you stretch the spring, the more you can feel its own urge to regain to the shape. Suppose a spring balance is suspended from the ceiling of your room. If you place an object over it, the spring stretches by some amount and remains stretched because the force with which the spring is trying to regain its shape is being balanced by the weight of the object. After proper calibration, you can tell the exact weight of the object with the help of a spring balance based on the length of the stretched spring.
Acceleration due to gravity or g
If the ceiling suddenly disappears, the spring balance and the object will start to fall towards the earth due to the pull of the earth or gravity. This situation is termed as a free fall. The spring balance and the object will experience no other force than gravity, if we consider that there is no air friction for the sake of the argument. In such a case, the spring balance and the object will both be accelerating towards with earth with ‘g’. If you check the reading of the spring balance at this time, it will be zero because no amount of spring will be stretched. This is known as weightlessness.
If you were experiencing the same condition in which you were feeling weightless, you will not feel your own weight because there is no force to provide a reaction to the pull by the earth. You must have noticed astronauts floating around in spaceships. If you were in a satellite going around the earth, you would feel the same weightlessness. This is because it is gravity which provides the centripetal force for the satellite to maintain orbit and move in a circle around the earth. This means that the satellite has an acceleration towards the center of the earth equal to the acceleration due to gravity or g. Everything inside the satellite is in a state of free fall and this phenomenon gives rise to cool imagery of water floating like a gel which astronauts have to catch and gulp down.