How do you know? You haven't measured it. By your own admission, you have only done half the experiment. You're too scared to try the other half.

In the **experiment** What is the force f in the scale when the human lifts his body? what its value and why?

This is just another simple application of Newton's second law of motion.

When a person stands on a set of scales, he pushes down on the scales with a certain force, N. The scales apply on equal and opposite upwards forward on the person.

The forces on the person, then, are the person's weight, W, downwards, and the force from the scales, N, upwards. As an equation, the upwards acceleration of the person is then given by

$$a=\frac{N-W}{m}$$

where $$m$$ is the person's mass.

When the person is standing still on the scales, he is not accelerating, so $$a=0$$ and then we must have $$N=W$$. Since the reading on the scales is essentially the $$N$$ value, the scales show the person's weight in this case.

If the person stretches upwards (e.g. as if trying to pick fruit from a tree), then his centre of mass will rise. For a short time, during the stretch, the person will be accelerating upwards, which means $$a$$ in the above equation has a positive value (as opposed to $$a=0$$ when the person is just standing on the scales). It follows that the force $$N$$ must, for this short time, now be

*larger* than the person's weight. Of course, when the person stops moving again, his acceleration goes back to zero and the scales again read his usual weight.

If the person holds his arms above his head and then lets them drop, as they are falling the person's centre of mass will be accelerating downwards for a short time. So, while this is happening the scales will momentarily read a value

*smaller* than the person's weight.

There is no mystery in any of this. There's nothing special about a human body.

You can test the same equation by taking your scales into an elevator and placing a rock on the scales. Observe how the reading changes when the elevator starts or stops moving up or down (i.e. when the elevator is

*accelerating*). Note that, when the elevator is moving at a constant speed, or is stationary, the scales just read the weight of the object on them.

Human bodies on scales in elevators do not behave differently to rocks on scales in elevators, in case you're wondering.