Cancellation of forces in feeling weight

Question

If we are on the ground, Earth's center attracts us by $mg$ force and the ground also exerts $mg$ force in the upward direction. Those two forces cancel out. So there is no net force acting on us now. Then how do we feel weight and not weightlessness? Since net force is $0$, we shouldn't feel any weight.

Answer 1

There is a force. Try forcing your two hands together very hard. They cancel each other but you can still feel the force or the pressure. When you apply more force they still cancel but the pressure grows. Like the Earth a force is meeting a resistance and you feel it. In free fall the force is there but there is no resistance to feel.

Answer 2

Since net force is 0,we shouldn't feel any weight.

This kind of statement can sound right intuitively, but actually has no basis in fact.

If two trucks hit you from opposite sides, then there's 0 net force on you. But you'll still probably feel something!

For a human to "feel" a force really means for something to press up against their nerves. Gravity applies to each molecule equally, so nothing presses against your nerves. This is why when you're in free fall, you feel nothing even though gravity is still pulling $9.8 m/s^2$. When you're resting in a chair, you feel the normal force of the chair because it applies to different parts of your body unevenly, and so certain nerves will be compressed to various degrees.

Answer 3

Also in free fall when there is a force on you, you feel weightless - isn't that ironic!

The reason is that there are relative stresses that you experience in your body that are not alleviated by the normal force. While the normal reaction is digging into your heals, gravity is trying to squeeze you from your head down, into the ground. Do you see how there is relative stress in the body? All our muscles and tissue are "hanging-off" our skeleton which stays in place and supports them. All these stresses induce the feeling of weight.

Imagine a flexible massive rod with masses at the end. Balance this rod on your finger tip. Is the net force on the rod zero ? Yes. But do you see the rod droop at the edges, slightly bent under the weight of the end masses. This relative stress is what constitutes the feeling of weight.

On a deeper level, is the net force really zero when you stand ? Is there really a force when you free fall?
Happy GR!

Answer 4

The sensed pressures Bill Alsept describes are called proprioceptive and the nerve impulses generated in this way tell you where your arms and legs are positioned even when you cannot see them. Proprioception also tells you where down is relative to your body; in this case, the pressures sensed by the nerves are caused by the pull of gravity on your muscles and bones.

Answer 5

If the sum of forces on a non rotating body is zero, that means zero acceleration, or: the body is at rest or with constant velocity regarding an inertial observer. Only in this sense $\sum \mathbf F_i = 0$ is equivalent to no force at all.

But there is another way to measure force, besides acceleration: elastic displacement from the Hooke's law: $F = -kx$. That is the way load cells work. When we are in a vertical position, the distances between several bones (and the length of some bones themselves) get shorter, (what can be painful if the cartilages of the knees are not in good state for example). This compressive strain is transformed in electric impulses to our brain. Our feeling of a compressive force results from the way our body measures that strains.

Answer 6

Why can’t we say the forces cancel each other ?

When you apply a force or push of 150 N on the wall. You will notice you move back. According to your statement , you should have had not moved. The point is , when you applied 150 N on wall. The wall also applied 150N on you. There are two forces. One is acting on wall and the other on you. Both have opposite direction but not the same body. Therefore , they cant cancel each other.

Like here : let’s say your mass = 50 kg and acc you applied on wall = $3 m/s^2$. If mass of the wall is $10^6$ kg. Then ,

$50 * 3 = 10^6 * Acc_{wall} = 150 N . $

Therefore , you notice that actually the acc of wall is very small.

Same is with earth. Acc of earth by is almost negligible evasive of such a huge mass of earth.

Edit : When is net force = 0 ?

Check what is free body diagram.

Brief definition : If you are writing forces acting on the body , then we do not write the forces which are acting by the body on other bodies.

If I keep my book on table. Forces acting on book are force by the earth downwards and normal force upwards.

NOTE : The force by book on earth is acting on earth. Not on book.

Therefore +N -mg(sing convention from Cartesian coordinates) = mass of book * acc of book(0). Therefore , N and mg can cancel out or N=mg.

I hope it is clear.