# Feeling the forces of nature

Why can't we feel any force unless that force is opposed by other forces ??

E.g. we don't feel our weight unless we stand on rigid bodies or other supports . When we are in air or when we jump from a height we feel weightless because there is no force opposing our weight.

So my question is why and how do such crazy things happen ?

Observation : I have felt this effect even while I was on my bicycle. When I was accelerated by the paddling faster , I barely felt any restriction in paddling and the paddles were rotating smoothly i.e. I couldn't feel any opposition . But due to the existence of rolling friction the tires slow down and paddling becomes harder and I could feel that hardness while driving.

Does this mean that we need a force in opposite direction of an existing force to feel that existing force ??

Is it proper to correlate the example and the observation ?

Kindly explain.

• I think you just stumbled upon Einstein's Equivalence Principle.
– PNS
May 20 '20 at 4:50
• Hi Ankit! Please do not post images of texts you want to quote, but type it out instead so it is readable for all users and so that it can be indexed by search engines. Also mention the name of the 'textbook'. Thanks! Jun 9 '20 at 6:12

You have indeed described Einstein's equivalence principle, but it does not just work for gravitational force. This is true of all inertial (or "fictitious") forces.

Newton's third law (and indeed Newton's formulation of mechanics) describes active forces, which act in the context of Newton's concept of Absolute Space and Absolute time. The reactive force is equal and opposite to the active force.

Huygens extended this to rotating and accelerating frames, by introducing non-active, or inertial, forces like the centrifugal force. Also g-forces. When you are in an accelerating car, you imagine a g-force pushing you back, but this is not a true force. There is only one active force, pushing you forwards. In a Newtonian frame, you are accelerating, there is no active force pushing you back. Instead one thinks of the g-force as an inertial force.

Einstein showed in general relativity that gravity is also an inertial force, not an active force. That is why it behaves the way it does. It is why we don't actually feel gravity pulling us down. We feel the surface we are standing on pushing us up, and imagine that this is counterbalanced by gravity.

You cannot feel the force if the force does not push/pull your molecules/cells apart from each other. As @PNS pointed out in his link, this holds for gravitation and free fall of small object.

In case of big objects, it no longer works. Like for example Earth is not supported by anything, it is in free fall, yet the moon produces tides on Earth. This is also true for you, but because gravitational inhomogenities around Earth are very small, you cannot feel it.

You feel the force only when you are supported, because the gravitation tries to pull your whole body down, but the support under your legs forbids this. The overall effect is then not to move your body as a whole, but to actually shrink you.

This however does not work for "any" force. It works only for gravitation. In case of electromagnetic forces different atoms/molecules/particles constituing your body have all different charge and therefore "try to move" with different accelerations. Because your body is more or less neutral, you do not normally feel this, but if you would apply huge electromagnetic force it could rip your molecules apart and it is very probable you would feel that.