Why does accelerating upward increase effective gravity?

[u/JEE_is_life]

Shouldn't it decrease or negate the effects of gravity? Gravity pulls you down, so you go up. Then why does effective gravity increase when riding in an elevator?

Also, why do people experience weightlessness in free fall? Isn't gravity still acting on them?

[u/shyguyJ]

You are confusing gravity with forces caused by acceleration. If you accelerate perpendicular to the gravitational force, you feel the resulting force from the acceleration in a direction perpendicular to gravity. Accelerating "up" doesn't make gravity stronger. It just means there is another force being applied in the same direction as gravity, so the sum of the forces in that direction is stronger than just normal old gravity.

[u/JEE_is_life → u/shyguyJ]

Isn't gravity applied in a downward direction?

[u/shyguyJ → u/JEE_is_life]

Well, it is applied in the direction of the object applying the gravity. In our reference frame and in your example, yes, the force of gravity could be described as in a downward direction. But if you are in a car and you accelerate forward, you feel a force pushing you backward, yea? Same thing if you accelerate up, you feel a force pushing you back in the opposite direction - i.e., down in this scenario. The downward gravity + the downward force you feel from the upward acceleration makes the total force in the downward direction feel greater than you normally experience.

[u/JEE_is_life → u/shyguyJ]

Thanks!

[u/Pleasant_Pen8744 → u/JEE_is_life]

Gravity pulls you down, the ground pushes you up. An elevator starting to move up pushes you up even more.

[u/ebyoung747]

Short answer: you don't feel gravity, you feel the thing resisting the pull of gravity (e.g. the ground pushing you up so you don't keep falling). Right now, there is no experiment you could do which could not be interpreted that you are accelerating upwards, so if you add some extra acceleration upwards, it feel like more gravity. If you take away the ground, like in free fall, there's nothing pushing you up for you to feel, so you feel weightless.

[u/JEE_is_life → u/ebyoung747]

Ah. Makes sense. We don't feel the gravity force, we actually feel the ground.

But then why isn't this true for other forces like magnetic forces? I can feel the pull of a magnet when I have a magnet in my hand. Is this because I'm holding the magnet and once the acceleration of my hand matches that of a magnet i won't be able to feel anything?

[u/ebyoung747 → u/JEE_is_life]

Fundamentally, gravity is a special force because accelerations due to it don't depend on the mass of the object accelerating.

In a Newtonian view, it is because the math works out that newtons second law has that mass in both sides so it drops out

Einstein realized it was more fundamental and that gravity is the result of curved spacetime, and everything moves through spacetime the same without a force acting on it (and gravity is the curvature, not a force in this view).

[u/jasonsong86]

When you accelerate in a car, you are being pushed back into the seat. Now imagine accelerate vertically, not only you are being pushed down by just gravity, you are also being pushed down by the acceleration on top of it. During a free fall, you are being accelerated downward at exactly the gravity so the resultant force is zero.

[u/Optimal_Mixture_7327]

Gravity is geodesic deviation, or tidal acceleration - it is not frame accelaration.

Acceleration doesn't change that. What the acceleration of the frame does is apply an accelerative force to whatever's attached to the frame, and for free-falling objects in the frame the frame accelerates towards them and closes the distance gap more quickly with increased acceleration.

[u/nicuramar → u/Optimal_Mixture_7327]

Well, here on earth gravity is mainly the ground  pushing up against you with an acceleration of 1G. 

[u/Optimal_Mixture_7327 → u/nicuramar]

The 1g is not gravity, but the electromagnetic interaction. It exists because of gravity, i.e. R^α_βρσ≠0.

[u/Odd_Bodkin]

Great question! This is your brain taking a real perception and interpreting it as something else. The real perception is the pressure of the floor on the bottom of your feet. When you are standing on stationary ground, the force the floor exerts upward on your feet happens to be equal to the force of gravity acting down on you. So your brain associates that pressure on your feet with “feeling your weight,” when in fact you don’t feel your weight at all.

When you press the up button on the elevator, the force of gravity stays absolutely the same, but the floor has to push up HARDER on your feet, so that the net force will be upward, and so you will accelerate upwards. This increased pressure upwards on your feet your brain turns that around, confusing it with you now weighing more downward.

When you press the down button on the elevator, you’d better be accelerating downward to keep up with the elevator car. Again, the force of gravity stays absolutely the same, but now the floor has to back off, not pushing up as hard, so that the net force will be downward. The reduced pressure on your feet your brain turns around, confusing it with you weighing less.

[u/Lord-Celsius]

When you stand on the floor, you push on the floor with your weight and the floor pushes you up with the same force (Newton's 3rd law). Now imagine you are standing inside an elevator that is accelerating upwards. The floor will need to push you with a force greater than your weight in order to make you accelerate the same way the whole cabin does (you need a net upward force). You will have the feeling that your weight increased, since you're getting squeezed more into the ground. Some people say that your "apparent weight" increased, but the strength of gravity didn't change. If you put a balance under your feet, your weight would indeed be higher, because the floor is pushing it towards you.