Question about force, acceleration, and gravity

Factual Questions
21

The force of gravity is a force of attractin between two bodies. You are pulling the earth toward you, and it is pulling you toward it. By the same force, but the difference in (m) mass is so incredibly large that when you jump off a cliff, only you appear to move.

Your question appears to be about the resultant force that pushes back so you don’t end up at the center of the earth.

Let’s consider several scenarios -

First, you in a bouncy castle… you try to stand on this big rubber air bag, you fall over, you lie in a slightly indented spot on this giant rubber balloon. Once the bouncing stops, you are in equilibrium - no longer moving. But your weight has squashed the airbag so the internal air pressure is slightly higher. That raised air pressure is pushing back at you. That force - X psi times the area - perfectly balances the force of gravity pulling you down - so that you are motionless, although essentially lying on (compressed) air.

Scenario B - you go jump in a lake, or the Dead Sea. You float. As you go deeper into water, the pressure goes up. Not by much, but spread over the whole body, the effect is the same as the bouncy castle airbag. The water pressure 33 feet down is 14.7 PSI (1 atmosphere), so 6 inches down for example it’s .22psi. That’s not much, but spread over your whole body, that extra force bouys you up until the force pushing you up from the deeper water pressure balances the force pulling down on your body. (If it isn’t more than the downward force, then you will continue down to Davy Jones. Aaarrr!)

For solids, the argument is a bit more complex. If you stand on a plank across a chasm, the plank is resting on solid ground at both ends. It presses down on the rock and dirt, until it deforms it enough to balance the downward force of your body. Plank breaks, there is no counter force and down you go.

Similarly, standing on rock. You deform the rock that infinitesimal amount, until the force of deformed rock times the area of your footprints equals you weight (gravitational force). This effect is best seen if you stand on a thick rubber mat or something that can deform visibly with pressure and rebound when pressure is removed.

22

It all gets back to the definition you choose for “real”.

Consider this: if F= m * a, there are two components for a force.

We are considering constant mass situations, not rocket thrust, etc, where mass is changing.

If a force is to be real, it must have an active part.

Mass is not active. It is the characteristic of the item being acted upon.

Ergo, the active part must be acceleration.

Thus, if a force is acting on you, it is accelerating you.

However, accelerations cancel in vector form. Just like you vector sum the Forces, you vector sum the accelerations.

Nancarrow is correct that you are confusing the body being acted upon. If we are discussing you, there are two forces on you. One is gravity, the other is the reaction force of the ground (or couch or bouncy house or whatever surface).

Your net change in movement may be zero, which you might call your “true acceleration”, as opposed to the “partial accelerations” of each force independently.

On the other hand, have you looked at the acceleration caused by Earth’s rotation, or the revolution around the Sun, or the Sun’s movement through the Milky Way? No? Then what is your “true acceleration”?