# If I’m hanging onto something, it’s being pulled down by my weight. But if I do a pull-up, does it get pulled down with more force or is it unchanged?

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I tried to do think it out but I can’t really wrap my head around the math and where to start.

In: 5

It gets pulled down with more force.

To give a similar but maybe more recognisable example, does jumping up from something push it down?

Also yes, that’s how trampolines work.

If you’re accelerating up due to the force of your legs or arms acting on your centre of mass, an equal but opposite force has to act on whatever your feet/hands are touching.

In vacuum, if you “pulled up” on something, you’d end up pulling it and yourself to your common centre of mass.

If you “jumped off” something, you’d both be accelerated away proportionally to your mass.

Another easy illustration is weighing yourself in an elevator. You’ll see your weight increase when the elevator accelerates up, because you’re exerting more force to stay upright. Otherwise you’d collapse towards the floor of the elevator at the same rate as a stationary reference point outside of it.

Imagine yourself floating in space without the earth below you, or the bar above you, doing the same motion as a pull up. You’re essentially pulling your arms down, and your legs up.

Even though the earth is below us pulling down, it’s still not that much different than being in space. If you consider yourself the “center,” of the system, then you’re still pulling the bar down, however the earth just so happens to be pulling both you *and* the bar down at the same time you’re pulling the bar towards you.

ELI13: As a physical object you still have inertia, meaning the mass of your body just like any other physical object resists speeding up or slowing down. This inertia acts as a sort of “anchor,” which wants to hold you in place, so as you try to pull yourself towards the bar, you’re not just overcoming gravity, but your stationary inertia as well, and the bar has to counteract that extra force. If it doesn’t counteract that force, then the bar will move towards you i.e. break and hit you in the face.

The breakdown of forces would be something like this:

You are hanging down with your hands straight: Your hand is pulling down the bar with your weight, and your hand (and body which is attached to it) experiences upward force equal to that, keeping you from falling, acceleration is 0, so net force is 0

You pull yourself up: You accelerate up, so net force cannot be 0 anymore, you pull the bar (and consequently earth itself that is attached to it) down, while you go up

When you start pulling, you accelerate up. By Newton’s 3rd law, the bar needs to be pulled down more. From then, if you ascend at a constant velocity, the bar is only experiencing your weight. When you reach the top and slow to a stop, you are accelerating downward, so the bar experiences less downward force until you stop again. Hanging with your arms flexed means the bar only holds your weight.

It would look like this on a graph

—-^—-v—-

First flat section is just hanging, then they start ascending, then the next flat section is the ascent, then they stop, and the last flat section is hanging with the arms flexed.