# Would making newton's 3rd law of motion unidirectional allow you to jump higher or will you break your legs?

Imagine a magic that allows the person to decide in which 'direction' the forces of an impact or imparted force are applied with regards to newton's 3rd law of motion. Either all of the force goes into that which the force is applied to and none into that which applies the force, or all of the force goes back into that which applies the force and none into that which the force is applied.

Think of a ball that is dropped and this magic is applied to it, with one direction the floor might crack if the ball had enough weight while the ball simply doesn't bounce, and with the other direction the ball bounces almost perpetually with all of the force of its bounces going back into it and none into the floor and the only thing that might eventually stop it is the drag it experiences(actually I don't know how drag would affect such an object, maybe it would not really? I haven't thought that far ahead with this magic, which is why this question exists XD).

One direction's applications are obvious, attacking something with a weapon would be more effective and you'll suffer no recoil, and no one would be able to move you even if they tried, and you'd easily survive falls with only the ground experiencing the forces associated with the impact. The other direction's applications are where I'm struggling a little bit with. Among other things I want the people to be able to use it to jump higher, with all of the force their legs impart going back into them instead of half into them and half into the ground, with the objective that they'd jump twice as high(or a little bit less, drag and gravity and everything), but I don't know if such forces going into the legs would have a risk of breaking them.

Would making newton's 3rd law of motion unidirectional in this manner allow you to jump higher or will you break your legs?

• When you fall from a height you can't stop without a force. And it's that force that kills you.
– L.Dutch
Dec 12, 2021 at 12:56

## Short Answer: Jump Higher? Yes. Break legs? Not likely.

Based on your description, you can rethink of it like this:

Anytime I perform an action, it's creating a reaction. In your case, you want the reaction to be magically adjusted to go in the same direction as the action. This is basically doubling the reaction.

Ignoring what it actually does to the other object, this results the basic scenario of "If I apply X force to an object, the result is 2X force back to me." The other alternative is "If I apply X force to an object, the result is 2X force to it."

Either way, you effectively double the force applied.

So your question boils down to "Can I naturally apply at least half the required force to break my legs?"

General google searching says it's about "1200 Newtons in an average jump" and about "4000 Newtons needed to break a bone". I'm not bothering with sources because this is just getting a benchmark. With those numbers, doubling your jumping force isn't going to break your bones.

Add in some other variables though (like not jumping and landing in the same spot, poor landing technique, running into something) and you've got plenty of scenarios that can cause some problems.

So yeah, you can probably jump higher without immediate bodily risk. But there's also a lot of other factors and implications to consider with the scenario that would probably need to be fleshed out. But realistically, it's probably the equivalent danger of doing a full-jump off a 3ft ledge and trying to land without hurting yourself.

• So if you jump, the reaction doubles your action to propel you up. then gravity kicks in to pull you back down and the effects to counteract that action propels you down faster. Then you hit the ground with tremendous speed and the reaction that would normally break your legs would be imparted in the direction of travel, shattering the ground beneath you Dec 21, 2021 at 14:52
• @Sonvar they don’t specify exactly how this change of force is triggered, but it’s implied that some force will be applied to your legs at some point. Also, if no force is applied to you, then you theoretically would not stop, which has other issues to explain with the premise. Going with Occam’s Razor, I’m assuming the least complicated scenario to be the expected one here. Plus, just doubling the force doesn’t result in ground-shattering force from just a jump. If this isn’t new tech too, this would should likely have made minor adaptations for it anyway. Dec 21, 2021 at 16:06
• I was mostly being facetious there, making a point the OP's idea is a bit far fetched. However, my 100kg ass falling from a height at near terminal velocity (since aero drag would actually accelerated me) would cause some damage to the floor I'm standing on. Like you said, there is issues here and conventional math doesn't work. Dec 22, 2021 at 14:49

## How drag could behave

I know it's a bit off-topic but you mentionned it and it's quite interesting.

If one was moving in a fluid (air, water, etc), they would apply a force on this fluid to make it bend so that it can move. In air we only feel this at high speed, but when you try to walk in water it's much more obvious.

• If one chose to receive no reaction from the force it exerces on the fluid, then it cannot be slowed down by it. They could run at full speed under water or fly without feeling any wind.

• If one chose to let the fluid receive no force from them, then the fluid would not bend, and they would be completely trapped and immobile, even if in air. This choice may not be very useful to use on oneself...

## Ability to walk or use cars

We can walk because by pushing on the ground, the ground pushes us back forward. Same thing for a cars tires.

• If you remove this reaction, you just slip, you cannot move anywhere, like if you were on ice.

• If you do the opposite and receive twice the reaction, then you could walk, run or drive faster with the same amount of effort/energy.