# What causes opposite motion in Newton's third law, or is the motion matter independent?

For every action, there is an equal and opposite reaction. Sample case me doing a push up I force myself to move up by pushing against a rigid body (the floor).

There goes I can not push myself sideways by thrusting my hands forward but as the case of a pushup. Or if I hit the water with an open fist it feels painful because the movement causes the liquid to feel like a rigid body (instantiating rapid opposing motion on a body).

So with that, moving to rockets, their motion (in earth) is caused by a heavy surge of air simulating the hard body (with the open fist slap on water logic) and when enough causes the rocket to move in the other direction.

My current logic in this argument is for the opposite reaction to occur, there is an opposing opposite matter that causes makes the opposite reaction to recache in the other direction?

As I am trying to understand how something would move in a vacuum. In the case of a bird also it pushes air down for it to go up... My mechanical physics is not that great I am tackling this in a more logic-based approach.So, a case scenario definition of all the definitive forces and direction of reaction would be well appreciated and is this logic correct (for a motion to occur there must be something resisting movement).

• When Chuck Norris does a push up he stays still and the Earth moves away... Mar 26, 2020 at 21:37
• Is there any chance of entering a few paragraph breaks (2 x Enter) and fixing the capitalisation and punctuation to make your post more readable? Mar 26, 2020 at 21:37
• @SolarMike From my logic, he moves up because the earth has more resistance to motion compared to the amount of force Chuck Norris is opposing on the body. If I get a table tennis racket the 3-4 size ration of the ball to racket lets assume and use 3-4 times the force to hit earth then I would expect the earth to move. The rations are not important the table tennis logic is important in this case thus this are two bodies one resisting motion one in motion a causes B to move and since B did not like the move A is forced to move in an opposite direction Mar 26, 2020 at 22:12
• everything moves, but a lot of times that movement is too small to see. When you do a push up, the stuff under your hands does indeed move, on a VERY small scale. That's easy to visualize, throw a pea at a hanging blanket and not much happens though it does a little bit, where the pea hit. Throw a brick at it, and you'll get more movement of the blanket
– Ack
Mar 26, 2020 at 22:29

Rather than think of it as 'resisting', think of it as force. In a vacuum, you are correct that there is nothing to push against, nothing resisting. The resisting, as you put it, comes from pushing out something that was taken into space with the rocket, the gas from the rocket motor. In force terms, it takes force to push that gas out. The force is in balance. The force that is pushed out the end of the rocket, the gas in the engine, is the same amount of force that is pushing the rocket forward.

• This insinuates that the surrounding matter does not contribute to the movement of a body in space where there isn't any? Mar 26, 2020 at 23:48
• For a basic viewpoint, yes that is true in the way that you mean it
– Ack
Mar 27, 2020 at 0:31
• In 1920 the New York Times famously said that rockets wouldn't work in space because there is no air to push against. They issued a correction in 1969 when Apollo 11 was launched forbes.com/sites/kionasmith/2018/07/19/…
– Mark
Mar 27, 2020 at 4:23
• The main consideration\method that various textbooks take is the conservation of momentum approach. This is because a force is defined as the change in momentum over time. Since the net momentum is changed in the rocket example (by the gas which causes propulsion), the force on the rocket changes or is created. So essentially, it is matter dependent but the matter does not initially have to be external.
– Amit
Mar 27, 2020 at 9:04
• Wouldn't one say that the moon is quite close to the earth and the Apollo getting there could be attributed to inertia coming from ( as my logic goes ) the push from an existing atmosphere and due to no friction in space the movement wasn't stopped... And how far has a fueled engine gone from earth. What's the furthest than the moon and come back as to move out can be somewhat inertia from movement exiting an atmosphere just something that moved let's say to mid-space ( not close to anybody mass ) and then came back? Mar 27, 2020 at 10:26

This is one of Newton's Axioms. I.e., strictly speaking the rule does not need to be able to be derived from some other physical law, but an empirical fact supported by a huge number of actual and possible experiments.

This axiom (together with the second of Newton's Axioms), leads to the inertial principle - the total momentum of a closed system is constant.

To avoid the reactive force, you don't need "vacuum" but rather "no other body that exerts force on you(r system)":

• A rocket works in open space (vacuum) as it does within earth atmosphere because the gas/smoke it ejects, forms the bodies it interacts with - the farther an amount of gas is ejected, the more the rocket is driven forward.
• For your push-ups, you need the the floor underneath (so you can push the earth a bit more downwards). If you are an astronaut floating in space, a push-up is only a push-apart of your space suit (and won't help your workout :-) ). To maintain a constant total momentum, earth is pushed a bit down while you are pushed up (if you push yourself up by about 0.1 m, earth is pushed "down" by about 10-22 m, which is less than the width of an atom's nucleus.

=> If you want to move in vacuum, you must either have some "contactless" force to some peer object that is beyond this vacuum (e.g., flying in your own orbit around the sun, which attracts you by gravitation even through vacuum), or you have to eject something in the opposite direction (as your rocket does for you).

• Thanks made me do more research into this and apparently it's as you say no background to derive from just observable results this is what made me ask the question in the first place there is no citation on why just presentation on effects via experimentation. As like from this thread Are Newton's Axioms really axioms? Mar 30, 2020 at 18:56

A potentially helpful visual is imagine sitting in a very flat rowboat on a placid lake. Now throw a javelin far away from you. The javelin will fly in one direction, and you and the boat will start to drift in the opposite direction. This is because when you threw it, you imparted a force to the javelin, while it imparted an equal and opposite force to you.

Now imagine you have a tank of water in the boat with a hose, and you spray the hose off the aft of the boat. Your boat will drift forward continuously – not because of the hose water hitting the lake, but because of the hose water exiting the hose. You could do this same thing in a large pool where the hose stream goes over the edge and never touches the pool water, and the result would be the same.

The rocket in space is exactly the same scenario, ejecting gas in one direction to be propelled in the opposite direction. The fact that it happens in atmosphere or in vacuum is not relevant.

Imagine you are a young astronaut, my favorite is Niel Armstrong, doing some repairs outside on your spaceship antenna and once you finish your job, to your horror, you realize your personal rocket is jammed.

Cool as ice and confidently you reach for the safety cord on your waist. Now you really shudder; the safety rope connection, which accidentally became disconnected when you were fighting your way with the big heavy wrench, is floating in the space some 20 feet away and is slowly going away.

you are racking your brains, what to do??

Then the spark ignites in your brain: Newton's third law.

You grab the heavy wrench from your belt and toss it violently in the opposite direction of the safety rope into the empty space.

I am sure you could see it in your mind. What's going to happen?

Edit

I decided to plug in some crude numbers here and respond to some comments by the OP. we assume there is no air, total vacuum and obviously no friction.

let's say the astronaut has done enough spacewalk that he knows to lean horizontally with his feet directed to the safety cord and throughs the wrench straight out from as close to his heart as to receive the least torque.

Let's say the force of his hand on the wrench is 30kgs and the length his hand swings before releasing the wrench is 75cm.

As per Newton's third law his hand, hence his entire body, absorbs exactly equal force in the opposite direction.

Assuming the astronaut with his spacesuit weighs 120kgms the force will accelerate his body back towards the safety cord.

$$W_{work}=E_{kinetic-energy}= \quad F*x=30kg*0.75m =22.5kg.m$$

speed of astronaut towards the cord is= V and can be calculated by his kinetic energy

$$E=22.5=1/2mV^2 \rightarrow V^2=22.5*2/m=45/120=0.375m/s \\ V=\sqrt{.375}=0.61m/s$$

And the astronaut will reach the cord in 20/.61= 32 seconds, without any friction or rocket. Just by ejecting the wrench and using the reacting force to his advantage.

• Are there any experimental situations that have been done within a vacuum and not with gases as the gas will fill the vacuum to fast and the vacuum will be compromised as a result Mar 27, 2020 at 9:16
• Imagine a trampoline when you jump on it you are thrown in a different direction as the rubber tries to regain its normal state by resisting your current movement state. The distance you will move up or down will be dependant on how hard you jumped on it ( force applied ) and due to the rubber resisting the change you are moved in a different direction the resisting factor here the nature of the trampoline... I do agree you do move the why of the scenario is what's interesting. Mar 27, 2020 at 9:35
• Because the laws also define that bodies tend to resist change from their current state is there a relationship between the body's resistance to change, to the amount of force applied in the opposite direction on a body trying to affect the change of a body. Mar 27, 2020 at 9:35
• And I do agree there will be motion. Trying to understand if there is motion isn't it do to something and if so isn't the cause the resistance to change state and if so isn't the resistance to change highly attributed to friction? Mar 27, 2020 at 9:38

Newton's Third Law is a consequence of the Principle of Conservation of Momentum, a law of conservation as fundamental as the laws of conservation of mass and energy.

When an object A applies a force on object B, object B exerts a force over A that is equal in magnitude and opposite in direction. At the same time, the change in momentum in object A is equal to minus the change in momentum in the object B. That way, the total change in momentum of the whole system is zero, momentum is conserved.

In symbolic terms:

$$F_{A\to B} = - F_{B\to A}$$ $$\frac{dp_B}{dt} = - \frac{dp_A}{dt}$$ $$\frac{d}{dt}(p_A + p_B) = 0$$ $$p_A + p_B = \text{constant}$$

As for your reasoning that Newton's 3rd Law (and by consequence, the conservation of momentum) depends on a material media... if you look from close enough, everything is a vacuum. An atom is mostly empty space, and yet the force a proton exerts over an electron is equal in magnitude and opposite in direction to the force the electron exerts on the proton.

You need to have matter in order have forces ($$F = ma$$) and momentum ($$p = mv$$), but you don't need to be around matter in order to Newton's Third Law make sense. Being in a vacuum or within material media is completely irrelevant for that matter.