Question: If everything follows Newton's third law that for every force there is an equal reaction force, how can things accelerate and move?

  1. You can accelerate things by doing work, right? If I push something along the floor I don’t move backwards because of friction. Friction is just lots of little tiny electrical forces between atoms, and they’re very strong. The force you apply to an object is actually just a force against two things, and the smaller one moves.

    Technically speaking if I push a box along the floor, friction transfers a force from my shoe to the ground, making the earth rotate under my feet to enable me to push. But the earth’s big, right, so you don’t notice… If you were standing on a ball the size of a car or something and tried to push something around it, the ball you’re standing on would move.

    If you use a rocket, or fuel, then you can make movement happen by using a “ratchet” and converting the energy into motion in one direction rather than another by a similar idea – a system of gears that goes one way. A rocket ejects stuff really fast in one direction, making something move in the other direction.

    I think I know what you mean, but if you tally up the numbers, it will work out every time 🙂


  2. Oh heavens. You wrote the last physics exam I took at uni didn’t you? Really, this is a common exam or assignment question in physics.

    Imagine if we go iceskating. If I pull on you, and we just look at you for the time being – you will
    accelerate and move forward. My hand is exerting a force on you, and yours is exerting a reaction force in the other direction, but what happens to me doesn’t matter. There is a net force on you and you move forward (technically I’m applying a force external to your system). So by Newton’s second law you start to accelerate and Newton’s third law is upheld as there is an equal force acting against me.

    If we look at us together, when I push against the ice and pull you along, you move forward when I move forward. The two forces we are exerting on each other do cancel each other out in ‘our’ system – you don’t accelerate towards me, and I don’t accelerate towards you (there are no external forces acting on the system, only internal ones). The force that I’m applying against the ice is what is pushing us along, which is a force external to us. There is an equal and opposite force acting on my foot from the ice, but I’m not to worried about what happens to the ice.

    Or another way to look at it. When we were ice-skating and standing still. When I pulled you towards me, you exerted a reaction force in the opposite direction. End result, we both moved towards each other. The forces cancel each other out – you just need to be careful which system you talk about.

    It’s a good thing that we’re only imagining this because in reality, I suck at ice skating.


  3. Normally, how we move is that friction allows us to push against something. As we step forward, we actually push what ever we are stepping on backwards. We see this on small boats. As we step forward, the boat is pushed backwards. But when the thing we are stepping on is big, big, big, then it does not move much so by comparison we accelerate and move a lot. Again we can see this when stepping off a water vessel, but if we are stepping off a huge ship, yes, we still push it backwards but it doesn’t move much. It depends on whether we have more mass or if the thing that we push against has more mass. The less massive object moves more, which is why we can step off the ship or take a step on the earth – the ship or the earth moves a tiny amount while we move a lot.