# How are unbalanced forces even possible, given Newton's 3rd law? [duplicate]

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The notion of an unbalanced force seems to contradict Newton's third law, entirely.

For instance, apparently, if you push a rock, then an unequal force is being applied in the opposite direction with respects to friction, in your pushing of the rock. To me, this makes complete sense, though then I was taught Newton's third law, which made no sense until I got deeper into the intuition behind that. For instance, the reason why it's hard to push, in particular a heavy rock, is because of this equal and opposite force, otherwise you could push it effortlessly, literally. So, I've gone from my initial intuition being shattered, to internalizing Newton's ideas, and now nothing makes sense anymore, because my initial intuition and my newfound insight is just battling out.

## marked as duplicate by Gert, user36790, dmckee♦Dec 20 '15 at 6:58

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• "the reason why it's hard to push, in particular a heavy rock, is because of this equal and opposite force". That is not correct. The force that the rock exerts on you has nothing to do with how hard it is to get the rock to accelerate, because the force that the rock exerts on you affects your motion, not the rock's. There's no problem with there being unbalanced forces on one object, because Newton's 3rd Law doesn't apply to the situation where you are considering just the forces acting on one object (the 3rd-Law forces are there, just not relevant). – march Dec 20 '15 at 3:29
• What Newton's law does is to simply formalize that pushing something with large mass results in a small acceleration while pushing something with a small mass with equal force results in a much larger one. That's a simple observation. Why does it not make sense to you? Mass is really just a measure for inertia, while force is a measure for the product of acceleration and mass. These definitions depend on each other and the naive observation that there is a real difference between objects of different masses. – CuriousOne Dec 21 '15 at 2:41

## 1 Answer

Newton's third law and the phrase "unbalanced forces" need context. The third law states the following: Whenever object A exerts a force on object B, object B exerts an equal and opposite force on object A. Notice that there are two different objects here, and each force is applied on a different one. Put in a different way, the law says that if you sum all the forces on all the objects in the universe you get zero, but it doesn't say that if you sum all the forces on each object you get zero.

Unbalanced forces usually refers to a single object. What it means is that the sum of forces on something is not zero. For example, if you're sitting still on a chair the forces on you are balanced: your weigth points downwards, the normal reaction from the chair points upwards, and they cancel. Both of them are applied on you; they are not an action-reaction pair, which are the concern of the third law. If you now imagine you're in free fall in a vacuum (please don't put yourself in free fall for the sake of experiment), there are unbalanced forces: weight pulls you down but there's nothing pushing up to compensate, and so you accelerate downwards. Newton's third law is not violated: the Earth is pulling you down, and you're pulling the Earth up with the same force.

The rock is not a great example because it being hard to move doesn't have a lot to do with this. One of the reasons it's hard to move is friction, which is roughly proportional to weight (in most circumstances). But even if you placed the rock on ice so it slipped, it would still be hard to push because of Newton's second law: since the rock has a large mass, it acquires a low acceleration. The third law is irrelevant because it concerns the force that the rock exerts on you when you push it. If you try to push while on ice skates, you'll just go flying backwards.