According to Newton's third law, the forces in an action-reaction pair must have the same magnitude and opposite directions. But do they have to be the same kind of force (gravitational, electromagnetic, strong, weak)?
[Physics] Does an action-reaction pair always contain the same kind of force
forcesnewtonian-mechanics
Related Solutions
Action/reaction pairs are describing momentum flow. Momentum is a vector, so it is more difficult to explain intuitively, so you should start with money, which is a scalar. Let me call a "payment" money that enters your posession. A payment can be negative, in which case you lose money, like when you buy a hat.
Newton's third law of finance says: for any payment, there is a negative equal payment associated to it on somone else (if you aren't a central bank!). So if you have a payment of -100 dollars, someone else got 100 dollars. This should be completely intuitive, because, outside of banking, on the personal level, money is a conserved quantity.
Newton's law is the same: the conserved quantity is momentum, and the momentum is flowing between objects. The flow is called the force, and the force is the "payment", it tells you how many units of momentum are incoming per unit time. The third law says that every payment is associated with a reverse payment going the other way (just like money, except the quantity is a vector).
So when the Earth pulls on you, it is paying you downward momentum, which means that you are paying the Earth upward momentum. That's the action reaction pair. If you are on a scale, the scale pays you up momentum (it pushes you up), and you pay the scale down-momentum (you push the scale down). The end result is that the force from the Earth and the scale cancel out, and the gravitational force on the Earth from you plus the downward force you exert on the Earth through the scale cancel out, and nothing ends up moving.
This is like a closed circuit of momentum, and elucidating the way in which momentum is flowing, even though the objects don't move, is the subject of statics. Newton's laws add to this the interpretation of momentum as a dynamical quantity, mass times velocity, so when an object accumulates momentum, you know how fast it is going.
This point of view is very useful, but it is not often explicitly taught.
Where does the 'pairing' in your first pair come into place? I.e. what is the force counteracting the earth's gravity pull on you (in your theory)?
The relevant force pair in your example is the attractive force between your body and the earth (gravitational pull) and the repulsive force between your body and the chair's surface (its lack of compressability).
You can either see the chair as part of the earth in this scenario OR you can use a force chain in which the repulsive force the earth has on the bottom of the chair transfers via the chair to your body.
It's getting more complicated by the fact that thereby the force the earth has on you is mostly translated into the deformation of your body.
The application of Newton's laws is very much about abstraction and simplification or in other words macroscopic effects, that are in fact the result of a LOT of microscopic effects. (Electromagnetic repulsion vs. attraction on atomic level vs. gravity making up the bulk but not all of the forces ar work here.)
Best Answer
We're talking about the "two sides" of the SAME force, so it must be yes, they are of the same type.
For example, if I attract you gravitationally, then you are also attracting me gravitationally. There is only one physical source of the force (in this case, gravity) and it is pulling us both equally and oppositely. Since there's only one thing, it has, and can only have, one type.
Or, to put it another way, according to Newton's third law: When I push against you, you are also pushing me back. It is the same physical thing doing both pushes, in this case electrical repulsion of our atoms.
Or like this: Imaging I support a book of 1N weight in my hand (against earth gravity). I can't support that book without supplying 1N of force to stop it falling. And that force will be provided by the electrical repulsion of the atoms in me vs the atoms in the weight. The book feels that 1N force upwards, and I feel it downwards on my hand. There's only one force doing that. There's ALSO a gravitational attraction pulling the weight towards the earth, balanced exactly by the gravitational attraction pulling the earth towards the weight. So there are (at least) two instances of the third law in play.