Learning about forces and frictions at the moment, one things I can't seem to grasp is what is the difference between the normal and reaction force? They both act perpendicular to a surface and away from it, but what is the difference between both of these forces? I have tried searching this up on the internet to no avail unfortunately.
Newtonian Mechanics – What is the Difference Between Normal and Reaction Force?
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At the beginning I found this question a bit naive. But now I think it is worth to think a little bit about it.
They definitely share some properties, at least at the microscopic level. Both have the same microscopic origin: the electromagnetic interaction. The upthrust requires gravity to create a pressure gradient on the fluid, resulting in an upwards net force. The normal force that a surface does on a body resting on it also has gravity as a "trigger", though this is not always required. If someone presses the body against the surface then the correspondent parcel of the normal force has nothing to do with gravity.
From the macroscopic point of view they are quite different. The upthrust has always the direction of the pressure gradient. The direction of the normal force can be any, defined by the normal to the surface. The upthrust on a (completely immersed) body of volume $V$ depends only on its volume and the density of the agent (fluid). The normal force acting on the same body when resting over a surface depend on its mass instead and does not depend on physical properties of the agent (surfaces).
I think this is enough to see differences and similarities.
The normal force is the force of the table pushing back against the book. Does this have a reaction force of the book pushing back on the table? And isn't this already accounted for with the gravitational force? What?
- The book feels weight downwards. Reaction: the Earth feels a gravitational pull upwards.
- The book feels a normal force upwards. Reaction: the Earth feels a push downwards.
Another example:
- An billard ball feels a "push" backwards at impact (equivalent to the normal force before). Reaction: The other billard ball feels the same "push" but oppositely (equivalent to the push before).
If everything has equal and opposite forces on each other then how can there be net forces? Seriously that is pretty weird. For example when you push a cube with a non-constant velocity along a horizontal plane then there are net forces which is confusing because according to the third law there are equal and opposite forces on every interaction.
There is a reaction force to every force exerted, yes. Just remember that you are talking about another object then.
- The book feels the normal force, but it's reaction is not felt by the book; the reaction is felt by the Earth.
- The billard ball feels the impact "push" and thus has a net force and flies back (accelerates). The reaction is not felt by the same billard ball but by the other ball, which then also has a net force and flies off (accelerates).
When setting up Newton's 2nd law, always look at only one object/system at a time.
Lastly if you walk along a floor it pushes out against you with an equal and opposite reaction normal force. But surely if you are really heavy such that the surface breaks and you fall through then there is no equal and opposite reaction force? So what's up with that?
- If a ninja karate-chops a plank without breaking it by applying force $F$, then the plank is able to hold back with the entire same force $-F$.
- If the ninja karate-chops a plank that breaks, then he did not need all the force $F$. He only applied force up until the plank stopped reacting with the same force. After that point he did not increase his force. So the force he exerted equals the force the plank was able to do (plus the acceleration term it also causes), which is smaller than the $F$ that he could have done.
Think of the difference between throwing your karate-hand through the air vs. throwing it into a falling piece of paper vs. throwing it into a wall: When hitting the wall, you exert a force on that wall. When hitting the paper, you exert a force but much smaller (and you move it as well). When not hitting anything, you don't exert any force on anything - you just move.
Best Answer
Reaction force refers to its physical content - a response to the action of an object. Normal force refers to its direction - it is perpendicular to the surface. Many forces can be normal forces and/or reaction forces, and do not necessarily be both. The elementary physics however often uses the term normal force in a reference to the force produced by a support on an object. In this case it is both a normal (in the general sense described above) and a reaction force.