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.
I believe the question is “If static friction is being attributed to adhesion alone, then shouldn’t it take a force correspondingly greater than an object’s weight to lift the object from a surface?” Is this correct?
If so, I agree. Atomic force microscopy experiments with probes touching surface also support this conclusion.
Of course, if one wishes only to describe the horizontal and vertical force components for an object that remains on a surface, then vertical detachment is irrelevant. The normal force is unaffected by the presence of any adhesion. In this way, the authors are describing relevant effects within their scope of interest.
In addition, a more sophisticated model of sliding friction involves deforming those asperities, not just detaching from them, and this deformation resistance is largely absent when lifting the object from the surface. Thus, it would be more misleading for an author to report that sliding and lifting are subject to the exact same effects.
So I don’t agree that not mentioning all implications of a model is somehow an attempt to deceive you or is evidence that the model is incorrect.
If you study contact probe microscopy, you’ll find due attention given to adhesion forces arising from separation detachment, as shown in your last three images.
Best Answer
It seems like you are misinterpreting the words 'applied' and 'normal'. You are thinking that applied force is some different type of force from normal force. You are thinking that there is some 'applied' force plus 'normal' force acting at same time between your foot and ball and that's where you are confused.
In reality the 'normal' force is the actual 'applied' force and it is the 'normal' force that is causing the motion of ball. The site you provided seems to be wrong because currently I am unable to remember any type of contact 'applied' force that is different from other mentioned contact forces.
Your foot apply normal force on ball and in return ball apply same amount of normal force on your feet that you feel when kicking. The normal force applied on ball cause an impulse and the ball start moving.