1. (a) Although static and kinetic friction are NOT reaction forces, they depend on the (normal) reaction. Try this out, you'll enjoy it.
Take a ruler or any other extended object. Keep your two first fingers at the two ends at the same distance from the center. Now try and move BOTH fingers together SLOWLY.... TOGETHER... See you can't do it, they move only one at a time until they reach the center.
The reason for this is simple. STATIC friction is always GREATER than kinetic friction (That's why it is always more difficult to set an object moving but easier to move it once it has started moving).Even when you try to keep both at the same distance, one finger is at a slightly greater distance than the other. The one closer has a greater share of the weight of the ruler(or any other extended object). It therefore has a greater reaction and therefore a greater static friction and the other finger starts moving.
Now as the other finger approaches the center, the weight on it begins to increase and therefore slowly the friction on it, although kinetic starts to increase. ( Friction is dependend on the normal reaction which is related to the weight). Now that finger stops and the other finger starts to move and this process repeats like a pendulum until both fingers reach the center.
(b) We're getting a lot of things mixed up here. Let's first settle the normal reaction first. NO, normal reaction is not always equal to the weight. For example on an inclined plane, the normal reaction is not weight but a component, weight cosine theta. If there is a force applied on a block at an angle, pull or push the normal reaction is different from the weight. If it is a pull, normal reaction is the weight - force sine theta, whereas if it is a push at an angle theta with the horizontal, the normal reaction is weight + force sine theta.
In case of fluids the forces that matter are the weight, fluid resistance during motion and upthrust. Depending on the problem we may neglect one of the upward forces. For example in case of a sphere falling in a viscous fluid, you may neglect the buoyant force. In case of an object floating on water we only consider the weight and the upthrust which depends on the density of the fluid. Therefore in studying the problem of floating or sinking, you'd say an object would sink if the weight is greater than the buoyant force (also called upthrust). The object would float if the upthrust is greater than the weight.
(c)
i) The answer to that is simply you and I are too big to squeeze between the spaces between the molecule in a solid. You do have diffusion taking place in solids though but after a very long time when two metals are placed close together.
2. The reason for this is a decrease in the normal reaction. It was earlier mg, the weight but when you tilt it the normal reaction is reduced to mg.cos(theta). Therefore when you tilt the board, although 'u' the coefficient of static friction remains the same the normal reaction changes.
You'll find it interesting to see my other answers on static and kinetic friction. You may be aware that kinetic friction is less than static friction.
3. The key words in this question are CONSTANT VELOCITY. If you've spotted those words, then you know that it follows from Newton's first law that the force of kinetic friction must be 55 (cos 35) N, the horizontal component of the pull force.
You can see my answer to Newton's first law to understand more about it. Coming back to this problem, if 55 (cos 35) N is the force of kinetic friction, you must now recognize that
Force of kinetic friction is f = (mu)N
Once again I would refer to one of my other questions about kinetic and static friction and their relationship to the normal reaction on one of my other answered questions.
N, the normal reaction in this case would be the weight {W - F * (sin 35)}
Therefore you can equate the two horizontal forces and write
55cos35 = (mu){200 - 55sin35}
Giving you the result for
"mu" as 0.27 (2 s.f.)
This is within an answer you might of kinetic friction.
ii) Fluid resistance is not the same thing as upthrust. For a sphere the fluid resistance is given by the Stoke's formula F = (pi).r.v.eta, the viscosity of the fluid whereas the upthrust is given by U = 4/3 (pi) r3 . sigma the density of the fluid.
iii) Weight is the action caused by the gravitational attraction of the earth acting at the centre of gravity of the object. The normal reaction is the force of reaction caused by contact between the object and the solid surface.
iv) Mercury is 13.6 times denser than water and you would not expect to fall through mercury. When you are buoyed up even by water, imagine a force 13.6 times stronger. Don't try it though because mercury is very toxic!
v) A fluid is something that can flow.
vi) Gas molecules such as hydrogen and similar light molecules do not fall through air but actually "escape" from the air because of thier high speeds easily overcoming the gravitational forces of attraction.
vii) Can you swim through dust and soil? ... Snow is basically dust and soil dressed up with ice.
viii) Air resistance is NOT a reaction to the weight of the object. It is simply fluid resistance which is dependent on the speed of movement of an object just like solid friction is dependent largely on the normal reaction and not on the area of contact or the speed of motion.
ix) At terminal velocity, air resistance is equal to the weight of the object, the weight which is not going to change significantly if the object is dropped close to the surface of the earth. You might know that objects weigh less when they are taken very high because the acceleration due to gravity decreases with altitude.
x) The EARTH experiences the reaction force to a body's weight in free fall. According to Newton's third law as you know,
"If body A exerts a force on body B, they body B will exert an equal force on body A along the same straight line in the opposite direction".
There are FOUR facts you can figure out from this statement of Newton's third law. That is, the two forces;
1. Come in PAIRS.
2. Are EQUAL in magnitude.
3. Would be pointing in OPPOSITE directions along the same straight line, and
4. Act on DIFFERENT BODIES.
The statement more popular in text books would be that to every action there is an equal and opposite reaction. This statement does not help you to understand the implications of the third law as delineated above.
The action in your case is the weight acting on the body undergoing free fall being attracted by the Earth. The reaction to this is the body attracting the Earth in its turn towards itself. You don't sense this attraction because the Earth is much too massive for you to see any appreciable change.
where N is the normal reaction.