Newton’s 3rd Law
This states that forces occur in pairs. If I exert a force of 10N on a weight pushing it up, it exerts a 10N force on me pushing me down. The two forces are equal in magnitude and opposite in direction. Note that one is the force I exert on the weight and the other is the force the weight exerts on me.
If I jump off the table, the earth exerts a force of 750N on me in the downward direction as I fall. This force causes me to accelerate downward. At the same time I exert a force on the earth of 750N in the upward direction. This force will accelerate the earth upward. Note that the force on me accelerates me and the force on the earth accelerates the earth. (Since a = F/M, my acceleration will be large, about 10m/s2 because my mass is small, 75kg. The earth’s acceleration is very small because its mass is very large, about 6x1024kg.)
Friction
It always opposes the sliding motion of one surface past another. The force is parallel to the surface. In some cases friction is a hindrance, but in other cases it is essential. For instance it is difficult to walk without friction, e.g. on wet ice there is very little friction and it is very hard to walk.
If I set a book on a level table, the book will just sit there. It is not being accelerated. However, there is a gravitational force on it pulling it down. If the net force is zero (no acceleration) then there must be another force acting on the book to counteract the gravitational force. This "other" force is due to the table that pushes up on the book to keep the book from "falling through the table". This is called a normal force because it is perpendicular to the table's surface. (A line perpendicular to a surface is called a normal to the surface.) In this case the normal force is equal in magnitude to the gravitational force and opposite in direction. (The two contact forces the table can exert on the book are friction, parallel to the surface, and the normal force, perpendicular to the surface.)
Terminal Velocity or Terminal Speed
If I drop a ball off a tall cliff, it will accelerate as it starts to fall. If gravity were the only force it would continue to increase its speed by 10m/s every second, but it does not continue to increase. The air resists its motion through the air and this force (called drag) gets larger as the speed increases. When the speed gets large enough this force cancels the gravitational force on the ball so the net force on it is zero and the acceleration is zero, i.e. the velocity, and the speed are constant, This speed where the forces cancel is called the terminal speed. The terminal speed of an object depends on its density and size. For objects of the same density, the smaller one will have a smaller terminal speed. That is why fog droplets, which are very small, have small terminal speeds compared to rain drops, which are larger.
What happens when I have a large weight on a piece of paper and suddenly jerk the paper out from under the weight? Normally, the weight stays close to where it was and the paper slides out from under the weight. Why does the weight remain behind? The main reason is inertia. The paper undergoes a large acceleration and it would require a large force to produce the same acceleration of the weight. The friction between the weight and the paper is not enough to produce that acceleration. Therefore the inertia of the weight keeps it close to where it started.
A similar effect occurs when you tear paper towels off a roll of towels. If the roll is full, it is much easier to tear them off. If it is almost empty and you yank on the towel to tear it off, the roll just unwinds. Can you explain this?
If you bump the wall with your hand, it usually does not hurt too much. However, if you have a large mass in your hand, it often hurts when your hand bumps against the wall. Your empty hand has less mass and less force to produce the acceleration to stop it. The larger mass requires a larger force.
If I punch an 8"x11" piece of cardboard with my hand, it doesn't hurt. If I punch a 8"x11" piece of wood (1" thick) it probably will hurt. What is the difference. The main difference is the mass. The wood has a much larger mass and it takes much more force to accelerate it when you hit it. The cardboard has much less mass (and it deforms easier) so the force necessary to accelerate it is smaller. (Large forces hurt!)
If I jump off a table and land on the ground, I bend my knees as I land. Why? I have a certain velocity just before I reach the ground. If I lock my knees and hold them rigid, I will stop very quickly (reduce that velocity down to zero in a short time). This will require a large acceleration and therefore a large force. If I bend my knees, I allow myself to stop over a longer time which means my acceleration is smaller. This reduces the force needed to stop me. The same principle applies to padding. The foam deforms as you hit it and the deformation means you have a longer time to change your velocity and consequently a smaller acceleration and smaller force. People often use the term absorbing the shock. That not a good expression, since you are not really absorbing anything, but rather reducing the force by stopping over a longer time.
If two cars collide, they exert forces on each other. By Newton's 3rd Law, the forces they exert on each other are equal in magnitude, but in opposite directions. If these forces are the main ones during the short time of the collision, i.e. they are larger than the frictional forces exerted on the cars by the road, we can consider the acceleration of the cars to be due to these forces they exert on each other. If this is true, the car with the smallest mass experiences the larger acceleration. In fact, a = Force/Mass for each car, or F = Ma. If the forces are the same magnitude, M1a1 = M2a2. This means that the car with the smaller mass experiences the larger acceleration, since a1 = F/M1 , and a2=F/M2 .