How does unbalanced force ever create?

Question

This question rather seems elementary but has been bugging me for a while! How does even unbalanced forces create? The main driving force of this question is Newton's third law

Let's take Free Fall:
It happens because the earth attracts the body and hence the body has an unbalanced force acting upon it, now we have the air molecules right, if the air molecules were to be in the way then by newton's third law the force exerted on the molecules must be the same force exerted back on the body, then if the person is falling with the force of gravity the force of gravity is exerted on the air molecules hence an equal and opposite force must be given to the person hence the body must not move (assuming this non realistic phenomenon happens at $t=0$ seconds), hence no unbalanced force is formed, but we know this isn't the case, What am I missing here?

Answer 1

Third law force pairs don't act on the same object. The third law states that if one object exerts a force on another, the second object exerts an equal and opposite force on the first. It doesn't state that if some force is acting on one object, an equal and opposite force acts on that object, as that would imply that net force is always 0 and acceleration is impossible, which is of course not the case.

Imagine two people standing on ice who push off each other - each pushes the other with an equal force, but each feels their own unbalanced force pushing them backwards. The third law doesn't suggest that it's impossible for there to be a net force on some object, just that there's always some reaction force on some other object. If A pushes B, B pushes A.

The earth pulls the skydiver down with the same force of gravity that the skydiver pulls the earth up. Drag from the air is a friction force that applies a force upward on the skydiver, but the skydiver also applies an equal and opposite force on the air - as air drags the skydiver up, the skydiver drags air down. Each of these force pairs must balance within the pair, but the pairs may be different from one another.

When the skydiver first jumps out of the plane, they are pulled down by gravity but don't experience much drag, so they accelerate toward the ground. As their speed increases, drag increases, and eventually becomes equal and opposite to gravity, at which point the skydiver has achieved terminal velocity and stops accelerating.

Answer 2

...hence an equal and opposite force must be given to the person hence the body must not move...What am I missing here ?

During free fall it is not the force of gravity that exerts a force on the air. The person exerts a force on the air that is equal and opposite to the force the air exerts on the person per Newton's 3rd law. But the motion of the person is based on the net force acting on the person, which is the sum of the downward force of gravity acting on the person and upward resisting force of air acting on the person, called air drag.

Air drag is a variable that increases with increasing speed. Initially the resisting force of air drag is much less than gravity and the person accelerates downward. But as the person's speed increases so does the air drag force. When air drag increases to the point where it equals the force of gravity on the person the net force on the person is zero and the person ceases to accelerate. But the person continues to fall at constant velocity, called its terminal velocity.

Hope this can help.

Answer 3

This is a really common mistake people make when they first learn Newton's Third Law. I think it's partly because teachers often say the law badly.

Newton's Third Law says "if object A exerts a force on object B then object B exerts an equal but opposite force on object A." That's all.

In your example we have three objects. Object A is the skydiver. Object B is the Earth, and object C is the air.

Newton's third law says that, because the Earth exerts a force on the skydiver, the skydiver also exerts a force on the Earth. $F_{\text{Earth on skydiver}} = F_{\text{skydiver on Earth}}$. Newton's third law also says that because the air exerts a force on the skydiver the skydiver also exerts a force on the air. $F_{\text{Air on skydiver}} = F_{\text{skydiver on air}}$.

The other key is that Newton's Second Law says that the motion of the skydiver is determined by the forces on the skydiver. Forces that act on the Earth do not affect the skydiver! Forces that act on the air do not affect the skydiver! $$ma_{\text{skydiver}} = F_{\text{Earth on skydiver}} + F_{\text{air on skydiver}}$$

Finally, notice that Newton's third law does not give us any relationship between $F_{\text{Earth on skydiver}}$ and $F_{\text{air on skydiver}}$. That's just not what Newton's third law does. Newton's third law never tells you about multiple forces acting on a single object. It only tells you about what's happening on other objects. It is never useful for determining the motion of a single object.

Answer 4

If A exerts a force on B, B exerts an equal and opposite force on A. Both A and B are accelerated.

In this case, a falling skydiver pushes air out of the way, accelerating the air downward. The air exerts an equal and opposite force on the skydiver, slowing his fall.

The skydiver falls at a constant velocity because drag increases with velocity. Gravity accelerates him to higher and higher velocities until drag grows big enough to match the force of gravity. The total force on the skydiver, gravity + drag, adds to $0$. This is different from 3rd law equal and opposite forces.

Answer 5

While there are several excellent answers already, I can't help but throw another answer into the ring in order to emphasize different aspects of the same situation:

1. There is a gravitational attractive force between the earth and the falling body. This is one "action/reaction" pair: The earth gravitationally attracts the body and the body gravitationally attracts the earth.

2. At an atomic scale, the electrostatic repulsion of the air molecules and the molecules in the falling body repel each other. This is the second "action/reaction" pair: The air molecules repel the body molecules and the body molecules repel the air molecules.

3. Incidentally, there is also a gravitational attraction between the air molecules and the body. But that is insignificantly small. Plus, the air molecules above the body will gravitationally attract in the opposite direction than the air molecules below the body. So this effect can be ignored.

Now, we know bodies do fall in air. So we know the gravitational attraction between the earth and body overwhelms the electrostatic repulsion between the air and the body. That's almost all there is to it. Except:

Why does the gravitational attraction overwhelm the electrostatic repulsion? That's hard to say since I'm sure there are a lot of complexities involved in the fluid flow of the air. It's probably not just that the gravitationally attractive force between the earth and the body is stronger than the electrostatic repulsion between the body molecules and the air molecules. There is also the effect of the fluid air molecules being able to flow around the falling body. They get pushed out of the way, so they can't counteract the gravitational attraction as effectively.

You can compare this to the stereotypical situation of a book laying on a table. It's mostly the same situation: The earth and the book attract each other gravitationally. And the atoms in the book and the atoms in the table repel each other electrostatically (the so-called "Normal" force). But in this case, the table isn't a fluid. It's atoms don't move out of the way as easily as the atoms of the air. So the normal force counteracts the gravitational force and the book doesn't fall.

Finally, an intermediate situation: A boat floating in a lake. Same situation: The boat and the earth gravitationally attract each other (one action/reaction pair). And the atoms in the boat and the atoms in the water electrostatically repel each other (the 2nd action/reaction pair). The boat does "fall" through the water, but only until it displace enough water to match its own weight. At that point the upward force of the electrostatic repulsion matches the downward force of the gravitational attraction and the boat floats.

I've left another answer about why the "pairs of forces" interpretation of Newton's 3rd law leads to confusion. If you think of Netwon's 3rd law as "Single forces must always operate between pairs of bodies" (as done above) I feel like doing that makes these situations easier to analyze.

Or, at the very least, remember that action/reaction pairs must always be the same type of force. From the body's point of view, the electrostatic repulsion of the air is not the reaction force to the gravitational attraction of the earth because they are different types of forces.

Answer 6

You are right, once air establishes equilibrium, you‘ll have $F = m \cdot a = 0$, which means $a=0$, which means $v>0$, as it took a while.

E.g. sky jumpers fall at a constant speed of roughly $250$ kmh in the end.

Just remember $v=a \cdot t$ as an approximation.