# Important Forces: Air Resistance

by Jared Rovny

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00:01 Now we've discussed four big forces gravity, the normal force, tension force and friction.

00:07 And the last one we introduce will actually be very simple because we're not going to go into detail.

00:12 This is the drag force and this comes when we include air resistance to anything that's moving through the air.

00:17 What happens in the case when we include air resistance is you take something like a projectile.

00:23 We've launched the projectile, and would normally follow the black parabolic path that we've already talked about.

00:29 When we include air resistance, what you need to know, is that this object that is flying instead of getting to follow its full path will be resisted by the air resistance and cut short, and start to fall to the ground.

00:39 Since air resistance turns out to be a very complex phenomenon, there's no way we're going to try to get into all the different nuance of what the air resistance force is, or the force of drag.

00:50 So instead there are just a few things that you should know about air resistance and how it works.

00:55 First of all, air resistance depends on the velocity.

00:59 In other words, if you're moving faster air will oppose you more.

01:02 It happens to depend on the velocity square so if you speed up by double for example, the force from air resistance will quadruple.

01:11 So air resistance is what we would say strongly dependent on your velocity.

01:15 The fact that the force is velocity dependent, regardless of the fact that it's depending on velocity square is something key to know that it is depends on how fast you're moving.

01:25 The reason this is important is because if your object is falling and picking up more and more speed through the air, if it is just a falling object, the air resistance force is going to increase until gravity is equal to that force.

01:40 In other words, if an object is falling, it has a gravitational force acting downwards like this.

01:45 If the object is moving, you will have what I call the drag force F-sub-d from air resistance acting upwards trying to fight its falling but because as you can see in this picture, there's a greater force on object downwards, that means there will be an acceleration of the object downwards so the object will pick up speed and go faster and faster downwards.

02:04 As we just said that means that the resistance from the air is going to increase because the object just increased its speed so the force will drag will be bigger than it was once before while the gravitational force stays the same.

02:17 So long as the gravitational force is still even a little bit bigger than the drag force, this argument will continue to hold, the object will pick up more and more speed as it has a non-zero acceleration, meaning that the drag force will get stronger and stronger in the opposite direction.

02:29 At some point the force downwards pulling on your object will become equal to the drag force fighting it, pulling upwards.

02:36 When these two forces are equal, the gravitational force and the drag force, your object will stop accelerating because the total force on your object is zero and therefore by Newton's second law, the acceleration of your object is also zero.

02:48 When your object isn't accelerating anymore, that means it's at some specific velocity, and it's just going to stay at that velocity.

02:54 We call that velocity the terminal velocity of your object and that is very important to know about air resistance is that as an object falls, if it has a net force downwards, it accelerates. If it's accelerating, it picks up speed.

03:07 That speed picks up, the drag force increases.

03:10 If the drag force increases until both forces are the same, your object has stopped accelerating meaning that it has reached what we call terminal velocity.

03:18 So this is the important thing to know about the drag force, not as details but that if an object is falling, so long as it has a net force downwards, from gravity, then it will be accelerating because it has a non-zero force.

03:33 If it's accelerating, it will pick up more and more speed or a greater velocity, meaning that the drag force will increase in the opposite direction.

03:40 At some point both of these forces, the gravity downwards and the drag force upwards will become equal meaning that an object has no net force acting on it and it will stop accelerating.

03:50 When it stops accelerating, it has a particular velocity that won't change and that is the terminal velocity.

03:56 The drag force is the last of the forces that we call important forces that are going to be used over and over again.

04:02 We'll be introducing a few more forces later on but these are the key ones to understand for now.

04:06 Thanks for listening.

The lecture Important Forces: Air Resistance by Jared Rovny is from the course Force.

### Included Quiz Questions

1. The drag force increases as the object speed up.
2. The drag force decreases as the object speeds up until it reaches terminal velocity.
3. The drag force is always equal to the gravitational force as an object speeds up to maintain terminal velocity.
4. The drag force is always equal and opposite to the gravitational force.
5. The drag force is always equal and parallel to the gravitational force.
1. Terminal velocity reaches when the drag force equals the gravitational force.
2. Terminal velocity is the negative acceleration for a stable object.
3. Terminal velocity is the fastest an object can ever move.
4. Terminal velocity is the square of the velocity of an object.
5. Terminal velocity is equal to the drag force at its maximum.
1. F_d ∝ v²
2. F_d ∝ v
3. F_d ∝ √v
4. F_d ∝ v³
5. F_d ∝ 1/v

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