… according to one of my teachers.

Let’s start off with a little bit of grade 10 physics:

There are four forces that act on an airplane: Lift, Weight (gravity), Thrust, and Drag.

All of these forces are equal in straight and level flight.

Rule of Flight #1: Lift always acts perpendicular to the wingspan (lateral axis).

So what happens when we turn the airplane? We bank our wings. Remember rule number 1? The lift component also banks to stay perpendicular to the lateral axis:

The two green arrows are the resolved vectors for the lift component (the red arrow). The horizontal green arrow is the horizontal component of lift (aka centripetal force), the green arrow is the vertical component of lift.

Newtons third law states: For every action there is an equal and opposite reaction.

And this is where (according to my teacher) is where the textbook gets it all wrong. The textbooks throw in that 5th force called centrifugal force.

Let’s go back to newtons third law. The key words in there being “action” not “force”. The law does not state: For every force, there is an equal and opposite force.

Let’s pretend for a minute that this is the case, that for every force there is an equal and opposite force, and lets resolve our vectors.

Do be doo… lets see… lift and weight will disappear.. what else.. hmm.. oh yeah, centripetal and centrifugal forces will also resolve to 0. So we’re left with something that looks like this:

So if centrifugal force existed (which, according to the textbook it does) then there would be no horizontal component of lift, and thus, no actual horizontal change in direction.

So what exactly is happening? I’m going to leave that up to you (hint: it has to do with momentum and not force). Please leave a comment with your thoughts on this, positive or negative.

## 6 thoughts on “Your Textbook is Wrong!”

1. Julian says:

Newton’s laws are only valid under an inertial frame of reference, i.e. where no acceleration is taking place.
The bogus “centrifugal force” terms are needed when trying to make Newton’s laws “work” under a non-inertial reference frame, such as a rotational reference frame as in this example.

2. Aviatrix says:

One comment:
The four forces are not “all equal” in straight and level flight. That is, a 12000 pound airplane does not produce four thousand pounds of thrust. The sum of the four forces is zero, however, and that’s likely what you meant.
And your teacher is right. That diagram in the flight manual is bogus. It’s there for people who don’t understand that forces only balance in unaccelerated flight. A turn is acceleration.

3. Blake says:

Aviatrix, Thanks for the clarification, and that’s what I meant.
It was funny. My teacher only brought this up this week (a few days after you had written your original post)

4. Anonymous says:

not true the 4 rule of flihgt don’t aply on space ships

5. Anonymous says:

not true the 4 rule of flihgt don’t aply on space ships

6. torokilopz says:

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G’night