"Net Forward Thrust": What does it mean in this problem?

[scottshannon]

Homework Statement

An airplane of mass 1.2 x 10^4 kg tows a glider of mass 0.6 x 10^4 kg. The airplane propellers provide a net forward thrust of 5.4 x 10^4 N. What is the glider’s acceleration?

Homework Equations

I

The Attempt at a Solution

Let T = tension on glider from airplane. M= mass of airplane, m = mass of glider. F = force on airplane generated by propellers.
I wrote two equations based on the two free body diagrams for the glider and the airplane.
Glider : Newtons 2nd Law: T=ma
Airplane : Newtons 2nd Law: F - T = Ma
The answer is of course
F/(M+m) = a

Here is my question: The problem uses the term NET FORWARD THRUST. That is confusing to me because I took that to mean that the NET FORWARD THRUST F was the SUM of the forces on the airplane so that a = F/M. Can anyone help me with the language here? Is the term NET FORWARD THRUST just bad terminology? What am I not understanding?

 

[FactChecker]

"net thrust" is the force of the engine forward after subtracting the drag of the air being forced into the engine. So it is the force that the engine applies to the airplane.

 

[scottshannon]

but is it the net force on the airplane? If F is the net force on the airplane then the equation I wrote for the Newtons Second law for the airplane isn't correct.
If F is the NET force after all other forces have been subtracted then the Newtons law equation for the airplane should be F =Ma not F-T=Ma

 

[FactChecker]

but is it the net force on the airplane? If F is the net force on the airplane then the equation I wrote for the Newtons Second law for the airplane isn't correct.
If F is the NET force after all other forces have been subtracted then the Newtons law equation for the airplane should be F =Ma not F-T=Ma

It's just the net force from the engines. You will have to take it from there.

 

[scottshannon]

Ok I am looking for a definitive answer..thank you though ..good effort on your part...it is an important question when one writes the Newtons second law equation for the airplane.

 

[FactChecker]

Ok I am looking for a definitive answer..thank you though ..good effort on your part...it is an important question when one writes the Newtons second law equation for the airplane.

You have everything you need to solve the problem. I think you are making it more complicated than you need to.

 

[scottshannon]

My question is a simple one..is the net forward thrust of the propellers the net thrust on the airplane? You weren't able to answer it. My contention is that the word "net" is confusing because it leads one to think that it is the net force. If you read my original post you would see I already HAVE solved the problem. My question revolves around the use of the word "net". Please read the post the next time before responding...and please don't respond to this post...

 

[haruspex]

My question is a simple one..is the net forward thrust of the propellers the net thrust on the airplane? You weren't able to answer it. My contention is that the word "net" is confusing because it leads one to think that it is the net force. If you read my original post you would see I already HAVE solved the problem. My question revolves around the use of the word "net". Please read the post the next time before responding...and please don't respond to this post...

Goodness, what a response!
FactChecker was completely clear in stating that his understanding is that "net thrust" tells you what the engines do for the plane after subtracting out any losses/drag on their part. If I had to bet on it I would say he is right.
But in the context of the problem, that leaves you with a difficulty because you know nothing about other drag on the plane and glider. FactChecker is saying no, it is not the net force on the plane+glider combo, but to answer the question you have to assume it is.
So here's the choice: either FactChecker is wrong about the definition or the problem setter is. Where you take it from there is up to you, but FactChecker has done all he can for you.

 

[scottshannon]

"Net force" is usually defined as the sum of all the forces and therefore because all of the other forces have been summed it is the only force operating on the object. But this can't be the case if Newtons second law for the airplane is F-T=Ma. The previous respondent did not in fact say that the net force wasn't the only one. He never answered the question. My point is that the term "net" is misleading . My response was well deserved. When someone says "I think you are making it more complicated than you need to. " my immediate conclusion is that "you are not thinking about it carefully enough" Unless you have something new and cogent to add, there is no need to respond further.

Reference https://www.physicsforums.com/threads/net-forward-thrust-what-does-it-mean-in-this-problem.913637/

 

[FactChecker]

"net forward thrust" is usually how I defined it. It is not the only force on an airplane towing a glider. Any force that is not related to the engine is not part of "thrust", net or otherwise. There is also gravity, aerodynamics, towing force, possible inlet effects (if they were not included in the net forward thrust). Many of these would have linear and rotational forces in all 3 coordinates for both the airplane and the glider. Depending on the coordinate system and model used there may be Coriolis effects (often treated as a force). There may be others that I can't think of right now.

Since there is not enough information to calculate the other forces and moments, I assume that the engine force (and possible gravity) are the only forces you can consider in this example. Without knowing the direction of the net thrust force (airplane flight path), it is not possible to combine the engine thrust and gravity.

 

[haruspex]

Net force" is usually defined as the sum of all the forces

Newtons second law for the airplane is F-T=Ma

I think I see your problem. You are confused about systems.

In ΣF=ma, ΣF is the sum of forces (i.e. the net force) on the system, m is the mass of the system, and a is the acceleration of the mass centre of the system.
But the system is whatever you define it to be.
If you define it to be the airplane, not including its engine (I'm using the term generically for now... I'll get into how that is defined later), then ΣF is the force from the engine minus the tension in the cable, (minus any drag, but that is being ignored). Hence F-T=Ma, where F is the net thrust from the engine on the plane and T is the tension in the cable. You can then define a second system for the glider and write another equation with T in it.

If you define the system as plane+glider, still without engine, and with F still as above then we can write F=(M+m)a without having to deal with the tension. Note that in these two analyses M does not include the mass of the engine.

The question tells you the net forward thrust from the propellers. To make it clear what that means, imagine the propellers as towing the plane by another cable. The net forward thrust provided by the propellers is the force that cable exerts on the plane.
So we need to define "engine" as just the propellers. We see now that there is a small flaw in the question - in order to solve it we must take the stated mass as not including the propellers.

 

[FactChecker]

The OP asked about thrust, which is engine only. In reality, there are a lot of other forces but the question does not include any information about them. The only possible exception is gravity, which could be included since the masses are given. The OP would have to make assumptions about the direction of the thrust to combine it with gravitational force.

 

[haruspex]

The OP asked about thrust, which is engine only. In reality, there are a lot of other forces but the question does not include any information about them. The only possible exception is gravity, which could be included since the masses are given. The OP would have to make assumptions about the direction of the thrust to combine it with gravitational force.

True, we also have to assume horizontal thrust and acceleration.