Lift and stuff.

[APenNameAndThatA]

2 hours ago, old man emu said:

aro, I am crying tears of blood. You keep saying that I know f-all about physics. I cry because you know f-nothing. 

 

In physics, a force is any interaction that, when unopposed, will change the motion of an object. A force can cause an object with mass to change its velocity (which includes to begin moving from a state of rest), i.e., to accelerate. Force can also be described intuitively as a push or a pull. A force has both magnitude and direction, making it a vector quantity. It is measured in the SI unit of newtons and represented by the symbol F.

 

Characteristics of Force:

Forces are due to an interaction of at least two objects.

It may change the state of motion of an object.

It may change the shape of an object.

 

The very definition of Force involves a change in velocity over time (acceleration)  F = m.a  Force is proportional to acceleration, which is defined as the rate of change of velocity. It is also proportional to mass for a particular acceleration, but we don't usually talk about varying the mass, except if we are talking about stall speeds at different amount of load a plane is carrying, but let's not go there right now.

 

Work is the energy transferred to or from an object via the application of force along a displacement. Work is a scalar quantity, so it has only magnitude and no direction. Work transfers energy from one place to another, or one form to another. The SI unit of work is the joule (J), the same unit as for energy.

 

To calculate the amount of work done we use the equation W = Force . distance (W = F.d = m.a.d) The units of Work are Joules which have the dimensions

which is the result of multiplying mass by acceleration by distance

 

In your example of a box being pushed across the floor, consider Newton's Third Law - equal and opposite forces.

If you want to move the box in a certain direction, you apply a force to it. Because of the surface of the box and the floor are not perfectly smooth (an impossible situation, ever an molecular level) they lock together.

To get the box moving you have to overcome that mechanical interlocking. That resistance to movement due to mechanical interference is called Friction, which is a Force acting in the opposite direction to the desired direction of movement. The level of friction that different materials exhibit is measured by the coefficient of friction. The formula is µ = f / N, where µ is the coefficient of friction, f is the amount of force that resists motion, and N is the normal force, or force acting at right angles to the desired direction of motion. In this case, it is the force of gravity.

 

Moving the box involves exerting a Force that is equal to Force to overcome the Friction force. Once that force has been overcome, the box will move and Work will be done. To keep the box moving you have to apply a force that is greater that Friction force. Since the force due to gravity never changes while you are applying the force to overcome Friction, you can assign it a value of 1 in calculations and add the real value in at the end of the calculations. Perhaps this diagram will help.

 

 

 

By your definition, if I am forcing a box steadily across the floor, I am not exerting a force on it because it is not accelerating. I think that we can agree that that is wrong. 

 

Also "To keep the box moving you have to apply a force that is greater that Friction force." Actually, the same as the friction force. If it was greater than the friction force you would accelerate the box. 

 

I also think that it is poor form to accuse someone on knowing f-ing nothing when you repeatedly state that something with a mass of 0.3 kg has a weight of 3 kg. 

[APenNameAndThatA]

Aro is right, BTW. You can have a force without anything being accelerated. At first I thought that Aro was being picky when he picked you up on the following. 

 

"I think we agree that the magnitude of a force is the result of the acceleration of a mass, which we can calculate using the equation F = m.a"

 

But on reflection, he is quite right.  Force might be expressed in in terms of acceleration, but clearly force can exist without a mass being accelerated. If I sit a book on a desk, it is not accelerating but the book and the desk are each exerting a force on each other. If you want to go around teaching people, you need to make sure that you are right. 

 

What Aro said was actually *important*. I used to be confused and thinking to myself, "If the gravity is accelerating the book on the desk at 9.8 m/s/s, then how come it isn't moving." The answer is that that amount of force would accelerate the book if there was not contrary force. Force can be expressed in terms of the acceleration that it would produce if it was unopposed, but it is not defined in terms of acceleration but in the units newtons. ***a newton is not an expression of acceleration but of force***.  

 

And, to repeat myself, I think it is poor form that you would ask a question, have people go to the trouble on answering it and not attempting to come up with a consensus or even reflecting on the correct bits of their answers. One answer in particular about the change in the amount of lift of a wing because of diagonal flow across the wing was changed. It means that if someone is side slipping steadily that the wings will be producing unequal lift. There are questions about pilots changing AoA, it happening automatically. There are questions about the roll axis being different from the axis along whitch the AoA is measured and that being different from the angle of incidence. It brings up the question of how much pilots doing hesitation rolls are motivated to not lose height and how much they are motivated to no move laterally across they sky. Thanks for nothing. 

 

Furthermore, when you said that the answer was C or "same", all you had to do to explain what you meant was  "Lift is defined in terms of being at right angles to chord of the wing". That means that a) Your answer was wrong, because it was too simple, and b) even if you accept the assumptions that would have made your answer right, your explanation was wrong. So, you have no business telling others that they know "f-g nothing". 

Edited April 1, 2021 by APenNameAndThatA

[Jabiru7252]

You can have a mass of 0.3kg but on the moon that mass will weigh only 0.05kg if my memory serves me correctly.

[facthunter]

Nothing that I write comes second hand OR from the net.. It's stuff that has saved my life a few times over what is now a lot of years. When some of you people talk about inadvertent stalls it scares me. Any inadvertent stall is potentially the end of anyone on the planes life.   I've never done one and hope to never  do it. (I know some here won't believe me) Too bad as  I know what the facts are and that's what matters to me.. I'm fortunate to have done a lot of LIMIT flying in planes capable of not coming apart under me.  That gave me a grounding few can get these days. I also survived with the help of some luck. That's for sure.. The planes did have lots of problems Systems failures Icing systems didn't do much and Big pistons destroy themselves quite thoroughly sometimes and were generally underpowered even with them all running.

    . Instructing is my greatest developer. It taught me to get the plane out of something I myself would never put it into.  Also, if the student subsequently comes to grief I would hate to have on my conscience the fact that My deficient instruction may have contributed to it..  I've flown damaged planes when things went wrong in the air but never damaged one. Some fellows and girls have near incident free careers. Others are on board when things go wrong, You never know what Lady? Luck will dish out till it happens. You don't know how you will respond till it happens, but the better you've prepared your self the better the outcome. I'm not a believer in the "When your times up" thing. Cracks and errors are real. The Laws of physics can't be ignored to fit some theological timetable.. 

  NEVER stop Learning. Nev

[old man emu]

" it turns out that the only thing that you were interested in was that sine 45 = 0.7." When and where did I ever indicate that I did happen to use a value of 45 degrees to illustrate a position of a wing relative to the longitudinal axis of an aircraft, or to indicate an angle relative to the longitudinal axis of a wind tunnel that an aerofoil could be placed?

 

I repeatedly said that my point was not related to anything more than that all the diagrams we are shown about how the Lift force produced by an aerofoil can confuse when you consider diagramming the Lift produced by an aerofoil from plan view as it is placed at different angles to the air stream coming at it. It was other people who wandered off talking about what happens when the aerofoil is attached to a fuselage and taken into the air to bank and roll and loop.

3 hours ago, APenNameAndThatA said:

if I am forcing a box steadily across the floor, I am not exerting a force on it because it is not accelerating

And we can have perpetual motion machines and free energy.

 

When the box is on the floor there is a mechanical interaction between the floor and the surface of the box that is in contact with it because both surfaces are not absolutely smooth even to the molecular level. This mechanical interaction prevent the two surfaces moving against one another and is called Friction.

 

In order to slide the box, that Friction force has to be overcome. Say you want to move the box from left to right. You apply a Force to the left side of the box, so you create a Force vector acting from left to right. In accordance with Newton's Third Law, the mechanical interaction creates a force vector acting from right to left. Since at all times the force on the box arising from the acceleration due to gravity is the same, we can recognise it mathematically by assigning a value of one unit of force to it.

 

Therefore, to move the box we have to apply a Force to the left side that will overcome the Friction force acting to the right. I've spent too much time trying to explain this, so you can do your own research and read this: https://x-engineer.org/undergraduate-engineering/physics/solid-mechanics/how-to-calculate-friction-force/

 

3 hours ago, APenNameAndThatA said:

you repeatedly state that something with a mass of 0.3 kg has a weight of 3 kg. 

An African forest elephant has a weight of 25,000 N

What is the mass of the African forest elephant?

Weight is another word for the force of gravity mg. We can solve for the mass using the formula W=Fg=mg. (use the formula for weight)

 

25,000 N=m(9.8 s2 m)(plug in values for weight and g)

m=25,000 N/(9.8 m s^-2)(solve for mass m)

m= 2,551 kg

https://www.khanacademy.org/science/high-school-physics/forces-and-newtons-laws-of-motion/newtons-first-law-mass-and-inertia/a/what-is-weight

 

And if you don't believe that, ask Jabiru7252. He understands.

 

 

3 hours ago, APenNameAndThatA said:

You can have a force without anything being accelerated.

I bet you can't explain that statement 

 

 

[facthunter]

Run the engines up on the brakes then release them. The thrust doesn't change. IF the brakes dragged the excess thrust would cause the plane to assume a certain speed. When the brakes are released the excess  thrust does more accelerating. but some of it is used overcoming the rolling friction and increasingly DRAG. The increasing drag will eventually stop the acceleration.. It doesn't stop the plane going at it's maximum velocity when it's in equilibrium. Nev

[Thruster88]

OME you could have just asked is anyone confused by the cross section lift diagram. I doubt that anyone would be. Changing the lift vector is the method used to keep the aircraft aligned with the centre line on final approach. Small subconscious adjustments of the lift vector with aileron control, everyone does it. 

Edited April 1, 2021 by Thruster88

[aro]

10 hours ago, old man emu said:

In physics, a force is any interaction that, when unopposed, will change the motion of an object.

 

Quote

The very definition of Force involves a change in velocity over time (acceleration)  F = m.a  Force is proportional to acceleration, which is defined as the rate of change of velocity.

You just defined force yourself, with the qualifier "when unopposed". "When unopposed" is the important bit.

 

F = m.a

BUT

F is the vector sum of all forces acting on the mass. We can only calculate a force using F= m.a if there is only one force, or all other forces are known.

 

It is very common e.g. for there to be 2 forces perfectly opposed, and therefore acceleration is zero, net force is zero, but significant and measurable force is being applied.

 

An example is standing on a set of scales. The scales measure a force, but there is no change in velocity.

 

If I use a spring compressor and apply a force of 2000 newtons and compress a spring by 50cm, I have applied a force. I have done work, as evidenced by the energy stored in the spring.

 

Where is the mass? Where is the acceleration? We know the value of F, but there are no values for m or a to use in the equation.

 

If the compressor suddenly breaks, we will have mass (the spring and parts of the compressor) and acceleration. F = m.a suddenly applies. Force is the potential to accelerate a mass when unopposed (as you said in your first definition) which is why the units of measurement incorporate mass and acceleration.

 

It does not mean that you can't have force without acceleration - which is what defining force as F = m.a implies: if a = 0, F = 0.

 

To repeat myself, to apply F = m.a, F must be the sum of all forces. The equation doesn't tell you the value of any specific force if there is more than one.

[APenNameAndThatA]

So can we just straighten this out? Does something on Earth that has a weight of 3 kg have a mass of 0.3 kg. Before you said it did. I don’t know what you think now. 

[APenNameAndThatA]

hours ago,  APenNameAndThatA said: 

You can have a force without anything being accelerated.

I bet you can't explain that statement 

 

It is easy to explain that statement. If I sit a 1 kg weight on my desk, it exerts about 10 N force on the desk but neither it nor the desk are accelerating. 

[APenNameAndThatA]

8 hours ago,  APenNameAndThatA said:

if I am forcing a box steadily across the floor, I am not exerting a force on it because it is not accelerating

 

Obviously I was demonstrating that you are wrong to suggest that if something is not accelerating then no force is being applied.

 

Anyway, please answer directly about the 3 vs 0.3 kg. 

[Jabiru7252]

If you push a box across the floor at constant velocity you are exerting a force - just enough to overcome friction. If no force were applied to the box then friction would slow it down to a stop. In a vacuum however, with the force removed the box would continue in a straight line at constant velocity.

[Jabiru7252]

58 minutes ago, APenNameAndThatA said:

So can we just straighten this out? Does something on Earth that has a weight of 3 kg have a mass of 0.3 kg. Before you said it did. I don’t know what you think now. 

I'm dazed and confused, but; An object with a mass of 3kg will have a weight of (about) 3kg on Earth. Gravity changes a little depending on where you are on the surface of the Earth. The mass never changes, no matter where you are in the universe.

[old man emu]

3 hours ago, APenNameAndThatA said:

If I sit a 1 kg weight on my desk, it exerts about 10 N force on the desk but neither it nor the desk are accelerating.

And the materials and structure of the desk are able to exert a force of 10 N as an equal an opposite force. Try putting 1000 kg on the desk and see if the 1000 kg accelerates downwards.

[APenNameAndThatA]

5 hours ago, old man emu said:

And the materials and structure of the desk are able to exert a force of 10 N as an equal an opposite force. Try putting 1000 kg on the desk and see if the 1000 kg accelerates downwards.

We need to hear about the 3 kg vs 0.3 kg, please OME. 

[APenNameAndThatA]

OME, you stated before that something with a weight of 3 kg has a mass of 0.3 kg. I am asking you if you still believe that. 

[old man emu]

Sorry that I didn't jump to reply to your demand for a reply, but I've wasted too much time trying to show someone who has no understanding of the manipulation of algebraic equations and a complete lack of primary school arithmetic ability to know that three divided by ten is the decimal fraction 0.3 .

 

How you are ever going to be able to do the simple calculations required for aerial navigation has me stumped.

[facthunter]

We shouldn't be into personal issues. It's not Prune .  Nev

[APenNameAndThatA]

17 minutes ago, old man emu said:

Sorry that I didn't jump to reply to your demand for a reply, but I've wasted too much time trying to show someone who has no understanding of the manipulation of algebraic equations and a complete lack of primary school arithmetic ability to know that three divided by ten is the decimal fraction 0.3 .

 

How you are ever going to be able to do the simple calculations required for aerial navigation has me stumped.

So you are saying that something with a weight of 3 kg has a mass of 0.3 kg? 

[facthunter]

Maybe on Jupiter?  Nev

[old man emu]

What's the point of posting links to explanations given by people other than myself when the enquirer obviously won't follow them?

[APenNameAndThatA]

20 minutes ago, old man emu said:

What's the point of posting links to explanations given by people other than myself when the enquirer obviously won't follow them?

I did follow the link. I am clarifying that you still say that something with a weight of 3 kg has a mass of 0.3 kg. 

[onetrack]

I've kept out of this fight up until now, but I feel the need to correct the idea that weight and mass always should be the same.

 

They're not, and the reason is; "weight and mass are different physical quantities, although they are closely related" ...

 

https://cnx.org/contents/aAA3IrFE@4/Mass-and-Weight

 

AFAIC, weight is the laymans everyday term, mass is the specific scientific term, which takes into account a lot more factors than just what the everyday person wants to know, when they throw something on a set of scales.

[old man emu]

1 hour ago, APenNameAndThatA said:

I did follow the link. I am clarifying that you still say that something with a weight of 3 kg has a mass of 0.3 kg. 

Yes. But you keep seeming to dispute that. There must be some words that are causing confusion because the maths is correct. Please indicate what is confusing you.

 

1 hour ago, onetrack said:

weight is the laymans everyday term, mass is the specific scientific term

Correct. Mrs Einstein told Albert to go an buy her three kilograms of potatoes for her cooking. Albert came back with a quantity of potatoes that Mrs Einstein recognised as about right for her version of three kilograms of potatoes. Albert said, 'There you are my Love, 300 grams of potato matter'. 

[APenNameAndThatA]

1 hour ago, old man emu said:

Yes. But you keep seeming to dispute that. There must be some words that are causing confusion because the maths is correct. Please indicate what is confusing you.

 

Correct. Mrs Einstein told Albert to go an buy her three kilograms of potatoes for her cooking. Albert came back with a quantity of potatoes that Mrs Einstein recognised as about right for her version of three kilograms of potatoes. Albert said, 'There you are my Love, 300 grams of potato matter'. 

So does that mean that if an RAAus aeroplane has a maximum takeoff weight of 600 kg it has a maximum takeoff mass of about 60 kg?