Does lift force decrease as speed decreases?

[Femme_physics]

Or, to be more blunt, does "lift" force even exists in statics?

I was rather surprised that our teacher told us that lift force is just as strong as the normal force in statics of an object against ground.

It made me wonder just how strong is it. Does lift force decrease when speed decrease? And, does it always equal the weight of the object that's moving at constant velocity? Does it exist in a vacuum?

Also, does it depend on the shape of the flying object? Because normal force does not.

 

[Doc Al]

What do you mean by 'lift' force? Do you mean aerodynamic lift? Why would that be relevant to statics? (Unless you're considering the effects of wind on a structure.)

 

[Femme_physics]

Well, wiki called it "lift (force)" :)

http://en.wikipedia.org/wiki/Lift_(force)

I didn't think it's relevant to statics. But, I'd like to have things said formally :) But what about my other question. Does it decrease as speed decrease?

 

[Doc Al]

I was rather surprised that our teacher told us that lift force is just as strong as the normal force in statics of an object against ground.

You'd better ask for an explanation of that statement. Sounds crazy to me.

It made me wonder just how strong is it. Does lift force decrease when speed decrease?

Yes.

And, does it always equal the weight of the object that's moving at constant velocity?

If an object is flying horizontally at constant velocity, then lift must equal its weight.

Does it exist in a vacuum?

Lift is a force exerted by the air (or surrounding fluid). No air, no lift.

Also, does it depend on the shape of the flying object? Because normal force does not.

Yes, shape and angle of inclination matters.

 

[Doc Al]

I was rather surprised that our teacher told us that lift force is just as strong as the normal force in statics of an object against ground.

Perhaps your teacher was illustrating that just like normal force from the ground supports a structure in statics, the lift force from the air supports a flying structure. I doubt he was saying that lift is relevant in statics.

 

[Femme_physics]

You'd better ask for an explanation of that statement. Sounds crazy to me.

I was referring to lift force in dynamics countered against normal force in statics, just for the record. Is it still crazy?

If an object is flying horizontally at constant velocity, then lift must equal its weight.

Now I'm confused. First you said that lift force decreases as speed decreases, now you say it must equal the weight. According to the last statement, it must be a constant. But you also say it changes.

Yes, shape and angle of inclination matters.

AHA!

So there are a lot of variables that need to match the weight of the airplane for W = N

So it's not necessarily W = N

 

[Doc Al]

I was referring to lift force in dynamics countered against normal force in statics, just for the record. Is it still crazy?

See my last post. I think it was just an ambiguously structured sentence.

Now I'm confused. First you said that lift force decreases as speed decreases, now you say it must equal the weight. According to the last statement, it must be a constant. But you also say it changes.

Note the condition for lift to equal weight: Flying horizontally at constant velocity. In general, of course, lift does not equal weight.

 

[Femme_physics]

See my last post. I think it was just an ambiguously structured sentence.

Well, I just asked it. The lecturer didn't say it himself. I'm not sure why it's so confusing...I thought it was a fair question. I could be wrong.

Note the condition for lift to equal weight: Flying horizontally at constant velocity. In general, of course, lift does not equal weight.

That clears it :)

 

[cjl]

Note the condition for lift to equal weight: Flying horizontally at constant velocity. In general, of course, lift does not equal weight.

Similarly, in a constant rate climb or descent, assuming a small climb (or descent) angle, lift force must equal weight.

 

[Femme_physics]

Similarly, in a constant rate climb or descent, assuming a small climb (or descent) angle, lift force must equal weight.

Interesting, and also makes sense :)


(*sighs* I love physics)

 

[Doc Al]

Similarly, in a constant rate climb or descent, assuming a small climb (or descent) angle, lift force must equal weight.

Lift is generally considered as acting perpendicular to the velocity. So in the case of a constant speed climb, the lift will not be vertical. (But for any constant velocity motion the net force must be zero.)

 

[chrisbaird]

If the aircraft is moving horizontal, there must be no net vertical force, so that the lift equals and cancels the weight. If the aircraft then speeds up horizontally, the lift increases because lift depends on the aircraft speed. The lift force becomes larger than the force of gravity, there is a net force upwards, so the aircraft accelerates vertically and ascends. Similarly, if from the horizontal state (cruising) the aircraft slows down, the lift becomes smaller than its weight, there is a net force down, and the aircraft descends. Speed up to take off and slow down to land. The lift also depends on the shape/orientation of the wings, so the aircraft can ascend and descend just by changing the wings without needing to speed up or slow the aircraft.

 

[cjl]

Lift is generally considered as acting perpendicular to the velocity. So in the case of a constant speed climb, the lift will not be vertical. (But for any constant velocity motion the net force must be zero.)

Hence the caveat about the small climb (or descent) angle. For large angles, the lift will deviate from the weight because of that fact.