Physics Question

Jokester said:
Because there is a constant acceleration parallel to the x axis that doesn't change. This means you have a fixed speed going up the y axis, but the speed along the x axis is continually increasing
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How can there be acceleration? (assuming still that the forces are balanced - no air resistance and no thrust)
If the speed (along the x axis) is continually increasing then surely, at some point, the resultant vector of the plane will soon be larger than it was as the started (but in a different direction)?
 
It will pretty much be heading in the x direction, flying sideways, but there will still be the velocity in the y direction, so it will never truely be just heading in the x-direction. As has been said, a parabola is never actually straight.
 
SiriusB said:
Wouldn't this be best described as a boat with two propellers? One at the rear moving the boat forward at a constant speed, and one on the left of the boat, right in the middle [so it doesn't rotate] pushing the boat to the right, with the power constantly being increased?

It makes sense in my head!
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It makes oh so much sense, although it would be better slightly modified. A single prop boat with tide/wind pushing it at an accelerating pace to the left. :p

As all your drawings in this thread are soo atrocious I decided to actually plot it properly and show a proper parabola and an "actual" path. :p

img4967p.jpg


(Unfortunately I don't have a scanner up here so you'll have to make do with a camera shot..)

Now (assuming I got it right *gulp*) that graph shows a boat moving at a constant 5 m/s forwards with an accelerating force (wind/tide/propellor) pushing the boat to the left at 2 m/s^2 (am I right so far? :p). That should mean at t=1 the distance travelled in one second is 2m, t=2 distance travelled in a second is 4m, t=3 is 6m and so on until it is travelling at 20m/s at t=10. Plotting those points with a constant forward speed shows the graph...


Personally looking at the first image I decided on path 2 as there was no chance of it being path 1 (if it was supposed to have been drawn accurately :p)!

Inquisitor said:
The conveyor belt jammed and the plane crashed. Oh, the humanity!
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I still don't understand how people still think aeroplanes are driven by their wheels and not the massive engines on the wings, meh... :p
 
A tide makes more sense I guess. I was trying to keep it simple before someone started mentioning phases of the moon and whether or not it was morning or evening blah blah blah. Pedantic lot in here! :p

Excellent graph and proves that my boat rules. Stupid aeroplane :eek::D
 
Ha, yeah I didn't think of that (although the only reason I could think of a tide accelerating was just after it had turned, but I doubt it would do so so uniformly... :p).

The original question is a perfect exam question however, way, way, way to complicated for what it is actually asking. As usual the hardest part is actually understanding what the question is actually asking you to do. :D
 
The usual kind of question where you are given a thousand different figures pertaining to the aircraft only to be asked what day it is! :p
 
Jeeez what a thread. I only scanned through it but you're all making a mountain out of a mole hill. For a start you are making a totally unrealistic situation by having a plane and then taking out all of the forces that a plane has. You need to either talk about a plane and the actual forces or just a simple object with the theoretical vectors you want, you can't mix and match both and expect anything useful to come from it.

As for the plane situation, the diagram of forces in a turn was posted and that is how it is. All this lift comes from CoG is rubbish, the lift is centered at the CoP. This is the main reason most aircraft have a horizontal stabiliser, to balance the forces from the CoG and the CoP.
 
The only forces in play are the lift and mavity
The wings are the same and the lift is constant on both wings
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It would fly straight on, and it would probably decrease in speed due to being less aerodynamic than a straight "arrow" shape.

It would only turn if lift was more on one wing than another.
 
Blinkz said:
Jeeez what a thread. I only scanned through it but you're all making a mountain out of a mole hill. For a start you are making a totally unrealistic situation by having a plane and then taking out all of the forces that a plane has.
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I think thats more realistic than this question ever occuring in the real world. ;)
 
Jonnycoupe said:
But path one goes forward, there is no thrust on the FBD.

People are assuming that because its a plane it has to be going forward.
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I think there is an assumption that because the wings are generating constant lift that the plane must be travelling forward at a constant speed.

If there was no forward flight the plane would just accelerate to the right in a straight line, again assuming the wing generates lift even with no forward motion.

You don't need a forward force if the plane is already in motion (and no air resistance).
 
Well to be honest, it is pretty standard in A levels to have a 'story', then to be told which forces to consider, and which to ignore. It really isn't that hard. A question doesn't have to be realistic to have a correct answer.
 
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