High School Physics Problems

Originally posted by Mexico
I think the problem was simplified to ignore wing lift and assume the 12 degrees was simply it taking off..... Like many other basic physics problems....

so the answer is: it won't take off. Unless it had an initial velocity, in that case it'll make a beautiful parabola until it collides in the ground.

Originally posted by Mexico
I think the problem was simplified to ignore wing lift and assume the 12 degrees was simply it taking off..... Like many other basic physics problems....

The wing has to have a positive angle of attack to produce lift, that is what he is assuming with the 12 degrees.

Originally posted by wittywonka
Feel free to correct me if I am indeed mistaken.

The question said that it was accelerating - not that there was a force acting on it (as implied in your calculations). So it will take off instantly.

Originally posted by twhitehead
The question said that it was accelerating - not that there was a force acting on it (as implied in your calculations). So it will take off instantly.

The fact that it is accelerating implies there is a force acting upon it, presumably the turbine or propeller.
If there were no force, there would be no acceleration as per Newton, F=ma.

It will not take off instantly, it has to first overcome the force of gravity.

Originally posted by mlprior
The fact that it is accelerating implies there is a force acting upon it, presumably the turbine or propeller.
If there were no force, there would be no acceleration as per Newton, F=ma.

It will not take off instantly, it has to first overcome the force of gravity.

I already answered, but here goes again in detail....
You decompose the vertical and horizontal components of acceleration. The vertical one will never be bigger then gravity. The horizontal one will make the plane accelerate forever.
THe plane will never take off with the data in the problem.

The 21.3 you got is: how bigger gravity than plane acell gravity is.
Dividing force by force gives a number or proportion.

You must have a change in acceleration and know when will the vertical acell of the plane be greater or equal than gravity to know when plane will take off...
hope I've been clear, now...

Originally posted by serigado
I already answered, but here goes again in detail....
You decompose the vertical and horizontal components of acceleration. The vertical one will never be bigger then gravity. The horizontal one will make the plane accelerate forever.
THe plane will never take off with the data in the problem.

The problem assumes the wings are at a positive 12 degree angle of attack, that means the plane will take off.

You could accelerate a door at a 12 degree angle of attack and it would fly.

The 21 seconds may not be entirely accurate, but the plane will eventually take off.

Originally posted by mlprior
The problem assumes the wings are at a positive 12 degree angle of attack, that means the plane will take off.

You could accelerate a door at a 12 degree angle of attack and it would fly.

The 21 seconds may not be entirely accurate, but the plane will eventually take off.

Then you have to know the force exerted in the wing by the air (it's proportional to the horizontal velocity). That force increases with increasing speed. That's why planes need a minimum speed to take off. But again, you would need more data.
But I doubt that kind of problem would be given in the US' high schools.
So the problem is badly formulated, or there's something missing, or the answer is really that simple: it won't take off.

Originally posted by serigado
Then you have to know the force exerted in the wing by the air (it's proportional to the horizontal velocity). That force increases with increasing speed. That's why planes need a minimum speed to take off. But again, you would need more data.
But I doubt that kind of problem would be given in the US' high schools.
So the problem is badly formulated, or there's something missing, or the answer is really that simple: it won't take off.

The problem is oversimplified, he already stated that he just made it up and it was not out of a textbook.

With a 12 degree angle of attack you do have horizontal and vertical force vectors and you are correct, that needs to be separated out.

Originally posted by wittywonka
An airplane weighing 175,000 kilograms begins accelerating at 2.2 meters per second per second at an angle 12 degrees above the horizon. How long will it take before the airplane takes off?

What is the humidity, which direction is the wind blowing, how hot is it (what is air density), and various other things would have to be known to be certain. That's why I hate my physics class... it is easy to memorize things, but I prefer astrophysics and conceptual, spatial concepts. You can't truthfully tell the answer to any of the stupid questions like this because of all the factors that go into it... and the fact that Quantum Physics makes it possible (however unlikely) that the plane will fall into the ground before it has the chance to accelerate anyway. If you walk into a wall 10000000000000000000000000000 times, you'll eventually walk through it, or get stuck in the middle. Nothing is impossible.

Originally posted by mlprior
I will eat the bread and a quarter pounder with cheese, thus increasing the overall mass by .3 pounds.

Conservation of matter... law #1.

Originally posted by wittywonka
Ha, well, I confess that I have had physics problems like this one, but this isn't specifically one of my homework problems. Sorry for that confusion.

As proof, I'll solve it (at least I hope).

Because the airplane is accelerating at 2.2 m/s^2 at an angle of 12 degrees, we start by finding the y-axis component of that acceleration, which we ...[text shortened]... y [b]21.3 seconds (1,716,750/80,500).

Feel free to correct me if I am indeed mistaken.[/b]

That's correct in a world without wind, changes in air pressure, etc... I had worked it out in my head and got 1700000/7500=6800/300=21.25, but one can't expect to get exact answers when you work with mental calculations, so I skipped the formalities and just got close answers... otherwise I'd have had to write it down, like you.

You are not mistaken!

Originally posted by Mexico
I think the problem was simplified to ignore wing lift and assume the 12 degrees was simply it taking off..... Like many other basic physics problems....

Correct, which makes me want to strangle whoever decided to teach this c**p anyway. Why not just teach us all that 5+2.25=7 because it's easier to ignore decimals?

Originally posted by serigado
I already answered, but here goes again in detail....
You decompose the vertical and horizontal components of acceleration. The vertical one will never be bigger then gravity. The horizontal one will make the plane accelerate forever.
THe plane will never take off with the data in the problem.

The 21.3 you got is: how bigger gravity than plane acel greater or equal than gravity to know when plane will take off...
hope I've been clear, now...

Wow, I totally missed that. Yikes, for all my skill in physics, I still stumble over simple problems, and assume stupid things. I feel like an idiot.

Originally posted by mlprior
The fact that it is accelerating implies there is a force acting upon it, presumably the turbine or propeller.
If there were no force, there would be no acceleration as per Newton, F=ma.

It will not take off instantly, it has to first overcome the force of gravity.

Read the question again. It is accelerating at an angle of 12 degrees. Acceleration is the result of the total forces adding up to a resultant force which accelerates the body.

So, as per Newton we have the a in F=ma and not part of the F as suggested by wittywonka. We also know m and the G part of the F so we could calculate the force created by the engines, the wings etc if we knew more about their directions, angle of attack, surface area etc etc, but that is not what we were asked.

The correct answer remains:

It will take off instantly.

Originally posted by wittywonka
Ha, well, I confess that I have had physics problems like this one, but this isn't specifically one of my homework problems. Sorry for that confusion.

As proof, I'll solve it (at least I hope).

Because the airplane is accelerating at 2.2 m/s^2 at an angle of 12 degrees, we start by finding the y-axis component of that acceleration, which we ...[text shortened]... y [b]21.3 seconds (1,716,750/80,500).

Feel free to correct me if I am indeed mistaken.[/b]

If the plan accelerates at 2.2 m/s^2 at an angle of 12 degrees, it takes off the instant it starts moving (assuming the runway is sloped less that 12 degrees). The result of accelerating at 12 degrees is a motion component in the vertical direction.

Not physically possible of course, but it is the result of assuming acceleration at 12 degrees.