High School Physics Problems

Originally posted by twhitehead
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 e ...[text shortened]... ut that is not what we were asked.

The correct answer remains:

It will take off instantly.

Absolutely correct, if you consider 2.2 to be the current net acceleration.

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?

This shouldn't be too bad; not sure how to get converted to N from KG

Originally posted by serigado
Absolutely correct, if you consider 2.2 to be the current net acceleration.

What else could it mean?
If it means the forces created by the aircrafts engines / wings + angle of attack etc have a resultant acceleration of 2.2 whilst ignoring gravity (ie not the current net acceleration) then the answer is "Never", as the acceleration due to gravity is greater than 2.2 and much greater than the component of 2.2 in the vertical direction. The mass of the aircraft is irrelevant.

Originally posted by twhitehead
What else could it mean?
If it means the forces created by the aircrafts engines / wings + angle of attack etc have a resultant acceleration of 2.2 whilst ignoring gravity (ie not the current net acceleration) then the answer is "Never", as the acceleration due to gravity is greater than 2.2 and much greater than the component of 2.2 in the vertical direction. The mass of the aircraft is irrelevant.

I understood the question "the airplane starts accelerating at 2.2" as the acceleration due the plane by itself , nothing more.

Well... but there was enough discussion to this. I think everyone understood it now.

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.

Assuming that the runway is level, I agree.

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?

which planet is it on?
may we assume it is not under water?

Originally posted by twhitehead
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 e ...[text shortened]... ut that is not what we were asked.

The correct answer remains:

It will take off instantly.

It might be TRYING to go up at a 12 degree angle but at 2.2 m/s^2, that is about .22 g's. That is a decent accel for a car but for a plane, it has to accelerate till it gets to about 280 Km/hr, which is about 77 m/s which looks to me like about 35 seconds. If you specify an acceleration, it doesn't matter what the mass of the plane is.

Originally posted by sonhouse
It might be TRYING to go up at a 12 degree angle but at 2.2 m/s^2, that is about .22 g's. That is a decent accel for a car but for a plane, it has to accelerate till it gets to about 280 Km/hr, which is about 77 m/s which looks to me like about 35 seconds. If you specify an acceleration, it doesn't matter what the mass of the plane is.

But that is assuming it is accelerating horizontally. We were told quite clearly that it accelerating at 12 degrees.
Acceleration=rate of change of velocity.
velocity=rate of change of position.
if position=0 and velocity=0 and acceleration exists then movement is instantaneous in the direction of the acceleration ie 12 degrees.
Therefore the plane takes off immediately.

Originally posted by techsouth
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.

Yes, yes, yes. Even an elderly granny, like myself, was able to ascertain the correct answer to this rather simple physics problem, even in my head. The physics is quite simple to understand and the math requires only a first grade education. My only question is.....What the hell is an airplane???

GRANNY.

Originally posted by twhitehead
But that is assuming it is accelerating horizontally. We were told quite clearly that it accelerating at 12 degrees.
Acceleration=rate of change of velocity.
velocity=rate of change of position.
if position=0 and velocity=0 and acceleration exists then movement is instantaneous in the direction of the acceleration ie 12 degrees.
Therefore the plane takes off immediately.

You have a bit of a dilemma, the problem is stated wrong. You cannot have a plane take off immediately with less than a quarter G of acceleration.
You seem to forget the earth imparts 1 full G of force downwards. So with 1/4 G going upwards, it doesn't take a rocket scientist to see the plane can only act like a car and move forward. The problem says the plane comes in at 175,000 pounds or Kg, forget which, but either way, you get less than 50,000 pounds or Kg of thrust, which even if it was pointed straight up, would get you nowhere except to burn off a lot of fuel for nothing. At 12 degrees, you would have some vector of vertical force, maybe 1/8th of the total thrust going upwards but the majority of it would be spent moving the plane forward. Some people have said he probably means the wings are at a 12 degree angle, front end up, but that is not needed to generate vertical thrust, the differential pressures above and below the wing with sufficient air flow by the wing is what gives vertical lift. So you can maybe say the whole plane is on a tilted runway, tilted upwards at a 12 degree angle. If so, and my numbers are anywhere near correct, the 12 degree angle would consume maybe half the forward thrust giving a net acceleration of around 1 unit of accel instead of two and change which would at least double the length needed to get up to takeoff velocity. I am not doing any math, just trying to give an idea of the vectors involved.