What change in kinetic energy does an airplane experience on takeoff if it is moved a distance of 500 m by a sustained netforce of 5000 N?

Answer 1

Tell you what: We'll calculate how much work is done by the force, not how

much kinetic energy the plane picks up, and then we'll tell you why.

The work done is (force) x (distance) = (5,000) x (500) = 2,500,000 joules.

Whenever we do these force/distance/work/energy problems, we always ignore

friction, air resistance, etc. because it's just too hard and complicated to take

all of them into consideration, and it would only get in the way of understanding

the basic principles anyway.

How valid is the answer we get when we ignore these things ? Well, it may be OK,

and pretty close to reality, for baseballs, stones, bullets, and bricks. But it's definitely

not OK for airplanes. Airplanes are designed specifically to have an intimate relationship

with the air, and that's the only reason they work at all.

If you do 2.5 million joules of work on a Baseball, a stone, or even a little red wagon,

it may have close to 2.5 million joules of kinetic energy when you stop pushing. But

an airplane won't. It's designed to use some of that K.E. to move air, to generate

LIFT, with the ultimate objective of getting off of the ground. That drains away

a big part of the work done to roll down the runway, and that's the big part that

will never show up as kinetic energy.