Well, there you have a very important principle wrapped up in that question.
There's actually no such thing as a real, actual amount of potential energy.
There's only potential <em><u>relative to some place</u></em>. It's the work you have to do
to lift the object from that reference place to wherever it is now. It's also
the kinetic energy the object would have if it fell down to the reference place
from where it is now.
Here's the formula for potential energy: PE = (mass) x (gravity) x (<em><u>height</u></em><u>)</u> .
So naturally, when you use that formula, you need to decide "height above what ?"
If you're reading a book while you're flying in a passenger jet, the book's PE is
(M x G x 0 meters) relative to your lap, (M x G x 1 meter) relative to the floor of the
plane, (M x G x 10,000 meters) relative to the ground, and maybe (M x G x 25,000 meters)
relative to the bottom of the ocean.
Let's say that gravity is 9.8 m/s² .
Then a 4kg block sitting on the floor has (39.2 x 0 meters) PE relative to the floor
it's sitting on, also (39.2 x 3 meters) relative to the floor that's one floor downstairs,
also (39.2 x 30 meters) relative to 10 floors downstairs, and if it's on the top floor of
the Amoco/Aon Center in Chicago, maybe (39.2 x 345 meters) relative to the floor
in the coffee shop that's off the lobby on the ground floor.
Answer:
The approximate magnitude of the force of air resistance is 540 N.
Explanation:
There's a short handy formula for that.
If the object is just dropped and not tossed, and it's not affected by air resistance on the way down, then the distance it falls in T seconds is
D = (1/2) (gravity) (T²)
For this problem . . .
176.4 m = (1/2) (9.8 m/s²) (T²)
Divide each side by (4.9 m/s²) :
T² = (176.4 m) / (4.9 m/s²)
T² = (36 s²)
Take the square root of each side:
<em>T = 6 seconds</em>
