# Kinetic, potential and mechanical energy question.

## Homework Statement

A 5.5 x 10^4 kg airplane, travelling at an altitude of 9.9 km, has a speed of 260 m/s relative to the ground. Nearing the end of the flight, the plane slows to a speed of 140 m/s while descending to an altitude of 2.1 km. Determine the plane's total loss of mechanical energy during this change of speed and altitude.

## Homework Equations

$$E_g = mgh$$
$$E_k = \frac{1}{2}mv^2$$

## The Attempt at a Solution

I have no idea how to start to solve this problem. If someone could point me in the right direction, it would be great.

Doc Al
Mentor
The mechanical energy is just the sum of gravitational PE and kinetic energy. You have the formulas for both--just crank it out.

The mechanical energy is just the sum of gravitational PE and kinetic energy. You have the formulas for both--just crank it out.

Thanks dude!

Ok, I did this:

a)
Eg = mgh
Ek = mv^2, where v I put 260 m/s
Et1 = Eg + Ek

b)
Same thing as above but with the second values.
Elost = Et1 - Et2.

The answer is supposed tob e 5.5 x 10^9 Joules but i get 4.98 x 10^9 Joules. What did I do wrong? Did I go about this the right way?

Doc Al
Mentor
Looks like the right way to me. If you post the details of your calculation, perhaps we can spot an error. (Or confirm your answer.)