# Kinetic, potential and mechanical energy question.

1. Jan 25, 2008

### solarwind

1. The problem statement, all variables and given/known data
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.

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

3. 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.

2. Jan 25, 2008

### Staff: Mentor

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

3. Jan 25, 2008

### solarwind

Thanks dude!

4. Jan 25, 2008

### solarwind

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?

5. Jan 25, 2008

### Staff: 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.)