A 60 kg man is sitting on the floor of an airplane 8000 m high traveling at 850 km/hr. How much total mechanical energy does he have relative to the reference frame of the plane?

Relative to reference frame of the airplane he's sitting in:

- - Gravitational potential energy = (mass) (gravity) (height)

His height above the airplane is zero, so his GPE is zero.

- - Kinetic energy = (1/2) (mass) (speed) ²

His speed relative to the airplane is zero, so his KE is zero.

Mechanical energy = (potential energy) + (kinetic energy)

Mechanical energy = (0) + (0)

The poor fellow's total mechanical energy is zero.