# Relative velocity query

Discussion in 'Mathematics' started by moontitan, Jul 7, 2015.

1. ### moontitanOccasional commenter

An aeroplane can fly at 600km/h in still air

Its ground speed is 632km/h

I am getting relative velocity in a muddle. 600 is the velocity of the plane relative to the air, but it is still air, why is not the same as the velocity of the plane-i.e. 632?

2. ### PaulDGOccasional commenter

Is this the whole question? The two statements are 'odd' but not mutually exclusive.

One way both can be true is if there is a tail wind of 32km/h. Then the aircraft is flying within its limits of a maximum airspeed and the ground speed will be 632km/h.

You're correct (for a relatively simple model... where is this question from?). But the opening statements do not quite say what you seem to think they say. (I read the 600km/h a bit like limiting friction in friction questions, not like actual experienced friction.)

("Airspeed" is much more complicated than just relative velocities. "Airspeed" is an indicator of the lift an aircraft experiences in particular circumstances and is dependent on actual speed through the air and the density of the air (temperature and altitude being important). If you're taking the example from a pilot training handbook, there might be more to their model of airspeed than we'd normally teach in maths A level.)