Question

Imagine a leaf floating down a creek (south) at a rate of 6m/s. You walk in the creek towards the leaf (north) at a rate of 2m/s. Relative to the ground, at what rate is the leaf moving?

Answer 1

It depends on what is the reference frame where the velocity of the leaf (6 m/s) is measured.

If it is measured with respect to the creek, then the answer is simply "6 m/s south".

If it is measured with respect to the observer moving toward the leaf, then the velocity of the leaf relative to the ground is actually the sum between the velocity of the leaf relative to the observer (6 m/s south, so 6 m/s) and the velocity of the observer relative to the ground (2 m/s north, so -2 m/s):

`v=v_l +v_o =6 m/s +(-2 m/s)=4 m/s`

and the direction is south.