# Relative Velocity of a ship

1. Sep 17, 2014

### kerol9904

1. The problem statement, all variables and given/known data
A ship goes from A to B at $$v_1=10 km/h$$ and from B to A at $$v_2=16 km/h$$ Find: (1) the average velocity of the ship, and (2) the velocity of the river current.

2. Relevant equations
$$v_{avg}=(v_1+v_2)/2$$
$$v_{AC}=v_{AB}+v_{BC}$$

3. The attempt at a solution
$$v_{Boat}-v_{Current}=10km/h$$
$$v_{Boat}+v_{Current}=16km/h$$
$$v_{Boat}=13km/h, v_{Current}=3km/h$$
The actual answer for $$v_{Boat}=12.3km/h$$ Why is that?

2. Sep 17, 2014

### Staff: Mentor

What's the definition of average velocity (really average speed)?

3. Sep 17, 2014

### kerol9904

Change in position/Change in time?

4. Sep 17, 2014

### Staff: Mentor

Try total distance / time.

This only applies to uniformly accelerated motion, which is not what's going on here.

5. Sep 17, 2014

### kerol9904

Ahh... I didn't realise that the equation is only for uniformly accelerated motion. I've got the answer now. Thanks!!