# Relative velocity

1. Jul 20, 2017

### dirk_mec1

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

2. Relevant equations

3. The attempt at a solution

The water skier makes an angle of 20 deg with the axis of the boat. I do not understand how to get the other angles.

2. Jul 20, 2017

### Staff: Mentor

Which angle do you want to find?

3. Jul 20, 2017

### J Hann

Apparently, you need to assume that the skier is moving perpendicular to his tow line.
That is, his total velocity is perpendicular to the tow line even tho his linear velocity (parallel to the boat)
is at 50 deg.w.r.t. the tow line.

4. Jul 21, 2017

### J Hann

I need to correct the above statement "his total velocity is perpendicular to the tow line".
A person riding in the boat would see the skier moving perpendicular the tow line.
That velocity then has to be added to the velocity of the boat to get the skier's total velocity.
Using geometry and the angles given you can construct the velocity triangle and solve with the Law of Sines.

5. Jul 21, 2017

### Staff: Mentor

I'm quite sure the person is moving along the length of the skier (relative to the water). (a) because that's how they work and (b) otherwise giving their direction would be pointless.

6. Jul 21, 2017

### J Hann

I agree that skier is moving at 20 deg to the path of the boat.
Since he is at a fixed radius (10 m) from the boat his instantaneous path w.r.t an observer
on the boat must tangential w.r.t that radius.
That assumption agrees with the answer (a) of 80.8 m/s but it doesn't agree
with the answer for part (b)???

7. Jul 22, 2017

### haruspex

Did you forget to convert from kph to m/s?

8. Jul 22, 2017

### J Hann

Thanks. I agree with the given answer.