Falling From a Boat: Relative Velocity Explained

[istock1]

All,

I am an avid kayaker, and recently got into an argument with another kayaking friend of mine over (what I think is) an issue of relative velocity. The question was this:

A person is sitting in a kayak and that kayak is in a constant velocity current (say, 3 knots), but the kayak is not moving relative to the water it is in. If he falls out of the kayak, at what speed will the person and boat separate?

My argument was that they will not separate; they will both continue to move at 3 knots with the current, but will not move relative to the water. Can anyone help settle this?

Thanks!

 

[cristo]

Ideally, you are correct. I say "ideally," since obviously when you fall into the water, you will probably have some acceleration in some direction, and so will start to move in that direction. However, the answer (I think you want) is that no, the boat and person will not separate, since both the boat and the person are moving with the same constant velocity of the water.

 

[russ_watters]

Falling out of a kayak, you'll probably push it away with your feet. Once you are in the water, it will drift with the wind and you won't...

 

[istock1]

Falling out of a kayak, you'll probably push it away with your feet. Once you are in the water, it will drift with the wind and you won't...

Russ,

Of course you are right, and I should have included that we were holding windspeed at zero.

Thanks!

 

[cesiumfrog]

you mean, holding windspeed at 3 knots?

 

[russ_watters]

Heh - right. Assuming the wind is moving the same speed as the current, there is no motion between you and the kayak.

 

[DaveC426913]

Assuming wind is discounted:

There's going to be no "residual" velocity. Both you and the kayak will virtually instantly achieve the same velocity as the current.

Now, in practice, you and the kayak will likely drift apart from the initial push of you falling in. This distance will be in whatever direction you fall in, independent of the current. And the kayak is so light that this could become a significant distance.