# Thread: Physics motion problem using two dimensions

1. ## Physics motion problem using two dimensions

Hello

I've been having trouble getting the hang of motion in two dimensions, as I've been trying to teach myself. There is a question in my booklet that I am stuck on, which goes as follows.

A dog wishes to swim across a slow-moving stream. The dog can swim at 2.0 m/s in calm water. The current velocity is 3.0m/s. The distance directly across the stream is 50m.

a. If the dog points himself directly across the stream, how long will it take to cross the stream?

b. How far downstream will the current have carried the dog when the dog gets to the other side?

c. What was the dog's velocity relative to the bank from where the dog started? (I don't even understand what this one is asking)

If somebody could help on any of these, I'd really appreciate it.

2. ## Relative velocity

Hello M23
Originally Posted by M23
Hello

I've been having trouble getting the hang of motion in two dimensions, as I've been trying to teach myself. There is a question in my booklet that I am stuck on, which goes as follows.

A dog wishes to swim across a slow-moving stream. The dog can swim at 2.0 m/s in calm water. The current velocity is 3.0m/s. The distance directly across the stream is 50m.

a. If the dog points himself directly across the stream, how long will it take to cross the stream?

b. How far downstream will the current have carried the dog when the dog gets to the other side?

c. What was the dog's velocity relative to the bank from where the dog started? (I don't even understand what this one is asking)

If somebody could help on any of these, I'd really appreciate it.
You need to understand the triangle of velocities that I have drawn in the attached diagram. I am assuming that the stream is flowing due North - straight up the page - and that the dog is pointing due East.

The water carries him downstream, his velocity relative to the ground being given by the hypotenuse of the right-angled triangle.

Can you complete the calculations now?