# Motion - help!

1. Feb 19, 2010

### twenty-ten

Hello everyone, I am new to this forum and very much interested in physics. As a student, I find motion is my weakness in this subject. Hopefully everyone can help me out :)

1. The problem statement, all variables and given/known data
While driving North at a velocity of 28ms^-1 relative to the ground, a man notices a cyclist crossing the road ahead. The cyclist is travelling West at 7ms^-1 relative to the ground.

2. Relevant equations
(a) What is the velocity of the cyclist relative to the driver?
(b) What is the velocity of the driver relative to the cyclist?

3. The attempt at a solution
For (a) I thought of subtracting the two values, however (b) has the values the other way around, therefore it doesn't make sense to me. Perhaps I could use the Pythrogem's theorem a^2 + b^2 = c^2 but would it be correct? I'm very unsure of quoting the directions in the answers.

Thankyou.

2. Feb 19, 2010

### diazona

As a first step, can you write the velocities of the man and cyclist (with respect to the ground) as vectors?

3. Feb 19, 2010

### twenty-ten

I'm not very sure how to write them as vectors, I think it's something I am quite unsure of. The question didn't say anymore than that.

I've been putting an effort to work my way through but I reached the block and nowseeking help.