# Dv/dr or v/r

1. Jul 13, 2011

### anmolnanda

in some cases we use dv/dr to find speed and in some cases we use v/r which 1 will be more appropriate to use?

2. Jul 13, 2011

### ZapperZ

Staff Emeritus
Er.. you can't use either one!

Speed, or velocity, is defined as ds/dt, where s is displacement. If speed is constant, then finding the total net displacement S, over a period of time T, will give you v = S/T (for v constant). If v isn't constant, then the instantaneous you use ds/dt at a particular time t.

Zz.

3. Jul 13, 2011

### Avijeet

What are v and r? If they are distance and time , then dv/dr is the appropriate quantity. v/r will work for constant velocities only.
However if v and r represent velocity and displacement and you are talking about angular velocity then v/r is the correct quantity.

4. Jul 13, 2011

### ZapperZ

Staff Emeritus
Very good point.

Consider this as a lesson, especially to new members, that if you neglect to present your question clearly and completely, you'll not only cause confusion, but also get confusing responses. If you wish that people who respond to your queries put in some effort and thought into presenting a response that is clear and well-thought out, then it is imperative that YOU do the same as well!

Zz.