# Why is changing direction called accelerating?

1. Sep 15, 2007

### Skhandelwal

I understand that in form of vectors but conceptually speaking, I don't get it.

Btw, why do accelerating charges emit energy?

2. Sep 15, 2007

### robphy

By the definition of the acceleration vector,
when, as time evolves,
there is a change in the velocity-vector [either in magnitude, in direction, or in both],
then there is a nonzero acceleration-vector.

3. Sep 15, 2007

### Skhandelwal

In english?

4. Sep 15, 2007

### cristo

Staff Emeritus
An acceleration is a change in velocity. So, as robphy says, when there is a change in the velocity (either its direction or its magnitude) then there is a non-zero acceleration.

5. Sep 15, 2007

### G01

A change in direction of velocity is still a change in velocity, so there has to be an acceleration.

6. Sep 15, 2007

### belliott4488

You can also think about it in terms of Newton's 1st law - without forces (which means accelerations, by the 2nd law, i.e. F=ma) things move in straight lines (or not at all). Therefore, if you're changing your direction of motion, you must be under the influence of a net force and therefore accelerating.

If you doubt this, consider the reaction to acceleration: you get thrown back in the opposite direction. So, if you accelerate forward in a car, you get thrown back against the seat, but if you take a sudden turn without changing speed, you get thrown to the side. Both cases are responses to acceleration.