# Introductory Rotational Motion Question

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1. Dec 23, 2014

### Jazz

I'm beginning the chapter of Rotational Motion and Angular Momentum and it says the following which got me confused:

When I was introduced acceleration at the beginning, it was stated that an acceleration $a = \frac{\Delta v}{\Delta t}$ can be a change either in speed's magnitude or direction; or both. In other words, in any case I would be dealing with an acceleration $a$.

Does it mean that this is not true?
Do I need to be more specific?
If linear acceleration $a_t$ is a change in the speed's magnitude and $a_c$ a change in its direction, how is the change in speed's magnitude and direction at the same time called? Just acceleration?

Thanks!!

2. Dec 23, 2014

### ehild

Velocity is a vector quantity, has both magnitude and direction. $\vec v = v \vec {e}$ where v is the magnitude of the velocity (called 'speed') and $\vec e$ is an unit vector in the direction of the velocity.

Acceleration is also vector, the time derivative of the velocity: $\vec a = \frac {d \vec v}{dt}$. Applying product rule $\vec a = \frac {d \vec v}{dt} = \frac {d (v \vec e)}{dt}= \frac{dv}{dt} \vec e + v \frac {d \vec e}{dt}$. The first term is acceleration in the original direction: linear acceleration at. The second term is the centripetal acceleration, corresponding to the change of the direction.

3. Dec 23, 2014

### Jazz

Aah, I see that the statement of the paragraph above does not contradict what I learned about acceleration, as I was thinking.

Now it's much clearer.

Thank you :)