# Question in rotational motion

1. Dec 27, 2012

### ehabmozart

In rotational motion what is the difference between angular acceleration, radial acceleration and tangential acceleration?? ... Now suppose a rotating disk about its center (axis) rotating and a mass attached to the string of the rotating disk. By what v does it move... Is it just like v-rW... If yes, y does it have the same w.. And suppose it is set free to fall but still attached to the disk, what acceleration would it have during the fall??? Thanks

2. Dec 27, 2012

### tiny-tim

hi ehabmozart!
radial and tangential acceleration are linear, they are two perpendicular components of the ordinary (linear) acceleration

they are measured in distance per time squared

angular acceleration is measured in angle per time squared
you mean the string is wound round the disc at radius r ?

then, if the string isn't slipping, the speed of the string at any point at which it is in contact with the disc must be the same as the speed of that point of the disc

since the speed of the disc (at distance r from the centre, with the centre fixed) is rω, the speed of the string must be rω also

(if the centre of the disc was moving, with velocity v … for example if the disc was rolling … then of course you would have to add v)