Year 12s: what will be the first way you research your future options? >>

Rotation using vectors? (Pleaase help, university exam tomorrow!)

A point mass, m, is attached to one end of a light inextensible string of length 5r with
the other end being fixed at (0; 0; 3r), rotates in a circle with an angular frequency
w (omega) about the z-axis: Calculate its linear velocity and its angular momentum when
the mass is at (4r; 3r; 3r) and at (-4r;-3r; 3r)

Please could someone help me do this question? I have no idea where to start and I have a university exam tomorrow :frown:

All I know is that tangential velocity= omega x r but I'm not sure how I can use that here

Reply 1

Pessimisterious

Original post by Mallika

All I know is that tangential velocity= omega x r but I'm not sure how I can use that here

Vectors? Those are co-ordinates.

You need to visualise or draw it.

It's basically a case of working out the the radius of rotation and you're onto a winner.

Picture it from above so you can see a 2D version of the circular path of its movement. Find the x and y co-ordinates, then use pythagoras to work out the radius

v=wr
L=mvr=mwr²

Reply 2

Mallika

Ah yes you're right I just assumed the velocity and angular momentum would be vectors :P
Would the x coordinate just be 5rcosw? I thought we'd been given the radius- is it not 5r?

Thank you btw :smile: