Vectors (cross-product?)

A mass point at r = 3i + 4j - 2k rotates with a rotational velocity ω = 2i - 3j + 5k around an axis. Calculate the linear velocity of the mass point given that the units for its coordinates are in metres and the units for the components of the rotational velocity are rad/s.

I think you have to do a cross product of v = r X ω.

This gives v = 14i - 19j - 17k.

If you find the magnitude of this you get v = 29.09.

But I got a feeling that isn't the right way of doing this. Can someone please help?

Thanks!

Reply 1

devesh254

anyone got any ideas with this one? all help will be really appreciated!

thx!

Reply 2

Supermerp

You have the cross product the wrong way round. It's v = ω x r

So your answer should be negativified. Other than that it's right, I think.

Reply 3

devesh254

its definately not the other way round? as in v = r x ω ?

we learnt that equation in physics as v = rω...just confused about the order...

anyway shouldn't make a diff in the final answer because you find the modulus of v...

Reply 4

Supermerp

devesh254

its definately not the other way round? as in v = r x ω ?

anyway shouldn't make a diff in the final answer because you find the modulus of v...

I'm sure I'm right. It doesn't matter if what you're doing is finding the modulus. But if you need to know the direction of the velocity then it does. I remember it as rω too. I worked out the order of the cross product by thinking about which way all the vectors should point relative to each other.