Mechanics - vectors question M1

This type of question comes up a lot but I never know how to answer it and don't really understand the mark scheme

Part b)

If the particle is traveling due north then that means its velocity vector must be pointing due north. So the velocity will be 0i + kj i.e. it has no i component.

Does that help? If not, please post what equations you already have.

Okay but how do I go about finding the j component?

I'm trying to use a suvat to work it out but the only known values that can be used are u and a

You can use v = u + at like you did in a) but with an unknown t. This will give you a vector with i and j components in terms of t.

Set the i component to 0 to give the time that the velocity is due north then use that to write down the velocity vector at this time.

v = -2j + 2i + (0.25i + 0.3j)t

i = 0

v = -2j + 0.3jt

v is still an unknown multiple of j so how do you get the time?

It's not that i = 0 because this doesn't make sense since i is a unit vector which is not 0. The i component is equal to 0.

v = -2j + 2i + (0.25i + 0.3j)t

You can rewrite this

v = (2 + 0.25t)i + (-2 + 0.3t)j

Now set the i component to 0 to find the value of t.

Oh

2 + 0.25t = 0
t = -8

Do I just ignore the minus and say t=8 since time can't be negative?