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# I don't see what this question's on about??!! watch

1. Hey guys, this M1 question's a pain; I can do the first and last parts to it easily but I don't get what this part is on about:

A particle moves on a smooth horizontal surface with acceleration (3i - 5j)ms^-2. Initially the velocity of the particle is 4j ms ^-1. Here's the part I can't understand:

Find the time when the particle is travelling in the i direction.

What does that mean? I think I'm missing something simple but what exactly are they asking? Is it when the particle starts moving horizontally from 4j or what??
2. You want to find the time when its velocity is a positive multiple of i, so the j-component of the velocity will be zero.
3. i would assume it wants you to find the point in time when the particle is solely travelling in the i direction (j component = 0)
4. (Original post by goldlock)
Hey guys, this M1 question's a pain; I can do the first and last parts to it easily but I don't get what this part is on about:

A particle moves on a smooth horizontal surface with acceleration (3i - 5j)ms^-2. Initially the velocity of the particle is 4j ms ^-1. Here's the part I can't understand:

Find the time when the particle is travelling in the i direction.

What does that mean? I think I'm missing something simple but what exactly are they asking? Is it when the particle starts moving horizontally from 4j or what??
Yes. i and j are two perpendicular directions (they're actually the unit vectors in the x and y directions, respectively).

When the particle is "travelling in the i direction" it means all its motion is in the x direction, and none of its motion is in the y direction.
5. so could the answer be expressed in the form t = v/3i ?
6. v = u + at
v = 4j + (3i - 5j)t
v = 3ti + 4j -5tj

Now just make the j part equal to zero to find t.

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