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    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??
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    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.
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    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)
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    (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.
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    so could the answer be expressed in the form t = v/3i ?
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    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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