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    The edexcel January 2006 question 6d they say B has a speed of 8 and is moving in the direction of vector (3i+4j) . originally B had a velocity of (3i+4j) and position vector of (-26i+4j) how did they get the new velocity to be 8/5 (3i+4j) ??
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    (Original post by hanhan19)
    The edexcel January 2006 question 6d they say B has a speed of 8 and is moving in the direction of vector (3i+4j) . originally B had a velocity of (3i+4j) and position vector of (-26i+4j) how did they get the new velocity to be 8/5 (3i+4j) ??
    originally B had a velocity of 3i+4j, if you find the magnitude of that you get the sqrt of 25 (3^2 + 4^2) so the magnitude is 5 units per second.we know that speed is 8 moving parallel to 3i + 4j so we need to find the velocity.

    From the previous bit we know the velocity was 5. We need the unit vector which is simply 1/5(3i+4j) then you multiply this by the new speed which is 8 to get 8/5(3i+4j)

    Remember that this only works when vectors are parallel
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    (Original post by hanhan19)
    Thank you so much. I don't member learning about unit vectors, how do i know when to use them and what exactly is a unit vector ?
    A unit Vector is simply a vector which has a magnitude of 1 unit. You can use them when you are given one vector of a certain magnitude in a direction like the 3i+4j in your question and asked to find the vector for a different magnitude which is parallel to that original vector.

    The important thing to know is that a unit vector is a multiple of any vectors that are parallel to it and this is useful when finding vectors with different magnitudes or finding vectors from their magnitudes.
 
 
 
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