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Kinematics- acceleration question watch

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    The question I'm working on is 2), for i) I got to the answer 20m/s when t=3 but Im getting stuck on ii) because the speed of 25m/s in a negative direction is equal to -25m/s velocity right? So it must be after 3 seconds so constant acceleration is -10m/s/s. I came to an answer of 4.5 seconds by doing -45/-10 = 4.5 . However the book I'm working it out from says that it is 7.5 seconds. I am quite confused, am i wrong or is it possible that the book is wrong?
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    Sorry you've not had any responses about this. Are you sure you've posted in the right place? Here's a link to our subject forum which should help get you more responses if you post there.
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    (Original post by Mermaidqueen)
    The question I'm working on is 2), for i) I got to the answer 20m/s when t=3 but Im getting stuck on ii) because the speed of 25m/s in a negative direction is equal to -25m/s velocity right? So it must be after 3 seconds so constant acceleration is -10m/s/s. I came to an answer of 4.5 seconds by doing -45/-10 = 4.5 . However the book I'm working it out from says that it is 7.5 seconds. I am quite confused, am i wrong or is it possible that the book is wrong?
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    could you send a picture of the question please? You made me confused
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    (Original post by GuyfromtheWA)
    could you send a picture of the question please? You made me confused
    This is it I think Name:  IMG_4413.jpg
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    v=u+at
    -25=20-10x
    -45=-10x
    x=45/10
    x=4.5
    However, since we didnt count the time that already passed, we have to add that too.
    Therefore, 4.5+3=7.5
 
 
 
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