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# Kinematics- acceleration question watch

1. 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?
Attachment 698422
2. 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.
3. (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?
Attachment 698422
could you send a picture of the question please? You made me confused
5. (Original post by GuyfromtheWA)
could you send a picture of the question please? You made me confused
This is it I think
6. 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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Updated: October 28, 2017
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