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# Mechanics 1 help Watch

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1. just when i thought i never have to do physics again

anyways, i've jsut started suvat equation/rectilinear motion

a train travelling on straight rail, from A to B.
acceleration = -0.6ms^-2
u = 72km/h
t = 25s
find the final velocity in km/h.

i know im using the equation v=u+at

v=72+(-0.6*25?)
do i use the time in seconds or convert it into hours? (and why? I'm curious)
2. (Original post by ihatePE)
just when i thought i never have to do physics again

anyways, i've jsut started suvat equation/rectilinear motion

a train travelling on straight rail, from A to B.
acceleration = -0.6ms^-2
u = 72km/h
t = 25s
find the final velocity in km/h.

i know im using the equation v=u+at

v=72+(-0.6*25?)
do i use the time in seconds or convert it into hours? (and why? I'm curious)
Use consistent units and you will get the answer in the same units.

Eg if you were adding 5 hours to 40 minutes then you would not say t = 5 + 40. You would either convert 40 minutes to 2/3 hours to give t = 5 + 2/3, so you know that the answer is in hours, or alternatively t = 5*60 + 40, so you know that the answer is in minutes. As you have acceleration (already in m/s^2) you would need to be very careful as well. Best to stick to seconds for that reason (unless your answer requires km/h which I have just seen )
3. You need to use the same units throughout a calculation.

If you are talking about km/h then convert all distance values to km and all time values to h before subbing them into the formula.

Alternatively, you can do everything in m/s and then convert the final answer to km/h.
4. (Original post by SeanFM)
Use consistent units and you will get the answer in the same units.

Eg if you were adding 5 hours to 40 minutes then you would not say t = 5 + 40. You would either convert 40 minutes to 2/3 hours to give t = 5 + 2/3, so you know that the answer is in hours, or alternatively t = 5*60 + 40, so you know that the answer is in minutes. As you have acceleration (already in m/s^2) you would need to be very careful as well. Best to stick to seconds for that reason.
so im matching the time with the unit they use in acceleration? what about km/h?
5. (Original post by BasicMistake)
You need to use the same units throughout a calculation.

If you are talking about km/h then convert all distance values to km and all time values to h before subbing them into the formula.

Alternatively, you can do everything in m/s and then convert the final answer to km/h.
so its best if i convert 72km/h into m/s? and then conver my final answer to km/h if they want it km/h?
6. (Original post by ihatePE)
so im matching the time with the unit they use in acceleration? what about km/h?
I have just seen that the answer requires km/h so you may as well convert everything to that.
7. (Original post by SeanFM)
I have just seen that the answer requires km/h so you may as well convert everything to that.
we all make mistakes just to clarify i change the seconds to hours? what do i do about the acceleration thats in m/s?

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