Another suvat

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KevinE

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Sorry guys thanks for the help yesterday but I've become stuck again:

A car has an initial speed of
u = 10m/s and accelerates at a = 6m/s^2
for s = 120 metres.

Using Using
s=ut+1/2 at^2

calculate the time taken t in seconds.

Again I'm just studying I know the answer is 4.87sec
 
KevinE said:
Sorry guys thanks for the help yesterday but I've become stuck again:

A car has an initial speed of
u = 10m/s and accelerates at a = 6m/s^2
for s = 120 metres.

Using Using
s=ut+1/2 at^2

calculate the time taken t in seconds.

Again I'm just studying I know the answer is 4.87sec
Click to expand...

A quadratic equation in t:
120 = 10 t + 3 t2
or
3 t2 + 10 t - 120 = 0.
 
Again I managed to simplify to 120 = 10t + 3t^2

I tried to just rearrange the formula and came up with an answer of 1

120/10 = t + 3t^2

12 = t + 3t^2

12/3 = t + t^2 (I'm not sure if I could do this?)

4= t+t^2

Square root 4 = 2 (I don't think I can do this either?)

2= t + t

T = 1
 
Obviously that was horribly wrong so I thought the equation when simplified looked a little quadratic

so I set it up - 3t^2 + 10t - 120 = 0


This made my calculator die... when I put the numbers into the quadratic equation, it gave me a math error!
 
Thanks Ishuda, I didn't see you reply - yeh I was thinking that...

Hmmm obviously I'm imputing the numbers wrong. Inserting negative values were there should be positives maybe? I often do this need to practice the old quadratics more...

Ok let me check here again
 
Roger on that Denis i'll follow it through and make sure i'm getting the correct value. Once again many thanks.
 
Perfect I know what I was doing now, I was trying to simplify 4*3*-120 on it's own first I think.
 
You need to understand the following (as you made similar mistakes above). We know that 15=12+3. If we divide the whole left side, which is 15, by 3, it does not make sense to divide part of the right side by 3. That is 15/3 does not equal 12/3 + 3 which is 4+3=7 and that is NOT 15/3=5
Rather we must divide each term by 3. That is 15/3 = 12/3 + 3/3 which is 4+1 or 5 and that is what 15/3 equals!!

You mentioned above that you were not sure if you could do what you did. OK, not a big deal BUT take the time and see if it works with numbers! These letters are just place holders for numbers, so if it works for numbers it works for letters! It it does not work for numbers it does not work for letters.
 
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