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# Which SUVAT equation do I need for this?

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1. A car is travelling at 36km/h when the driver has to perform an emergency stop. During the time the driver takes to appreciate the situation and apply the brakes the car has traveled 7m. It then pulls up with constant deceleration in a further 8m, giving a total stopping distance of 15m.

a) Find the initial speed of the car in m/s and the time that the driver takes to react

S=15
U=36
V=0
A=A
T=T
2. (Original post by ℓove)
A car is travelling at 36km/h when the driver has to perform an emergency stop. During the time the driver takes to appreciate the situation and apply the brakes the car has traveled 7m. It then pulls up with constant deceleration in a further 8m, giving a total stopping distance of 15m.

a) Find the initial speed of the car in m/s and the time that the driver takes to react

S=15
U=36
V=0
A=A
T=T
Convert to m/s first

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3. First part of question is converting from km/h to m/s, so 36'000m/h then divide that by 60^2 (=10m/s).
The reaction time is the time before he applies the brakes, so it is before deceleration begins therefore aA= 0.
So set up S = 7, U = 10, V = U (bc A = 0) A = 0 T = ?
A good equation might be s = ut + 1/2at^2, since a is 0 and the whole second term disappears
4. (Original post by ℓove)
A car is travelling at 36km/h when the driver has to perform an emergency stop. During the time the driver takes to appreciate the situation and apply the brakes the car has traveled 7m. It then pulls up with constant deceleration in a further 8m, giving a total stopping distance of 15m.

a) Find the initial speed of the car in m/s and the time that the driver takes to react

S=15
U=36
V=0
A=A
T=T
Well the first part is just converting km/h into m/s.

For the second one, you know that the car travels 7m before the brakes are applied, so the displacement is 7.

You know S, U, V and A which is the majority of the variables so you can use pretty much any SUVAT equation you want but not all of them will be able to tell you the time. I'd personally go for but if you notice shortcuts like above since a=0 then you may use that as well.

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Updated: October 11, 2016
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