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babi_gurl · Mar 16, 2007, 11:39 PM

A car travelling at 100km/h takes 65 m to stop after the driver sees a child run on to the road chasing a ball. If the driver's reaction time is 0.25s, calculate the deceleration of the car.

Thanks

Capuchin · Mar 17, 2007, 02:38 AM

you need:

$s = vt$

this will help you find the distance that he is decellerating over
and

$v^2 = u^2+2as$

this will help you find the decelleration

let me know if you need further help.

Evil dead · Mar 17, 2007, 10:52 AM

Hmm I don't get it. Aren't we supposed to consider the reaction time of the driver?

s=vt
65 = 100xt
t = 0.65s

now how does this fit into the suvat equation?

Capuchin · Mar 17, 2007, 11:52 AM

s=vt

s=100*0.25 = 25m

so the driver has 65-25 = 40m to decelerate in, plug s=40, v=0, u=100 to get out t