Maths/Physics HELP!!?

Question

I have had a problem set for me to solve. Trouble is, I don't know the EQUATIONS to solve the problem. I cannot find them anywhere. Even google seems to be getting confused by what it is I'm looking for. Below is the problem. Can anyone help me by letting me know the equations for each part. The answers I'm hoping to arrive at by myself (otherwise I'm being lazy, and I do want to learn!)


A pedestrian is hit on a pedestrian crossing. Measurements show that the pedestrian had walked 4m from the start of the crossing before being struck by a car. (Assume the pedestrian's walking speed was 1.2m/s)

Police measurements show that the car skidded 20.4m before hitting the pedestrian, and came to rest a further 25.6m beyond the point of impact. The driver claims that he was travelling at less than 50mph (80km/h), and that the pedestrian came out of no where. 

Using that above information, calculate whether the vehicle was travelling less than 50mph, and investigate the driver's claim that the pedestrian came out of nowhere. 



Any help on this would be MUCH appreciated! Thank you!

Fireman · Accepted Answer

time taken(t) by the pedestrian to cover 4 m 
=>t = 4/1.2 = 3.33 sec
If the speed of the car was 80 km/hr or (80 x 1000/3600) 22.22 m/s
Thus the distance of car from the impact point (s) = 22.22 x 3.33 = 74 m
It shows that that the car driver has plenty of time to see the pedestrian.
To know the exact speed of car before impact, by work energy relation:-
=>W = ∆KE
=>F.s = 1/2mv^2
=>µk x N x s = 1/2mv^2
=>µk x mg x s = 1/2mv^2
=>v^2 = 2 x µk x 9.8 x 25.6
=>v = √[501.76µk]
=>v = 22.4√µk ----------------(x)
put the value of µk in it and you can calculate v of car just before the accident.

? · Answer

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Maths/Physics HELP!!?

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