# Physics problem!!

This discussion is closed.
18 years ago
#1
I would be grateful on how to go aobut this problem:

A car has been caught in a box junction by police and you need to contest that you couldn't have stopped before entering it. I have the following info:

Width of junction: 8m
Time orange light displayed: 3s
Speed Limit: 30mph
Car length: 2m

I'm taking for fact that the car has 120 horsepower=90 kW
and that the car weighs 1000kg

I'd be grateful for any help.
0
18 years ago
#2
u=30mph = 30*8/5 = 48 km/h = 48000/3600 = 13.3 m/s
v=0
KE of car = .5mv^2 = .5(1000)(13.3)^2 = 88888.888888888...... J

=> time taken to stop = KE/P = 0.99 s

=> he could have stopped within 3s of seeing the amber light.

but it depends on where he started. if the light changed amber while he was entering the junction, he would have entered it anyway.
0
18 years ago
#3
(Original post by elpaw)
=> time taken to stop = KE/P = 0.99 s
explain elly
0
18 years ago
#4
(Original post by edders)
explain elly
p = E/t => t = E/p
0
18 years ago
#5
(Original post by elpaw)
p = E/t => t = E/p
ah of course. 1second seems too short to me though, 2 sec more like with good brakes and an emergency stop lol
0
18 years ago
#6
can any1 answer this biology 1:

suggest an explanation, in terms of natural selection, for the change in brain size during the evolution of homo sapiens
0
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