# 1d motion question

## Homework Statement

at the instant the traffic light turns green, s starts from rest and accels at a_s. at the same time k passes s with constant velocity v_k.

given a_s, v_k,
find distance before the cross again
time it takes to do so
velocity of s at this point

plot the position as a function of time for both. take x=0 at the intersection <- no idea what last sentence means.

## Homework Equations

v_s = a_s + t
d = 1/2 a_s * t^2
v_s^2 = 2a_s*d

d= v_k * t

## The Attempt at a Solution

t = sqrt(2*v_k/a_s)
d = v_k * sqrt(2d/a_s)
v_s = a_s*sqrt(2d/a_s)

as for this plotting, does this mean I "start" the graph at their intersection? which one?

as for this plotting, does this mean I "start" the graph at their intersection? which one?
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Yes.
k with constant velocity-a straight line.
s with constant acceleration- a parabola.

TSny
Homework Helper
Gold Member
plot the position as a function of time for both. take x=0 at the intersection <- no idea what last sentence means.

...as for this plotting, does this mean I "start" the graph at their intersection? which one?
I think the "intersection" is the intersection of the streets where the traffic light is positioned. So, take x = 0 to be the location where k initially passes s.

wanted to be sure, thanks.