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2.71.
Model:
Treat the car and train in the particle model and use the constant acceleration kinematics
equations.
Visualize:
Solve:
In the particle model the car and train have no physical size, so the car has to reach the crossing at an
infinitesimally sooner time than the train. Crossing at the same time corresponds to the minimum
1
a
necessary to
avoid a collision. So the problem is to find
1
a
such that
2
45
x
=
m when
2
60
y
=
m.
The time it takes the train to reach the intersection can be found by considering its known constant velocity.
20
02
2
2
60 m
30 m/s
2.0 s
yy
vv
t
tt
t
−
==
=
=
⇒
=
−
Now find the distance traveled by the car during the reaction time of the driver.
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 Fall '09
 geller
 Acceleration

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