While driving on the (straight) highway at a constant speed v0, significantly ab
ID: 2282526 • Letter: W
Question
While driving on the (straight) highway at a constant speed v0, significantly above the speed limit, you pass in front of a parked police car without noticing it. After a few seconds, during which time you have moved a distance d, the police car starts chasing you with a constant acceleration a0. Give your answers in terms of v0, d, and a0 and check the units of your answers. Assume that you keep moving at a constant speed.
(a) How long will it take for the police car to reach you?
(b) How far has the police car traveled when it reaches you?
(c) What is the speed of the police car when it reaches you?
Explanation / Answer
Given,
You -
Initial Speed = v0
Acceleration = 0
Police -
Initial Speed = 0
Acceleration = a0
Distance between you & Police = d
(a)
Let time taken by Police to Catch = t
Now,
Distance travelled by Police in this time is given by Formula
S = u*t + 1/2 * at^2
Sp = 0 + 1/2 * a0*t^2
Sp = 1/2 * a0*t^2
Distance travelled by you in the Given time,
S = u*t + 1/2 * at^2
Sy = v0*t + 0
Sy = v0*t
Now, as police catches you in this time, we can say
Sp = Sy + d
Subsituing Values -
1/2 * a0*t^2 = v0*t + d
1/2 * a0*t^2 - v0*t = d
t * (1/2 * a0*t - v0) = d
t = d , (2*(d+v0))/a0
Time taken by Police car to reach, t = (2*(d+v0))/a0
(b)
Distance travelled by Police,
Sp = 1/2 * a0*t^2
Sp = 1/2 * a0 *((2*(d+v0))/a0)^2
Sp = 2*(d+v0)^2 /a0
(c)
Speed of the police car when it reaches you,
V = u + a*t
V = 0 + a0 * (2*(d+v0))/a0
V = 2*(d+v0)
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