A police officer on a motorcycle chases a speeding car on a straight highway. The car’s speed is constant at 30 m/s, and the officer is at a distance of 300 m behind; when she starts the chase with a speed of 40 m/s. How long will it take her, to catch up with the car and what is the relative velocity at that moment?
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The origin of co ordinate system (x-axis), we assume it is the point where speeding car passes the police car
x
speeding
(t)=30t→1
The motion of the speeding car is the motion with constant speed of 30㎧
The motion of the police car is the motion with constant acceleration
∴x
police
(t)=v
initial
(t)+a
2
t
2
v(t)=v
initial
+at
v
2
(t)−v
initial
2
=2ax
police
(t)
a=3㎨
Initial velocity=0=v
initial
∴x
police
(t)=1.5t
2
→2
v(t)=3t
v
2
(t)=6x
police
(t)
x
speeding
(t
0
)=x
police
(t
0
)→3
Substituting equation 1,2
30t
0
=1.5t
0
2
t
0
=
1.5
30
=20s
Thus after 20s the police car catches the speeding car.
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