Physics(acceleration/velocity)

2009-01-06 2:46 am

The speed limit on most roads in HK is 50km/h. A speeding car traveling at a constant velocity of 60km/h passes a police car, which was by the road side. the police car accelerates at an average rate of 6m/s^2 to chase the speeder.

(a) How long does it take for the police car to reach the velocity of the speeding car? (I've done this and the answer is 2.78 sec)

(b) How much additional time will the police car need in order to catch up to the speeder?

(c) What is the speed of the police car when it catches up with the speeder?

Steps are needed.

回答 (1)

Gabriella Montez

2009-01-06 5:35 am

✔ 最佳答案

b) 60km/h = 16.667m/s
Suppose the time required is t s.
Displacement of the speeding car = Displacement of the police car
16.667t = ( 0 )( t ) + ( 0.5 )( 6 )( t2 )
t2 - 5.556t = 0
t = 0 ( rejected ) or 5.56s
Hence the additional time required is 5.56 - 2.78 = 2.78s
c) v = u + at
v = 0 + 6 ( 5.556 )
= 33.3m/s

2009-01-05 22:28:21 補充：
Hey, you haven't changed to the correct unit!!!
As I hv converted in the 1st line: 60km/h = 16.667m/s
So it is reasonable for the police to catch up the car with a speed of 33m/s.

2009-01-05 22:30:18 補充：
Please beware of the units in the question. You may need to convert them.

參考: My Phy Knowledge

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