a mc physics questions about mechanics 2

2007-10-20 11:45 pm

(With detailed steps for the following two questions please)

A bus starts from rest and accelerates aling a straight road with acceleration of 2m s-^2. A motorbike 100 m behind the bus is traveling at a uniform speed 10m s-1. The nearest distance between the motorbike and the bus should be?

A.10m B. 15m C. 75m D.150m

~thank you

回答 (1)

NCY

2007-10-20 11:57 pm

✔ 最佳答案

the answer is C
For the bus, u = 0ms-1 , a = 2ms-2 , Take v= 10ms-1
v=u+at
10 = 0+2t
t = 5
∴velocity of bus is 10ms-1 after 5s. Motorbike is faster than the bus at the first 5s.

Displacement of motorbike in the first 5s
= velocity×time
= 10×5
= 50m
Now calculate the displacement of the bus in the first 5s.
s=ut+0.5at2
=(0)(5) + 0.5(2)(5)2
= 25m

[Alternative: s = (u+v)t/2 = (0+10)(5) /2 = 50/2 = 25m]
↑(s=average velocity×time)

∴Nearest distance
= 100+distance travelled by bus﹣distance travelled by the motor bike
= 100+25﹣50
= 75m

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