physics 唔識做

2008-10-02 1:03 am

1) A racing car accelerates from rest a speed of 100km h^-1 in 3.2 s. Find
the average acceleration of the car.

2) A car travels along a straight road from A toB with a uniform acceleration .The speed of the car is V1 at the instant when the half of the journey time
from A to B is elapsed and its speed is V2 at the mid-way of A and B.which
of the following is correct?

A. V1 is always smaller than V2
B. V1 is always greater than V2
C. V1 and V2 are always equal.
D. wherther V1 is always greater or smaller than V2 depends on the initial
velocity of the car at A.

第2條需要詳細而簡單既解釋同埋點解其他吾岩...

THANK YOU !!希望識既人幫幫手^^

回答 (1)

Audrey Hepburn

2008-10-02 1:52 am

✔ 最佳答案

1. Initial speed, u = 0

Final speed, v = 100 kmh-1 = 27.78 ms-1

Time, t = 3.2 s

By v = u + at

27.78 = 0 + a(3.2)

Average acceleration, a = 8.68 ms-2

2. The answer is A. (But in fact the question is badly designed, because acceleration can both be +ve or -ve. If option D is whether v1 is always greater or smaller than v2 depends on the direction of acceleration of the car, I will choose D.)

As the car is moving with uniform acceleration, its speed must increases with time.

Hence, if we consider a journey from A to B. The time required to travel from A to mid-way of AB, it must be larger than that of from the mid-way to B.

And for front half of the journey time, the distance travelled by the car must be smaller than half the way of AB.

That means v2 must occur later than v1, in which v2 is larger than v1.

Hence v1 is always smaller than v2.

參考: Myself~~~

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