一個F.5 PHYSICS 問題

2009-09-12 4:17 pm

An object starts from rest and travels 14m in the fourth second of its motion. If the acceleration is the same throughout the ibject's motion (uniform acceleration), calculate:
a) the average acceleration of the object.
b) how far the object travels in 4s
c) the distance travelled in the tenth second of the motion

我想問4s和第4s的計法有什麼不同? 第四秒走了14M和4秒走有14M何不同??

搞到我計極也計不到a) part

回答 (1)

Prof. Physics

2009-09-12 5:17 pm

✔ 最佳答案

a. Let v be the speed of the object just after the third second

By v = at

v = 3a ... (1)

Now, consider the motion in the fourth second

by s = vt + 1/2 at2

14 = 3a(1) + 1/2 a(1)2

a = 4 ms-2

b. Consider the whole journey,

By s = 1/2 at2

s = 1/2 (4)(4)2 = 32 m

c. Consider the motion in ten seconds

by s = 1/2 at2

s = 1/2 (4)(10)2 = 200 m

Consider the motion in nine seconds

by s' = 1/2 at2

s' = 1/2 (4)(9)2 = 162 m

So, the distance travelled in the tenth second = 200 - 162 = 38 m

Or you may draw a v-t graph to find out the answers.

4 s means from time = 0 to 4 s, the fourth second means from the 3rd second to the fourth second, so they are different.

We just consider part of the journey in the fourth second, but not from the beginning to the fourth second.

參考: Physics king

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