f4 phy

👨🏻‍🏫 學術台其他 - 科學

Crystal

2012-01-23 7:44 am

圖片參考：http://imgcld.yimg.com/8/n/HA04937267/o/701201220068913873422830.jpg

Consider time interval 0-5s and 20-30s.The net force is zero.
The answer of my book said that the object is moving at a constant velocity at those time intervals.Can the object at rest instead of moving at constant velocity between at t=0-5s and 20-30s?

更新1:

但direction of net force from 0-30s 都係一樣嗎? 5-10s 同10-20s 的net force會唔會唔同direction?

回答 (3)

myisland8132

2012-01-23 8:44 am

✔ 最佳答案

若果只是就圖f來說﹐在 0 - 5s 中﹐若果物體本來是靜止的﹐則仍然會保持靜止狀態﹐若果原本是動的﹐則保持勻速狀態。但因為在5s後物體獲得了力而且力的方向永遠是正的﹐因此物體獲得了速度﹐而當淨力變成零後﹐根據牛頓第一定律﹐物體只能保持勻速狀態而不會靜止﹐除非再加一個相反方向的力。

2012-01-24 14:53:54 補充：
不會。力是有方向的﹐若果淨力是反方向﹐則數值上是負數。

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[匿名]

2012-01-23 6:34 pm

Apply F=ma; when F=0, a =0
So there is no acceleration when F is zero, and the object is in constant velocity. It can be moving or rest.

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天同

2012-01-23 8:31 am

The object is definitely not at rest during the time interval 20 - 30 s as it has been subject to a net force F, and hence has an acceleration and velocity.

But you cannot tell whether it is at rest or moving at constant speed during 0 - 5 s before the net force is applied.

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