Space Time & Motion A.2 Forces & Momentum (id: d7827558d)

本题目来源于试卷: Space Time & Motion A.2 Forces & Momentum，类别为 IB物理学

[单选题]
An object of mass M on a string movetr+q hoa6ng jq/( vnr c8h o:uhm;:x;4s in a ci+sbo*gd v(gs )7tua 0trcle of radius r at a constant speeossv*dttg 7)(gab0+ud.



Which of the following statements is true about the motion of the object:
I. The resultant work done on the object is zero.
II. The velocity of the object is constant.
III. The object is accelerating

A. Ⅰ only
B. Ⅱ only
C. Ⅱ and Ⅲ only
D. Ⅰ and Ⅲ only


参考答案:  D


本题详细解析:
Explanation:

I is correct. The accelerating force is always perpendicular to the direction of motion, therefore no work is done. $W = F_s \cos (\theta)$, and $\theta$ is always 90°. Also, $W = \Delta E_k$ and the speed of the object is constant so there is no change in kinetic energy.
II is incorrect. Because velocity is a vector quantity and the direction is constantly changing, velocity is not constant.
III is correct. Because velocity is constantly changing, the object is always accelerating.

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