phy (SHM) - Ans.fyi 參考答案

phy (SHM)

2011-01-04 8:50 pm

圖片參考：http://imgcld.yimg.com/8/n/HA01002325/o/701101040036413873435980.jpg

the graph shows how the potential energy (U) of a body varies with its displacement (x) from a fixed point. which feature of the graph shows that the force acting on the body is directed towards the fixed point?

A. the fact that the grapg passes through the origin
B. the fact that the value of the potential energy is always positive
C. the fact that the graph is symmetrical about U-sxis
D. the fact that the potential energy increases as the body moves further away from the fixed point.

ans: D

PS i want to ask the ans y not C?

回答 (1)

天同

2011-01-04 11:08 pm

✔ 最佳答案

Since force F equals to the negative of the potnetial gradient,
i.e. F = -dU/dx
hence, on the right hand side of the origin (region of +ve x), the slope of the curve is +ve. Hence, the force F is -ve. That means the force is directed towards the -ve x direction, i.e towards the origin.

On the left hand side of the origin (i.e. region of -ve x), the slope is -ve, hence -dU/dx is +ve. That means the force F is in the direction of +ve x, i.e. towards the origin. The force is thus a restoring force.

Syummetry alone is not a sufficient condition to qualify a restoring force. It only shows that the forces are equal on both sides of the origin.

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