oh....

Question

Two blocks are connected by a string passing a frictionless pulley. The inclined plane is smooth. If the block of A (mass= m) goes down a distance h , which of the following statement is / are correct ? (A is suspended , B is on the slope(mass=M). )

(1) the potential energy lost by A is mgh .
(2) the PE gained by B is Mgh .

A (1) B(1) and (2)

the answer is A . Why ? Why not B ?

Answer 1

因為 B 唔係 suspended (半天吊), 而係係個斜枱上面, 吊住 B 的那部份繩因為斜枱的關係, 角度會變, 而令到 B 未必同樣升高 h 咁多, 所以 B 升高的高度唔保証係 h, 答案 (2) 咪唔啱囉!

Answer 2

By law of conservation of energy, the potential energy (PE) lost by A must be equal to the PE gained by B.

Unless the masses m and M are equal, m(gh) can never be equal to M(gh).