? asked in 科學及數學其他 - 科學 · 1 decade ago

physics....

1. two scales are separeated by 2.00m, and a plank of mass 4.00 kg is placed between them. each scale is observed to read 2.00kg. a preson now lies on the plank , after which the right scale reads 30.0kg and the left scale reads 50.0kg. how far from the right scale is the person's center of gravity located?

2. a wire of diameter 0.0.mm stretches by 0.20% when a 6.28-N force is applied. what is the elastic modulus of the wire?

Update:

2. a wire of diameter 0.20mm stretches by 0.20% when a 6.28-N force is applied. what is the elastic modulus of the wire?

1 Answer

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  • 天同
    Lv 7
    1 decade ago
    Favorite Answer

    1. Weight of person = (30+50-4) kg = 76 kg

    Let y be the distance of the person from the right scale.

    Taking moment about the right scale,

    clockwise moment = 50gx2 Nm (where g is the acceleration due to gravity)

    anti-clockwise moment = (4x1 + 76y)g Nm

    Equating clockwise and anti-clockwise moments

    hence, 50x2 = 4x1 + 76y

    this gives y = 96/76 m = 1.26 m

    ---------------------------------------------------------------

    2. Stress=force/cross-sectional area= 6.28/[pi x (0.1x10^-3)^2] N/m2

    where pi=3.14159 ...

    Strain = 0.2/100

    Therefore, Young's modulus Y = stree/strain

    Y = {6.28/[pi x (0.1x10^-3)^2]}/(0.2/100) N/m2 = 10^7 N/m2

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