physics....

用戶98670

2006-12-04 9:52 am

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?

更新1:

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?

回答 (1)

天同

2006-12-05 12:39 am

✔ 最佳答案

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

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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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