Physics Easy 10 points ?

[匿名]

2016-07-30 11:52 am

To practice Problem-Solving Strategy 8.1 Static equilibrium problems.

When you lift an object by moving only your forearm, the main lifting muscle in your arm is the biceps. Suppose the mass of a forearm is 1.10 kg . If the biceps is connected to the forearm a distance dbiceps = 2.50 cm from the elbow, how much force Fbiceps must the biceps exert to hold a 650 g ball at the end of the forearm at distance dball = 35.0 cm from the elbow, with the forearm parallel to the floor? How much force Felbow must the elbow exert?

Picture - http://d2vlcm61l7u1fs.cloudfront.net/media%2F47e%2F47e9030e-e631-4e97-b668-33a4f589c738%2FphpbhL7Fg.png

1. What is the horizontal distance dforearm between the elbow and the point where the weight of the forearm acts?

回答 (1)

NCS

2016-07-30 6:52 pm

✔ 最佳答案

I think there is something wrong with the problem statement. In order to answer the question about Fbiceps, we need to know the distance at which the weight of the forearm acts. But that's posed as a question rather than a bit of data.

If we ASSUME that d_forearm = 17.0 cm, then summing the moments about the elbow results in this equation:
ΣM = 0 = Fbiceps*2.50cm - (1.10kg*17.0cm + 0.650kg*35.0cm) * 9.8m/s²
Fbiceps = 406N·cm / 2.5cm = 162 N ◄

Sum the vertical forces to get Felbow:
Felbow = 162N - (1.10 + 0.650)kg * 9.8m/s² = 145 N ◄

If on the other hand we know what Fbiceps was, we could calculate exactly where the weight of the forearm works.

Hope this helps!

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