Hold your upper arm vertical and your lower arm horizontal with your hand palm-d

Question

Hold your upper arm vertical and your lower arm horizontal with your hand palm-down on a table, as shown in the figure. If you now push down on the table, youll feel that your triceps muscle has contracted and is trying to pivot your lower arm about the elbow joint. If a person with the arm dimensions shown pushes down hard with a 90 N force (about 20 lbs), what force must the triceps muscle provide? You can ignore the mass of the arm and hand in your calculation. Please show all work and any equations used.

Hold your upper arm vertical and your lower arm horizontal with your hand palm-down on a table, as shown in the figure. If you now push down on the table, you?ll feel that your triceps muscle has contracted and is trying to pivot your lower arm about the elbow joint. If a person with the arm dimensions shown pushes down hard with a 90 N force (about 20 lbs), what force must the triceps muscle provide? You can ignore the mass of the arm and hand in your calculation.

Explanation / Answer

1) Define system

Pivot = lower arm bone

F(down) = 90N with distance .3m to the pivot point

F(tricep) = F with distance of .024 m to the pivot point

2) Calculate torques about the pivot (remember the torque = 0 abou the pivot)

Torque of force down = -90N * .3m

Torque of tricep = F(in newtons) * .024

Torque of Pivot = 0

Now put together : 0 = (-90N * .3m) + (F*.024)

3) Solve for F

F= 1125

Source: just completed that question on mastering physics

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