Forearm Homework: Finding Forces, Tension, Moment WRT Elbow Point E

[Ronaldo95163]

Homework Statement

Figure 5 below shows an approximation of the system of forces acting on the human forearm. The weight of the forearm is 25N and acts 15cm from the elbow of a girl lifting a dumbbell of weight 50N

Find the
1)Force, F, exerted on the human forearm by the muscle in the upper arm
2)Forces acting at the point, E of the elbow
3)Tension if theta is 45 degrees

Homework Equations

sum of ACW = sum of CW
sum of upward forces = sum of downward forces

The Attempt at a Solution

My Attempt is in the pictures below

 

[NascentOxygen]

Hi Ronaldo95163. You could use ∑ forces iff you knew the force acting at the elbow, but we don't know anything about force components at E (apart from the fact that they exist). So your first part is wrong. You will have to approach it differently.

In any case, because of the big mechanical disadvantage seen in this type of lever, you know that the upwards force needed close to the pivot point is going to be >> the two downward forces you have considered.

 

[Ronaldo95163]

Thanks a lot man...wasn't sure if to consider thet force at E first...what I figured tho was that
Fsin70 = 75 + E

Solve for E using moments and then substitute it into find for F