What is the lenght of the lever arm? (when torque and force are given)

[DeeAussie]

Homework Statement

If a torque of 55N.m is required and the largest force that can be exerted by you is 135N, what is the length of the lever arm that must be used?

Homework Equations

Lever Arm=(r)(sin)
Torque=(Force)(r)(sin)

The Attempt at a Solution

To be honest, I'm stumped

 

[kelly_kelly]

The answers are in the formulas you've posted. If lever arm = r sin theta, and torque = force * r sin theta, then torque = force * lever arm. Since you know torque and force, you can solve for lever arm.

 

[PhanthomJay]

The answers are in the formulas you've posted. If lever arm = r sin theta, and torque = force * r sin theta, then torque = force * lever arm. Since you know torque and force, you can solve for lever arm.

That's not quite right, and neither is the problem statement very clear. if T=rFsintheta, then you need to know theta in order to solve for r, where r is the length of the lever arm, unless one assumes that the force is applied perendicular to the arm. Is theta given in the problem?

 

[DeeAussie]

That's not quite right, and neither is the problem statement very clear. if T=rFsintheta, then you need to know theta in order to solve for r, where r is the length of the lever arm, unless one assumes that the force is applied perendicular to the arm. Is theta given in the problem?

Everything I listed was what was given so I guess you do have to assume perpendicular?

 

[PhanthomJay]

Everything I listed was what was given so I guess you do have to assume perpendicular?

I looked up the definition of 'lever arm' on several web sites, and discovered that some call it the length of the position vector between the point of application of the load and the axis of rotation (Wiki) while others call it the perpendicular distance from the line of action of the force to the axis of rotation. I tend not to trust Wiki most of the time (they take a simple topic and make it as complex as is humanly possible), so let's go with the latter. In which case the lever arm is __________(fill in the blank).