What Forces are Involved in Torque and Equilibrium Problems?

[odie5533]

Homework Statement

http://img404.imageshack.us/img404/248/sdaft7.png

The Attempt at a Solution

The arrows on the diagram are drawn in, as are the labels (except for the angles).
$$\sum F_{x} = P_{x} - Tcos30 = 0$$
$$T = \frac{P_{x}}{cos30}$$
$$\sum F_{y} = N_{p} - 8900N - 650g - Tsin30 = 0$$
$$N_{p} = 15274N + Tsin30$$
$$N_{p} = 15274N + P_{x}tan30$$

I think I'm missing forces, and probably making up a few. If anyone could check them over before I continue I'd really appreciate it.

 

[calcisforlovers]

you're off to a good start. make sure you have the weight of the box in kg. now you need a third equation, which comes from summing the torques about the pivot and setting that equal to 0.

 

[odie5533]

Are all the forces right though? Am I missing any?
Here's the third equation:
The angle between the strut and the tension from the cable is 16 degress: 180 - 30 - (180 - 46) = 16
$$\sum\tau = Tlsin16 - \frac{1}{2}l650g(cos46) - 8900l(cos46) = 0$$
$$\frac{P_{x}}{cos30}lsin16 = 325lg(cos46) + 8900l(cos46)$$
$$\frac{P_{x}}{cos30}sin16 = 325g(cos46) + 8900(cos46)$$
$$Px = \frac{325g(cos46) + 8900(cos46)}{sin16} = 26381N$$
$$T = \frac{26381}{cos30} = 30462N$$
$$N_{p} = 15274 + P_{x}tan30 = 15274 + 26381(tan30) = 30505N$$
$$P_{tot} = 40330N$$
$$\phi = 49 degrees$$

 

[calcisforlovers]

i just noticed. in your summation of the forces in the y direction, you have the mass of the strut. mass is not a force, so it shouldn't be included in the summation of the forces. you have to find the weight of the strut. (i made a bad mistake by saying that the weight of the crate should be in kg. mass and weight are not the same thing). same deal in your summation of torques. torque is defined as the force times the moment arm, not the mass times moment arm. i think you should have it though. just play around with your three equations till you get it.

 

[odie5533]

When I listed 600g, I meant the g as in 9.81 not grams or kilograms. When I computed the answers, I multiplied things by 9.81 wherever g is. I use g so that I get a more accurate answer. Am I using all the right forces though? Tension in cable pulling down and to the left, normal force from pivot on the strut pushing up and to the right, weight of strut going down, and weight of crate going down. Is that all?