A 3.0 m long rigid beam with a mass of 130 kg is supported at each end. A 65 kg student stands 2.0 m from support 1. How much upward force does each support exert on the beam?
Net Forces = F1 + F2 - w(student) - w(beam) = 0
where F1, F2 are the upward forces of support 1 and support 2; w indicates weights of beam and student
The Attempt at a Solution
I was able to finally solve this question but there is something I am still not clear on. During the initial part of our physics course (dynamics combined with kinematics, for example) we used normal forces when a mass was resting on top of a surface, including it in the force diagram even when there was no acceleration in the "y" direction and so the normal force was not relevant to the problem being solved. However, in statics problems similar to the one above, the normal force supporting a mass on a beam is not included. For example, in the problem given above, the student contributes to the weight of the beam, however the beam does not provide a normal force on the student - or at least it is not included when summing the vertical forces in the problem, anyway. In these types of "beam" static equilibrium problems, why is it that only the weight of the mass is included when summing forces, and yet never the normal force on the mass? Isn't the beam providing a normal force on the mass?