Exam Discussion!

[Ya Habibi]

For the arch question in materials and structures, did people get that no steel reinforcing was needed because the blocks were all in compression?

The arch needed no steel reinforcements. Arches can support themselves because everything is in compression, and usually the material is strong against compressive stress.

[silverpixeli]

Very confident. Some will say Q8 is b, however I think that if the wall wasn't there, the cantilever would wing left and down, so the wall must provide a force right and up, hence a. However, it's probably the only answer I'm unsure about. The rest are definitely right.

Someone described this question to me and I think you're right.

If you consider torques about the point of connection to the wall there's the torque from the weight of the cantilever itself and also from the tension in the cable, or effectively its vertical component. I understand that these were at different distances from the pivot. That being the case, their magnitude must have been different.

If the vertical component of tension up through the cable and the vertical weight force of the cantilever don't have the same magnitude, then there must be some other vertical force acting on the cantilever else it would not be in horizontal equilibrium. That force will be the vertical component of the force from the wall, meaning that overall it will point diagonally (with a horizontal component that cancels out the horizontal component of tension).

Only caveat is that I'm assuming the cable was not connected at the centre of mass of the bar, else the forces would cancel and the force at the wall would be horizontal only.

[Plitzer]

Someone described this question to me and I think you're right.

If you consider torques about the point of connection to the wall there's the torque from the weight of the cantilever itself and also from the tension in the cable, or effectively its vertical component. I understand that these were at different distances from the pivot. That being the case, their magnitude must have been different.

If the vertical component of tension up through the cable and the vertical weight force of the cantilever don't have the same magnitude, then there must be some other vertical force acting on the cantilever else it would not be in horizontal equilibrium. That force will be the vertical component of the force from the wall, meaning that overall it will point diagonally (with a horizontal component that cancels out the horizontal component of tension).

Only caveat is that I'm assuming the cable was not connected at the centre of mass of the bar, else the forces would cancel and the force at the wall would be horizontal only.

You would be correct however the cable was supporting the bar at the centre of it's mass, so yes the force from the wall would be horizontal on the beam

[silverpixeli]

You would be correct however the cable was supporting the bar at the centre of it's mass, so yes the force from the wall would be horizontal on the beam

Cheers for clearing that up, I'm still keen to see a paper and I'll make up some worked solutions if someone can get me a scan!

[silverpixeli]

Full scan in process! My physics teacher got me a copy of the paper. I'll add it to the main post when done.

Edit: done (5.17MB)