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So this is what we are working with: enter image description here

Basically, the spring and the lever are used to press in the plug (part no 2). I need to find the force that is acting upon the welding (I painted it in red). Now I don't need you to calculate anything for me, just explain why is the free body diagram of the lever like this: enter image description here

I don't understand, if the spring is pushing the lever upwards, why is the force Fb drawn downwards? I can only guess that Fb is the force inside the bolt, but still, why are they ignoring the effect that the spring has on the lever? Also, on the left side, why isn't there a reactive force that the plug has on the lever? I would appreciate if someone could help me out here.

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    $\begingroup$ If this is a free body diagram of the lever as shown in the diagram, then ALL of the forces are drain in the wrong direction. $\endgroup$ – James Jan 25 '17 at 20:17
  • $\begingroup$ Who drew the FBD? Why don't you ask them? ... The diagram looks ok to me. The spring can pull down as well as push up. Same for the force on the plug and the weld. If the spring is pushing up, the arrows should be reversed. $\endgroup$ – sammy gerbil Jan 25 '17 at 20:23
  • $\begingroup$ I don't know who drew it, I bought this "book" with solved exams... Yes but the spring is pushing up, yet the force Fb is going down $\endgroup$ – M. Wother Jan 25 '17 at 20:27
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The spring will always push because there is a contact on the right, and a contact can only push and never pull.

Similarly on the left the contact force is always upwards or otherwise the rocker arm will hop out of place.

The FBD is drawn upside down for some reason. The force on the bolt is downwards, and hence the need for a weld or threads. Otherwise you could just seat the bolt in a hole and it would be fine.

Here is how I see it

fbd

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