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I have a question about reaction forces when a body is hinged to a wall.

See the picture for a random example I just plugged off the internet.

When a mass is on a horizontal surface, the reaction force acts normal to the surface, in the direction of the line of action of the gravitational force exerted by the mass on the ground.

In this case, thus, you would expect the hinge to exert a force into the rod, because this is normal to the wall. However, there is also a component parallel with the wall? Why is this?

Edit: the vote to close is silly - I am not asking about a problem without giving any thought. I have said the picture is just an example off the internet, I am asking about a fundamental physics concept here... please tell if me I am doing anything otherwise...

enter image description here

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closed as off-topic by Gert, garyp, Kyle Kanos, Jon Custer, Yashas May 21 at 16:58

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  • $\begingroup$ "you would expect the hinge to exert a force into the rod, because this is normal to the wall." Just draw the force vectors that act on the system. $\endgroup$ – Gert May 20 at 20:10
  • $\begingroup$ @Gert That is the problem I'm having? I want to know why the hinge doesn't work in the same way as a normal reaction force, acting completely normal to the wall due to the rod? $\endgroup$ – PhysicsMathsLove May 20 at 20:11
  • $\begingroup$ @PhysicsMathsLove I think the problem you may be having is thinking the cable can alone support the forces of the weight of the rod and hanging ball. But you also need the sum of the moments about the end of the rod be zero. That can't happen without an upward reaction by the wall. See my complete answer. $\endgroup$ – Bob D May 20 at 20:38
  • $\begingroup$ "I am asking about a fundamental physics concept here" No, not really. This kind problem requires applying N2L to all parts of the system. $\endgroup$ – Gert May 20 at 20:41
  • $\begingroup$ @Gert Just curious. What is "N2L"? $\endgroup$ – Bob D May 20 at 21:18
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In this case, thus, you would expect the hinge to exert a force into the rod, because this is normal to the wall. However, there is also a component parallel with the wall? Why is this?

I think the problem you may be having is thinking the cable can alone support the forces of the weight of the rod and hanging ball. But you also need the sum of the moments about the end of the rod be zero. That can't happen without an upward reaction by the wall. If that is still not clear, read on.

The reaction forces at the wall consist of a normal component because there will be a horizontal leftward force the cable exerts on the rod that needs to be opposed by a horizontal component reaction force of the wall directed rightward.

There also needs to be a vertical upward (parallel) reaction of the wall on the rod that, together with the vertical upward force of the cable on the rod is needed to balance the downward forces of the weight of the rod and the hanging weight ball, as well as needed so that the sum of the moments about the end of the rod is zero.

Do you know how to draw free body diagrams (FBDs)? If you do, do a free body diagram of the rod showing the forces exerted on it by the cable, the wall, its own weight, and the weight of the ball. Take the sum of the moments about the wall and set to zero. Take the sum of the horizontal and vertical forces and set each of them (two equations) equal to zero. That will give you enough equations to determine the normal and vertical (parallel) reaction forces of the wall. If you need help, get back.

Hope this helps.

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I'm not sure that I'm understanding your question "Why is this?" I'll take a stab at one interpretation of the question.

The hinge is a hinge. By its construction in can apply forces perpendicular and parallel to the surface. Your graphic shows the pin in the hinge. The pin can propagate force in any direction.

In many examples, the normal force is displayed between two flat surfaces, often without friction. If there is no friction, the only possible force is normal to the surface. Keep in mind that "normal" is a mathematical term that is a synonym for "perpendicular". In these cases there is no parallel force, of course.

There could be friction between the two flat surfaces. That would be another example of a situation where there is a parallel force as well as a normal force.

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