# Why isn’t the reaction force perpendicular in this case?

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#1
On Q9, I tried resolving forces (assuming that R was perpendicular to the wall) to get the answer but i got the wrong answer. Then by moments I get the right answer. And after reading part b and the answer, the reaction is said to be at 60° to the vertical, but I thought reaction force was always perpendicular to the surface the object is acting on? I’ve attached the question and my working out as well as the solution.

0
11 months ago
#2
Vertically, there would have to be friction to stop the climber's feet slipping
Horizontally, the wall would have to push back to stop the climber swinging through the wall.
So it must be diagonally as it has both a horizontal and vertical component.

The reaction is perpendicular in simple cases like a stationary block on a flat surface. The reaction only counters the weight in that case.
0
11 months ago
#3
(Original post by Mathstermind)
On Q9, I tried resolving forces (assuming that R was perpendicular to the wall) to get the answer but i got the wrong answer. Then by moments I get the right answer. And after reading part b and the answer, the reaction is said to be at 60° to the vertical, but I thought reaction force was always perpendicular to the surface the object is acting on? I’ve attached the question and my working out as well as the solution.

mqb2766 has it correct, but more than that, I would say that this is a semantic point and that not everyone follows the same rules. I would agree with you that the reaction force always acts normally to the surface at the point of contact. There is also a frictional force acting at the point of contact, and it is the resultant of these forces that the question is calling the reaction force. This is not uncommon, but I think that it is better to think of the reaction force and friction as two separate things.
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