# Maths Question Help Urgent!

Hey guys, do you mind helping me with q13 from mixed exercise 7. Why do they not include the reaction force on the rod which acts vertically upwards at B in the calculation when working out the tension of the light inextensible string, BC? Also, what about he reaction force at C?
(edited 11 months ago)
Original post by Nithu05
Hey guys, do you mind helping me with q13 from mixed exercise 7. Why do they not include the reaction force on the rod which acts vertically upwards at B in the calculation when working out the tension of the light inextensible string, BC? Also, what about he reaction force at C?

Dealing iwth the second part first. What component is being considered? The rod. C is not part of the rod, so we're not interested in any forces at C.

As to your first question. What reaction force? There is some weight acting down and a tension in BC supporting the rod. Since we're taking moments about A, those are the only forces we need to consider.
Original post by ghostwalker
Dealing iwth the second part first. What component is being considered? The rod. C is not part of the rod, so we're not interested in any forces at C.

As to your first question. What reaction force? There is some weight acting down and a tension in BC supporting the rod. Since we're taking moments about A, those are the only forces we need to consider.

Thank you! Is there not a reaction between the string and the rod?
Original post by Nithu05
Thank you! Is there not a reaction between the string and the rod?

Yes. Reactions are equal and opposite. The string is exerting a force (the tension) on the rod, and in turn, the rod is exerting an equal and opposite force on the string (this is the reaction - at an angle, not just vertical). We're only interested in forces acting on the rod.
(edited 11 months ago)
Original post by ghostwalker
Yes. Reactions are equal and opposite. The string is exerting a force (the tension) on the rod, and in turn, the rod is exerting an equal and opposite force on the string (this is the reaction - at an angle, not just vertical). We're only interested in forces acting on the rod.

I see. Thank you, I was really struggling to get my head around this. Just want to clear this up: for ladder questions, we consider the reaction force of the ground acting on the ladder and the reaction force of the wall acting on the ladder? So, it's all about the forces acting on the object in question?
Original post by Nithu05
I see. Thank you, I was really struggling to get my head around this. Just want to clear this up: for ladder questions, we consider the reaction force of the ground acting on the ladder and the reaction force of the wall acting on the ladder? So, it's all about the forces acting on the object in question?

In bold, yes, you've got it.
Original post by ghostwalker
In bold, yes, you've got it.

Do you mind also confirming that the perpendicular distance of T is what I’ve drawn
Original post by Nithu05
Do you mind also confirming that the perpendicular distance of T is what I’ve drawn

Sin(45) or cos (45)
Original post by Nithu05
Sin(45) or cos (45)

Yep, either, as they're both the same.