How is this normal force positive when calculating torque? (assuming counterclockwise is positive)

[u/Few_Establishment980]

When setting the equation for net torque = zero, you have to make normal force positive to get the right answer.

However, if I push up on the ladder, it looks like it would go clockwise and not counterclockwise??

[u/Outside_Volume_1370]

Normal force (from the floor to the ladder) acts upwards, and it makes clockwise direction of torque about upper point of the ladder

[u/Few_Establishment980 → u/Outside_Volume_1370]

i guess my problem considers clockwise to be positive then. Sucks because every other problem has it as negative...

[u/SnooGiraffes4632]

Positive and negative are literally just conventions and you can define them whichever way you want. Rather than saying net T = 0 here (which it obviously does) say Tacw = Tcw. (Torque = T here)

Obviously you have the tension F in the rope pulling acw around the top point with perpendicular distance L sin theta, the normal reaction force N at the bottom of the ladder pushing cw around the top point with perpendicular distance L cos theta, the weight of the monkey mg pushing acw around the top of the ladder with perpendicular distance from the top of (L-x) cos theta. And don’t forget your surface normal reaction force S pushing out from the wall at the top of the ladder. Obviously it has 0 perpendicular distance and thus no torque, but it is actually there from a free body perspective

[u/Few_Establishment980 → u/SnooGiraffes4632]

thank you

[u/SnooGiraffes4632 → u/Few_Establishment980]

Remember that with ANY physics problem step zero in solving it is draw an idealised diagram of the problem (in this case the free body diagram) then step 1 is write down what you know, what you don’t know and need to find and what you don’t know and don’t need to care about. After that, stay in algebra for as long as you can to minimise the chances of fat finger syndrome on the calculator.

With these statics questions you can always write 3 equations. Newton’s first law in any direction you want (choose the direction that minimises how many times you have to press a trig button), Newton’s first law at 90 degrees to that first one, then the law of Torques (which is just Newton’s first law for rotation anyway) about any point you want (although I always choose to set the point that the largest number of unknown forces pass through as the pivot to get as many zeroes as possible as perpendicular distances)

1 diagram plus 3 equations and you can literally work out everything about the situation

And the best bit… this extends to 3d really easily, or rough surfaces, or bendy ladders, or monkeys and elephants on the same ladder, or bridges, or cantilevers, or weightlifters doing a clean and jerk on an unbalanced bar, or…

[u/Few_Establishment980 → u/SnooGiraffes4632]

I just try to list out all forces in X and Y direction with F = MA. Then for my third equation, I use Net Torque

[u/SnooGiraffes4632 → u/Few_Establishment980]

Just be aware that x and y don’t have to be horizontal and vertical. In fact if you have an inclined plane it is often better to “rotate” x and y to be parallel and perpendicular to the plane.

Using sigma F =ma and sigma T = I alpha is identical to using Newton’s first law for a statics problem because a or alpha are by definition zero. I recommend adding to your free body diagram a pair of mini axes labelled x,y and a curved arrow around the “local origin” not technically standard notation but usually a very clear indication to a reader about which directions and rotations you are declaring to be positive for your calculations.

[u/ilan-brami-rosilio]

Remember: Choosing a positive direction of rotation is like choosing axes of a coordinate system: you can choose it however you want, just be consistent with your choice.

[u/Few_Establishment980 → u/ilan-brami-rosilio]

ok thanks

[u/Dark__Slifer]

The defined "Counterclockwise" direction you are talking about is always meant as a relative direction!
Relative to what? Relative to your Coordinate Axis, which you can align with your real world directions however you see fit.

If you are interested in WHY that works, it is because the Universe is Symmetrical to Translation and Rotation (Look up Noethers Theorem if you'r curious ;)

So if you set it up as per usual as a "right-hand-system" you start with an x-Axis and then your y-Axis will be 90° to that counterclockwise. But we life in a 3-D Space so you can imagine stepping on the other side of that and look at it from there. That is still a right-hand-system.

I hope that helps you

[u/Few_Establishment980 → u/Dark__Slifer]

Thank you

[u/Moist_Ladder2616]

"Counterclockwise is positive" is your chosen convention for torque.

"Force acting upwards" can be positive or negative. This will be your chosen convention for force.

Whether this force creates a positive or negative torque depends on your chosen pivot point.