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# Applied maths- moment about a point watch

1. Been given a question about a square (1m sides) luggage case being pulled by a strap of tension T and an angle Theta. There are wheels on each corner with friction forces F1 and F2 aswell as reaction forces of R1 and R2, the bag ways 15kg. i am also given that the coefficent of rolling friction is U. And am asked to find moments about the rear wheel to find R2.
The formula i have been using is the vector cross product of the position of the wheel with the force applied. I am not sure how to find the position of the wheel. Is it as simple as taking the strap as the origin and the 1m sides as the axis therefore giving the rear wheel position as (-1,-1,0). Please help
2. (Original post by Nb13g14)
Been given a question about a square (1m sides) luggage case being pulled by a strap of tension T and an angle Theta. There are wheels on each corner with friction forces F1 and F2 aswell as reaction forces of R1 and R2, the bag ways 15kg. i am also given that the coefficent of rolling friction is U. And am asked to find moments about the rear wheel to find R2.
The formula i have been using is the vector cross product of the position of the wheel with the force applied. I am not sure how to find the position of the wheel. Is it as simple as taking the strap as the origin and the 1m sides as the axis therefore giving the rear wheel position as (-1,-1,0). Please help
If you take the location of where the strap connects to the luggage as the origin, and it's the top front corner, then (-1,-1,0) is correct.

However, I'd be inclined to take the rear wheel as the origin - it would simplify taking moments.

If you need further help, I'd like to see a diagram to make sure we're on the same page.
3. Attachment 476715476717
(Original post by ghostwalker)
If you take the location of where the strap connects to the luggage as the origin, and it's the top front corner, then (-1,-1,0) is correct.

However, I'd be inclined to take the rear wheel as the origin - it would simplify taking moments.

If you need further help, I'd like to see a diagram to make sure we're on the same page.
Taking the cross product of the origin would result in a moment equal to zero wouldn't it. Heres the diagram.
Attached Images

4. (Original post by Nb13g14)
Attachment 476715476717
Taking the cross product of the origin would result in a moment equal to zero wouldn't it. Heres the diagram.
You're taking moments about the rear wheel. The moment of any forces acting at the real wheel will be zero, regardless of whether the origin is there or not.

The position of the origin will not effect the moment about the rear wheel, but it will simplify the calculations.

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