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# torque question help please :) watch

1. hi, just want to say thanks to the guys that seem to answer all the questions. my question is written on the attachment for ease of convenience. thanks.
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2. In A-Level physics, a couple is two forces equal in magnitude but opposite in direction. The torque of a couple is the magnitude of one of the forces (just one of the forces, both have the same magnitude) multiplied by the perpendicular distance between them, the mid point of this is the pivot, so think of it as a circle with two opposite arrows on each side of the circle. The center is the pivot, the arrows are the forces and the diameter is the "perpendicular distance". Torque would be multiplying the magnitude of one of the forces by the dimater of the circle.

But if you look at the situation as moments (the product of force and the perpendicular distance it is from the pivot) you have two forces and therefore two moments, there must be two distances, these just happen to be equal in size. The net moment his is equivalent to the torque. Going back to the circle analogy will explain why. Here we are multiplying the magnitude of one force by the radius of the circle and doubling that to get the net moment. This is equivalent to multiplying the magnitude of one force by the diameter of the circle because diameter is always twice the radius.
3. (Original post by Primus2x)
In A-Level physics, a couple is two forces equal in magnitude but opposite in direction. The torque of a couple is the magnitude of one of the forces (just one of the forces, both have the same magnitude) multiplied by the perpendicular distance between them, the mid point of this is the pivot, so think of it as a circle with two opposite arrows on each side of the circle. The center is the pivot, the arrows are the forces and the diameter is the "perpendicular distance". Torque would be multiplying the magnitude of one of the forces by the dimater of the circle.

But if you look at the situation as moments (the product of force and the perpendicular distance it is from the pivot) you have two forces and therefore two moments, there must be two distances, these just happen to be equal in size. The net moment his is equivalent to the torque. Going back to the circle analogy will explain why. Here we are multiplying the magnitude of one force by the radius of the circle and doubling that to get the net moment. This is equivalent to multiplying the magnitude of one force by the diameter of the circle because diameter is always twice the radius.
i get how the values are attained but whats significant about them being equal the object has torque so it will spin, and has a momet so it will rotate around pivot but what specialy about the 2 quantities being equal?
4. (Original post by nmjasdk)
i get how the values are attained but whats significant about them being equal the object has torque so it will spin, and has a momet so it will rotate around pivot but what specialy about the 2 quantities being equal?
Torque of a couple is a specific case, torque in general is the rate of change in angular momentum.

Newton's second law is for linear motion and says that the net force acting on an object is equal to the rate of (linear) momentum.

So in rotional motion; the torque (which is calculated by adding together moments) about a pivot is equal to the rate of change in angular momentum.

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