[maged sameh]

If a body is held by two trestles can the colckwise moment be equal to the unticlockwise moment if the upwards force is not equal to the downwards force ?:confused:

[ebaines]

Not sure exactly what you are asking here - you say a body is "held by two trestles," which sounds like the body is stationary and has two points of support - correct? OK, that means the weight of the body is supported by two points, and depending on the arrangemet it could be that each support holds one half the body's weight. The sum of the two forces from the two supports must equal the weight of the body - otherwise the body would fall. The torques on the body presented by the two supports must cancel each other out - otherwise the body would start to spin. For example: let's suppose the body is weight W, and length between suports of L, and each support provides an upward force of W/2. Consider the torques about the cener of the body: they are (W/2)(L/2) for the support on the left (positive because they present clockwise moment about the center of the body), and -(W/2)(L/2) from the suport on the right (negative because it presents an anti-clockwise moment). The sum of the two torques is 0. Alternatively, you can consider the torque as measured around one of the supports - let's consider the left support: the two torques it feels are W(L/2) due to the weight of the body, and (W/2)(L) due to the support on the right - the sum is again zero. Hope this helps.