A level maths mechanics moments and forces question

Hi, so when you resolve forces acting on a particle let's say on a slope, you would include both its weight and reaction force. However, with moments if you, for example, put a block on a horizontal bar, you would only include the weight, not the reaction force on the bar. Could someone please explain why this is? I think it is due to the forces acting on different things and you are only resolving for one object, but I can't get my head round it. Thanks.

If you put a block on a bar then the reaction is on the block not the bar. So if you are resolving forces on the bar then you wouldn't include any reaction force at the point where the block is.

But when you resolve forces acting on a block e.g. on a slope, you include weight acting on the block. However, in moments why do you take weight as acting on the bar?

But when you resolve forces acting on a block e.g. on a slope, you include weight acting on the block. However, in moments why do you take weight as acting on the bar?

For the block, the weight and the reaction from the bar are equal and opposite (assuming equilibrium). For the bar only, there is the weight of the block pressing downwards at the same place as the rod pushes back (force pair) Only the block's weight acts on the bar. If you were analysing equilbirum for the block you include both forces, but for the bar only the (translated) weight as the reaction acts on the block.

Imagine you are standing up. Your com is your stomach, but the ground pushes your feet back with an equal and opposite force. The COM is acting through a solid body (you) and acts like it at your feet and hence the reaction and (translated) weight form a force pair.