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In my applied mathematics class, our teacher told us that there are no reaction forces acting on a weightless connected rods. But ,as in the this picture

as in the this picture, I have a weightless rod (AB) hinged to a wall by A, and (BC) rod which has a weight of W is held in equilibrium with a T horizontal force. My question is, if there are no reaction forces with a weightless rod (I've shown those forces as x and y) ,when considering the system BC, how would the rod BC stay in equilibrium.

Sorry if this is a silly question...I know weightless rods are ideal, but, I need a clarification on this...

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It's not that there are no forces acting on the rod, it's that the net force must be zero or else the rod (being presumably massless as well as weightless) would accelerate instantly to an infinite speed.

Since that isn't happening (the rod being fixed to the wall at the hinge) we know the force exerted on the rod by the hinge must exactly balance the force exerted on the rod by the second, weighted, rod.

Knowing that, there's no point doing any calculations to determine the forces on the weightless (and massless) rod.

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  • $\begingroup$ Sorry, but I am still unclear about the above situation. In that picture I have shown x and y as internal forces when considering the whole ABC system. What is unclear to me is that , when we consider BC rod as a system ( BC has a weight of W ), which force is going to neutralize the weight if we did not employ x and y forces. $\endgroup$
    – donthababakka
    Commented Nov 4, 2021 at 15:04
  • $\begingroup$ Rod BC exerts forces on rod AB. But the hinge also exerts forces on rod AB. These forces must balance or rod AB would accelerate instantaneously (due to having no mass). Rod AB also exerts forces on rod BC, but that's not part of your question, because the question asks about whether there are forces on a weightless rod, and rod BC is not a weightless rod. $\endgroup$
    – The Photon
    Commented Nov 4, 2021 at 16:40

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