Suppose a chain that is 20 m long hangs over the side of a building. Find the work required to lift the chain to the top of the building if the chain has a linear density of 5 kg/m. against gravity

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The answer is 4f======since electrostatic force is given by    we let f be the electrostatic force when  r = 1 m    if we let  denote the electrostatic force when the distance is decreased so that r = 0.5m, then    we established earlier that f = kq₁q₂. therefore    the force will now be 4f.