How the tension is calculated?

A massless rod of length L attached to mass m and with axle to cart of mass M. The cart has a shape of equilateral triangle (edge L). the cart is at rest and its center of mass is above x=0 and a rod is perpendicular to the ground. the cart is free to move without friction. at time t=0 the mass m is released and starts to fall to the left. at time t=τ the rod is parallel to the ground. Why the tension from the stick is T=mv^2/L=2mg and not T=mv^2/L=mg?

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