A frictionless pulley of negligible mass is hung from the ceiling using a rope, also of negligible mass. Two masses, m1 and m2 (m1 < m2), are connected to the rope over the pulley. The masses are free to drop. The magnitude of the tension Ttop is the sum of the tensions T1 and T2.

Ttop = T1 + T2

Explanation:

The three tension forces act on the pulley. With the assumption that the pulley has a negligible mass, the force of gravity on it is zero. So only forces Ttop, T1 and T2 act on the pulley.

The tension force Ttop acts upwards on the pulley while the tension forces T1 and T2 act on the pulley downwards. The pulley is in equilibrium so the sum of upward forces acting on the pulley and the sum of downward forces acting on the pulley must be equal.

So

Ttop = T1 + T2.

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