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.

the minimum amperage that the current in this wire would have to have so that its weight is completely supported by the magnetic force due to earth's field, assuming that no other forces except gravity act on it

it's when something is going so fast it makes a loud noise, breaking the sound barrier, for example: a jet makes a loud noise, not only cause of the jet but because they reached a point where they went so fast that you can year the noise from various parts, kinda like a police car, lets say the car was going   so fast that you hear the siren of the cop car right next to you, but the car is probably a mile a way, it's cos the siren was next to you a few seconds if you understand me