If a hot-air balloon hovers at a fixed altitude the net weight of the balloon must be

The net weight of the balloon must be zero.

The forces acting on the hot-air balloon when it hovers at a fixed altitude are gravitational force and the upthrust of the hot air. As the balloon is at a fixed altitude, the two forces balance each other resulting in zero net force. Thus, the weight of the balloon is balanced by the upthrust, so the balloon is weightless at this time of the flight.

c because the action was the roller coaster reaches bottom of hill (action) reaction would be blood tends to drain away from head

hope this !

probably not the "magnets"in the magnet, so to speak, are thought to be electrons orbiting in shells. vurrent loops two sides to the loop