A special electronic sensor is embedded in the seat of a car that takes riders around a circular loop-the-loop ride at an amusement park. The sensor measures the magnitude of the normal force that the seat exerts on a rider. The loop-the-loop ride is in the vertical plane and its radius is 21m. Sitting on the seat before the ride starts, a rider is level and stationary, and the electronic sensor reads 770N. At the top of the loop, the rider is upside down and moving, and the sensor reads 350N. What s the speed of the rider at the top of the loop?