Final answer:
Pulsars are the fastest spinning objects in the universe, with speeds reaching hundreds of revolutions per second. The force that keeps you in your seat during a loop-the-loop rollercoaster is the centripetal force.
Step-by-step explanation:
The fastest spinning objects known in the universe are pulsars. Pulsars are highly magnetized, rotating neutron stars that emit beams of electromagnetic radiation. Due to their rapid rotation, pulsars can spin at incredible speeds, with some reaching hundreds of revolutions per second.
As for the second question, the force that keeps you in your seat during a loop-the-loop rollercoaster is centripetal force. When the rollercoaster enters the loop, it experiences a downward centripetal acceleration, which causes a net force directed towards the center of the loop. This force pushes the passengers inward, keeping them pressed firmly into their seats.