As we know the relation between linear speed and angular speed as

we will say


so here we can say that angular speed will be given as


so it is rotating at angular speed 160 rad/s
Down stream it would be going 20 mph and up stream 10 mph
Half mass car because it's traveling faster
Objects should be cooled before their mass is determined on a sensitive balance because it could damage the balance. Also, because it would give you wrong reading of the mass. Hot objects would warm the air around it. A warm air would expand and would produce convection as it rises causing to give the object a mass that is less than the actual. Another reason would be it would cause instability in the readings, the mass would fluctuate every now and then due to the convection currents around the object. It is always recommended to weigh the masses of objects that are in room temperature.