this time you saw the bull moving off to one side falling down the vertical reference line as it did in the constant velocity case but suppose you were in the six telerate a frame of reference how could you explain this gravity is the only force acting on this fall so it should fall straight down but at the law of inertia is to hold there must be a farce pushing sideways on the ball in this direction accorded to deviate from the vertical path but what kind of a force is it it is not gravitational or an electric or nuclear force in fact it is in the force at all as we know what story left to conclude but it gets sensor is no force that could be pushing in this direction on the ball but the law governorship just does not hold this is a strange frame of reference we call a frame of reference in which the law of inertia holes and inertial the law how the nurse or holds in the third frame of reference so it is an inertial frame the car moving at constant velocity relative to the earth is an inertial frame but the car which is expel a rated is not an inertial frame because the frame of reference that we're used to living in is one in which the law of inertia holds when we go into a non inertial