Preparing for Lower Sixth Physics #11: Newton's First Law

Everybody knew that if no forces acted on an object that was stationary, it would remain stationary. It took Isaac Newton to extend that to moving objects. Take the car in the photograph, going at a steady 70 mph in a straight line. Newton pointed out that the forward force of the engine must be exactly equal to the frictional forces acting on it. So overall, taking into account that backwards forces are given a negative sign, the forces all add up to zero. We say that there is zero RESULTANT force. Newton said that if there was zero resultant force, an object that was stationary would remain stationary and one that was moving in a STRAIGHT LINE at constant speed would continue to do so. Being stationary and going in a straight line at constant speed are stable conditions.