We have said before that you carry on in a straight line at constant speed if there is no resultant force on you. Newton's Second Law deals with the case when there is a resultant force. If you carry on in a straight line in those circumstances, you either accelerate or decelerate. The size of the acceleration or deceleration is inversely proportional to mass. In other words, if your car is twice as heavy the acceleration will be half as big. Now consider what happens if there is a resultant force but the speed remains the same. The photographs show the top of the wonderfully named Spaghetti Junction on the M6 in Birmingham. You can see a road over the top in the first photograph. In the second photograph it loops round and down, going underneath - hence the name because other roads twist round these two. If you go round the flyover at constant speed, you are changing DIRECTION so your velocity changes. So you are still accelerating. This time the resultant force is directed towards the centre of the loop and is called CENTRIPETAL force.
