1. The problem statement, all variables and given/known data Suppose you are in the back of a truck with a ball hanging by a string from the ceiling of the truck. If the truck is moving at a constant velocity on a smooth track, you will not know that you are moving. But if the truck is accelerating, you will see the ball and string make an angle, and the ball will tend to want to "stay behind". Why is this? 2. Relevant equations 3. The attempt at a solution I just don't understand why this happens. If I draw a free body diagram of the ball (suppose the truck is accelerating to the right), I can see that there is a diagonal tension force, which can be split up into x and y components. The y component of the tension force will balance out with the force of gravity. And all that will be left is the x component of the tension force, which will cause the ball to accelerate with the truck. Maybe this will help me clarify my question. Suppose the ball is taped to the end of the rigid metal rod and suspended from the ceiling of the truck. Then when the truck accelerates, the ball and rod will remain straight (perpendicular to the ceiling). What is the difference between this example and the string example?