When a ball is dropped in a moving car, why does it not fall to the back of the car?
(Lansing State Journal, December 21, 1994)


The ball does not fall to the back of the car because the ball has inertia.  Inertia is a property of all objects that have mass.  If an object is moving then it will keep moving unless something stops it, or if it is at rest, it will stay at rest unless something causes it to move.

When the car is moving at a constant speed, a ball in the car is moving at the same speed.  When the ball is let go, it wants to keep moving in the same direction that it was moving, which is in the same direction as the car.  Gravity causes the ball to fall down, but the ball also keeps moving in the same direction as the car.  In the car, the ball appears to fall straight down and not to the back of the car.  But, the ball's path will curve with respect to the ground.

You can demonstrate this phenomenon with a ball and two people.  One person shold hold the ball and run, then drop the ball at some point while still running.  Just let go of the ball; do not throw it.  The second person should watch where the first person lets go of the ball. and then watch where it lands.  If the second person observes carefully, he/she will see that the ball does not fall straight down.  Instead the ball falls downward and in the direction the person was runnin.  The path of the ball follows a geometric curve called a parabola.

Lawrence E. Su contributed to this article

[ Back to Ask Science Theatre | Back to Ask Science Theatre Date Index ]
Back to MSU Science Theatre Home Page