When a baseball is thrown or hit, the resulting motion of the ball is determined by Newton's laws of motion and the relative strength of the forces acting on the ball. A force may be thought of as a push or pull in a specific direction. A force is a vector quantity so a force has both a magnitude and a direction. When describing forces, we have to specify both the magnitude and the direction. This slide shows the forces that act on a baseball in flight.

Weight
Weight is a force that is always directed toward the center of the earth. In general, the magnitude of the weight depends on the mass of an object as first determined by Newton's law of gravitation. By rule, the weight of a major league baseball is 5 ounces. The weight is distributed throughout the ball. But we can often think of it as collected and acting through a single point called the center of gravity. In flight, the ball rotates about the center of gravity. A baseball is made with a solid core, a string wrapping around the core, and a stitched covering. To first order, the center of gravity for a baseball is located at the exact center of the ball.

Strictly speaking, the ounce (oz.) is a measure of mass and not of weight. Weight is a force, mass times acceleration, and is not equal to the mass of an object. Unfortunately, humans often use the units for weight and mass interchangeably; the assumption being that we talking about the weight at the surface of the Earth where the acceleration is a constant (32.2 ft/sec^2 or 9.8 m/sec^2). So when the rule states that the ball weighs 5 oz, it should more correctly specify that the weight is 5/16 lb. The pound is a measure of force. On Mars, the mass of a baseball is the same as on Earth. But since the gravitational acceleration on Mars is 1/3 that of the Earth, the weight of a baseball on Mars is 5/48 lb.

As the baseball moves through the air, there is a large aerodynamic force acting on the ball. Aerodynamicists normally resolve the single aerodynamic force into two components; drag acts in a direction opposite to the motion, and lift acts perpendicular to the motion and to the drag. Let's consider each of these forces separately.

Drag
As the ball moves through the air, the air resists the motion of the ball and the resistance force is called drag. Drag is directed along and opposed to the flight direction. In general, there are many factors that affect the magnitude of the drag force including the shape and size of the object, the square of the velocity of the object, and conditions of the air; particularly, the density and viscosity of the air. Determining the magnitude of the drag force is difficult because it depends on the details of how the flow interacts with the surface of the object. For a baseball, this is particularly difficult because the stitches used to hold the ball together are not uniformly or symmetrically distributed around the ball. Depending on the orientation of the ball in flight, the drag changes as the flow is disturbed by the stitches. To determine the magnitude of the drag, aerodynamicists use wind tunnel tests to measure the drag.

The aerodynamic force acts through the center of pressure of any object. For an ideal, smooth ball, symmetry considerations place the the center of pressure at the center of the ball along with the center of gravity. But a baseball in flight is neither smooth nor symmetric because of the stitches. So the center of pressure for a baseball moves slightly about the center of the ball with time, depending on the orientation of the stitches. This motion is the source of the "dancing" knuckleball that confuses both batters and catchers alike.

Lift
Lift is the component of the aerodynamic force that is perpendicular to the flight direction. Airplane wings generate lift to overcome the weight of the airplane and allow the airplane to fly. A rotating cylinder and a spinning ball also generate aerodynamic lift. For a spinning ball the lift force is generated perpendicular to the axis of rotation. The magnitude of the lift depends on several factors related to the conditions of the air and the object, and the velocity between the object and the air. For a spinning ball, the speed of rotation also affects the magnitude of the lift. And the stitches on a baseball introduce some additional complexity in the generation of lift, just as the stitches introduced complexity in the generation of drag. To account for the complexities when making predictions of the lift on a baseball, or of the lift of an airplane wing, aerodynamicists make an ideal prediction using theory, and then correct the prediction using experimental data.

The motion of the ball through the air depends on the relative strength and direction of the forces shown above. We have built two major simulation packages that look at the physical problem of pitching a curve ball, and of the flight of a baseball that is hit from home plate. The curve ball problem involves all three forces with the lift force producing the side force that causes the ball to curve. The simulation calculates the magnitude of the lift force and it can be shown that even big league pitchers can not generate enough lift force to overcome the weight of the ball. There are no rising fast balls. The hit baseball problem considers only the forces of drag and weight. the simulator demonstrates the important role that atmospheric conditions play on the flight of a baseball. The flight trajectory is very different from the idealized ballistic flight that occurs when drag is neglected.

The figure on this web page was created by Elizabeth Morton, of Magnificat High School, during a "shadowing" experience at NASA Glenn during May of 2007.

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