G force refers to a unit based on the Earth’s gravity: the constant, invisible pull that keeps you from floating off into space. Because the value of this force is well-known, scientists use it as a convenient yardstick to measure other forces, such as the acceleration of a car, the impact of two colliding football players or a fighter jet pulling out of a steep dive.

In physics, acceleration is a change in speed or velocity. When you hold a ball at arm’s length and drop it, the ball’s initial speed is zero, but with the pull of gravity, it accelerates downward with an ever-increasing speed. Gravity supplies the force, and the ball responds by accelerating. Under the influence of gravity, all objects, regardless of weight, accelerate at the same rate on Earth.

Researchers measure speed in units of distance divided by time, such as meters per second; The units for acceleration are speed divided by time, such as meters per second per second, or meters per second squared. For gravity on Earth, the value is 9.8 meters per second squared or 32.2 feet per second squared. This means, if you throw a ball from atop a very high building, the ball has a speed of 9.8 m/s after one second, 19.6 m/s after two seconds, and so on until it hits the ground. For convenience, scientists refer to this value by the letter “g.”

You might run into some confusion when you talk about G force, as big “G” means one thing and small “g” means another. Small g is the constant gravitational force on Earth, 9.8 m ÷ s². Big G is itself a convenient unit based on the force of gravity, while 1 G is the force that produces 9.8 m ÷ s² of acceleration. Big G is convenient because virtually everyone knows what 1 G of force feels like from everyday experience.

All forces have a direction in which they push or pull. For example, you open a door by pulling the handle towards yourself. Or when you throw a ball to a friend, you push it in his direction. The force of gravity on Earth, g, always points down, towards the center of the planet. When working out problems dealing with motion, scientists give g a negative sign to show that it pulls in the down direction, counteracting motion and forces in the up or positive direction. However, arbitrary G forces can point in any direction: up, down, side to side or at an angle.

A car that accelerates from zero to 60 mph in 5 seconds exerts a force of about 1/2 of a G on its occupants. Fighter pilots can experience forces greater than 7 Gs during extreme maneuvers. Impact forces can be much higher, though they last only a brief moment. Collisions between football players can exceed 150 Gs. In a car crash, the forces on the human body can also reach 150 Gs, though wearing the right type of seatbelt can reduce it to about 20 Gs.