Press the gas pedal and the car speeds up. Press the brake and it slows down. Turn the wheel and it changes direction. All three are acceleration, because acceleration is any change in velocity, not just speeding up. Velocity has both speed and direction, so a change in either counts. A car going around a curve at a steady 60 mph is accelerating, even though the speedometer never moves.

Acceleration is measured in meters per second squared. A value of 2 m/s² means the velocity changes by 2 meters per second every second. Gravity near Earth's surface produces a downward acceleration of about 9.8 m/s². That number shows up everywhere, from falling apples to orbiting satellites. Newton's second law ties acceleration to force and mass: force equals mass times acceleration. Double the force on the same mass and you double the acceleration. Double the mass and you halve it.

Key points

• Vector quantity. Acceleration has both magnitude and direction.
• Units. Meters per second squared (m/s²).
• Gravity. 9.8 m/s² downward near Earth's surface.
• Newton's second law. F = ma, linking force, mass, and acceleration.

A fighter pilot pulling 9 g feels nine times their normal weight. A roller coaster at the bottom of a drop does the same thing, briefly.