Summary
Highlights
Physics frequently asks about an object's location, direction, and speed. These questions are answered using position, velocity, and acceleration. Position is an object's location in space, usually expressed in meters relative to a reference point. Velocity is the rate of change of position over time (e.g., 5 meters in 5 seconds equals 1 meter per second). Acceleration is the rate of change of velocity over time (e.g., speeding up to 5 m/s in 5 seconds equals 1 meter per second squared).
Position can be described using distance or displacement. Distance is a scalar quantity, representing the total path traveled. Displacement is a vector, representing the straight-line path from start to end, including direction. For example, two people can walk different distances to reach the same destination but have the same displacement because they finish at the same point. Displacement can be expressed with coordinates or as a magnitude and direction.
Velocity is the rate of change in position, and it's a vector, unlike speed, which is a scalar. Speed only has magnitude, while velocity has both magnitude and direction. Kids running from a seeker might have the same speed but different velocities because they're moving in different directions. Average speed is total distance divided by time, while average velocity is displacement divided by time.
Acceleration is the rate of change in velocity and is always a vector, indicating a change in velocity in a specific direction. Pressing the gas in a car causes forward acceleration, increasing velocity at a fixed rate. Slamming on brakes is deceleration, which is acceleration in the negative direction, causing a rapid decrease in velocity.
By observing a marble rolling to a stop, we can visualize these vectors. The displacement vector elongates with the marble's movement. The velocity vector points forward, decreases in magnitude as the marble slows, and disappears when it stops. The acceleration vector points in the negative direction, indicating deceleration due to friction, and maintains a constant magnitude.