Speed
Also called Average speed
Speed is a scalar measure of how fast an object is moving, with no direction attached. Average speed is total distance divided by total elapsed time, and instantaneous speed is the magnitude of the velocity at a single moment.
Symbol , measured in meters per second. Speed is a scalar, so it never carries a sign or a direction: a value of 5 m/s tells you nothing about which way the object is headed.
Two different quantities share the name.
- Average speed is total distance traveled divided by total elapsed time.
- Instantaneous speed is the magnitude of the velocity at one instant, which is what a speedometer reads.
Average speed is not the magnitude of average velocity. Drive 60 km out in 1.0 h and the same 60 km back in 2.0 h. Total distance is 120 km over 3.0 h, so average speed is 40 km/h. The displacement is zero, so average velocity is zero and so is its magnitude. The two agree only when the motion never reverses, because only then does distance equal the size of the displacement.
Speed and distance are the scalar pair; velocity and displacement are the vector pair, which is how the AP Physics 1 CED sorts them in EK 1.1.A.3. Topic 1.2 Displacement, Velocity, and Acceleration sets the AP framing, and the kinematics problem set makes you read both off the same motion.