Fix the mistake: Motion and Energy

How to Represent Speed and Motion

Speed represents how fast an object moves. In physics, several types of speed are distinguished.

Basic Knowledge

Speed is the distance traveled per unit of time:
Speed v = distance / time ( v = s / t )
Units include m/s (meters per second) and km/h (kilometers per hour).
Conversion: 1 m/s = 3.6 km/h(36 km/h = 10 m/s).
There are two types of speed:
・Average speed: Distance traveled over an interval ÷ time taken. A representative value for the entire interval.
・Instantaneous speed: Distance traveled in a very short time ÷ very short time. The value shown by a speedometer.
A quantity that also includes direction is called velocity(in middle school, speed and velocity are often treated as nearly synonymous).

Important Terms
Speed v（Distance traveled per unit of time. A scalar quantity）
Velocity（Speed including direction. A vector quantity）
Average speed（Total distance over an interval ÷ time）
Instantaneous speed（Speed over a very short time. The value shown by a speedometer）
m/s（Basic unit of speed. A speed of 1m per second）
km/h（How many km are traveled per hour. 1m/s = 3.6km/h ）

Further Study (Background・Significance)

Examples of familiar speeds:
・Human walking: About 1.2 m/s ≒ 4 km/h
・Bicycle: About 4〜5 m/s ≒ 15〜18 km/h
・Car(ordinary roads): About 40〜60 km/h ≒ 11〜17 m/s
・Shinkansen: About 80 m/s ≒ about 288 km/h
・Jet airliner: About 250 m/s ≒ 900 km/h
・Speed of sound(in air): About 340 m/s
・Speed of light(in a vacuum): About 3.0 × 10⁸ m/s
Repeated practice converting m/s and km/h helps you understand speed intuitively.
A ticker timer is often used to record speed. A powered ticker timer makes dots on tape 50 times per second(once every 0.02 seconds) in eastern Japan and 60 times per second(about once every 0.017 seconds) in western Japan. Instantaneous and average speeds can be calculated from the distances between dots.

Key Points
  • Speed v = distance ÷ time
  • Units: m/s、km/h(conversion: 1m/s = 3.6km/h)
  • Average speed ≠ instantaneous speed
  • Instantaneous speed can be measured with a ticker timer
  • East 50Hz=0.02 seconds/dot, West 60Hz=about 0.017 seconds/dot
  • Speed of sound about 340m/s, speed of light about 3×10⁸m/s
  • Velocity is a quantity that includes direction(vector)

Points to Note (Common Confusions)

① Unit conversion of speed: m/s → km/h is ×3.6, and km/h → m/s is ÷3.6. ② Distinguish between average and instantaneous speed. ③ Distance traveled(actual distance moved) and displacement(straight-line distance from the starting point to the endpoint) are different. ④ Make the units consistent before calculating. Mixing seconds and hours causes mistakes.

Practice

  1. Write the formula for finding speed.
  2. How many m/s is 72 km/h?
  3. Find the average speed in m/s of a person who runs 50m in 8 seconds.

Physics (Junior High): related lessons