Motion, Speed and Velocity

Motion, Speed and Velocity

Physics is the branch of science that studies matter, energy, and how they interact. One of its first topics is motion — a change in an object's position with time.

Key concepts

• Distance is the total path length travelled (scalar). Displacement is the shortest straight-line distance from start to end, with direction (vector).
• Speed = Distance ÷ Time. SI unit: metre per second (m/s).
• Velocity = Displacement ÷ Time — speed in a given direction.
• Acceleration = Change in velocity ÷ Time. SI unit: m/s².
• To convert km/h to m/s, multiply by 5/18.

Worked example

Question: A train covers 180 km in 2 hours. What is its average speed in m/s?

Solution:

1. Speed = Distance ÷ Time = 180 km ÷ 2 h = 90 km/h.
2. Convert to m/s: 90 × 5/18 = 25 m/s.

Answer: The average speed is 25 m/s.

Analogy

Analogy: A walk to the market and back

Imagine you walk 2 km to the market and 2 km back home:

• Distance is what your step-counter shows: 4 km in total. It only cares how much you walked — a scalar.
• Displacement is how far you are from where you started: 0 km, because you are back home. It cares about direction — a vector.
• Speed = distance ÷ time. If the trip took 1 hour, your average speed was 4 km/h.
• Velocity = displacement ÷ time = 0 km/h for the round trip!

So a speedometer tells you speed, while a GPS arrow pointing "3 km north" is closer to velocity.