The International System of Units (SI) provides standard units for measuring physical quantities. These units are used universally in science and engineering. Below are the SI units for key physical quantities — acceleration, distance, heat, speed, velocity, and weight — which are commonly asked in Class 9–12 Physics examinations.
SI unit of acceleration = m/s² (metre per second squared).
SI unit of distance = metre (m).
SI unit of speed = m/s (metre per second).
SI unit of velocity = m/s (metre per second) — same as speed.
SI unit of heat = joule (J) — heat is a form of energy.
SI unit of weight = newton (N) — weight is a force (W = mg), not mass.
Mass is in kilograms (kg); weight is in newtons (N).
Physical Quantity | SI Unit | Symbol Acceleration | Metre per second squared | m/s² or m·s⁻² Distance / Length | Metre | m Speed | Metre per second | m/s or m·s⁻¹ Velocity | Metre per second | m/s or m·s⁻¹ Heat (Energy) | Joule | J Weight (Force) | Newton | N Mass | Kilogram | kg Time | Second | s Temperature | Kelvin | K Pressure | Pascal | Pa
SI unit of acceleration: metre per second squared (m/s² or m·s⁻²)
Definition: Acceleration is the rate of change of velocity with time. Formula: a = (v – u) / t (change in velocity ÷ time)
Derivation: velocity/time = (m/s) / s = m/s²
Examples: • Acceleration due to gravity: g = 9.8 m/s² ≈ 10 m/s² • A car accelerating from 0 to 20 m/s in 4 s → a = 5 m/s²
Other units (non-SI): • cm/s² (CGS unit) • ft/s² (imperial unit)
SI unit of distance: metre (m) • Distance is a scalar — how much path is covered • Displacement is a vector — straight-line distance from start to end
SI unit of speed: metre per second (m/s) • Speed = Distance / Time = m/s • Speed is a scalar (only magnitude)
SI unit of velocity: metre per second (m/s) • Velocity = Displacement / Time = m/s • Velocity is a vector (magnitude + direction) • Both speed and velocity have the same SI unit (m/s)
Conversions: • 1 km/h = 5/18 m/s • 1 m/s = 18/5 km/h = 3.6 km/h
SI unit of heat (energy): Joule (J) • Heat is a form of energy — SI unit = Joule • 1 Joule = 1 Newton × 1 metre = 1 N·m = 1 kg·m²/s² • Other energy units: calorie (1 cal = 4.186 J), kilowatt-hour (kWh) • Specific heat capacity unit: J/(kg·K)
SI unit of weight: Newton (N) • Weight is the gravitational force on a mass: W = mg • It is a force, so its SI unit is Newton (not kilogram) • 1 Newton = 1 kg × 1 m/s² • W = m × g; if m = 1 kg, g = 9.8 m/s², then W = 9.8 N
Common confusion: • Mass (kg) — how much matter is in the object (does not change) • Weight (N) — gravitational force on the object (changes with gravity)
The SI unit of acceleration is metre per second squared (m/s² or m·s⁻²). Acceleration is the rate of change of velocity. Formula: a = (v–u)/t. Example: g = 9.8 m/s² (acceleration due to gravity).
The SI unit of weight is Newton (N). Weight is the gravitational force acting on a mass: W = mg. For a mass of 1 kg, weight = 1 × 9.8 = 9.8 N. Note: Mass is in kilograms (kg), but weight is a force measured in newtons (N).
The SI unit of heat is joule (J). Heat is a form of energy. 1 joule = 1 N·m = 1 kg·m²/s². Another common unit is calorie (1 cal = 4.186 J). The unit of specific heat is J/(kg·K).
Yes. Both speed and velocity have the same SI unit: metre per second (m/s). However, speed is a scalar (only magnitude), while velocity is a vector (magnitude + direction). E.g., speed = 10 m/s; velocity = 10 m/s due North.
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