Quick answer: write the conversion factor beside the number, cancel units like algebra and keep the final unit visible.
Speed
Use 1 m/s = 3.6 km/h. Therefore 5 m/s = 18 km/h, while 72 km/h = 20 m/s. To convert km/h to m/s, multiply by 5/18.
Density
Density = mass ÷ volume. If mass is in grams and volume in cm³, the unit is g/cm³. Since 1 g/cm³ = 1,000 kg/m³, a density of 2.5 g/cm³ is 2,500 kg/m³. Convert both mass and volume together; changing only one creates a wrong factor.
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Pressure
Pressure = force ÷ area. One pascal is one newton per square metre. If F = 80 N and A = 0.4 m², P = 80/0.4 = 200 Pa. For 400 cm², first convert area: 400 cm² = 0.04 m².
Dimensional check
Write the unit equation before calculating: (N)/(m²) = Pa. If a length is squared, the conversion factor must also be squared. This catches many area and density mistakes.
Practice
Convert 36 km/h to m/s and find pressure for 50 N over 0.25 m². Answers: 10 m/s and 200 Pa.
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