Evan's Space

Wonders of Physics

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Units conversion involving prefixes

Prefixes are used to simplify the writing of very big or very small numbers.

For instance, instead of having to write 12 500 000 m every time, you can simplify it by using the prefix mega (M), hence it can also be written as 12.5 Mm. Similarly, 0.00054 s can be written as 0.54 ms.


If you are unsure or confused with the unit conversions, you can refer to the following videos.

1) Prefixes can be used for different physical quantities units.

2) Unit conversion with examples

3) Unit conversion for units of area and volume

4) Unit conversion for units of speed and density

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Density (Units Conversion) and Floatation

Density is the mass per unit volume.

Common units for density are g/cm3 or kg/m3. SI unit is kg/m3.

Density of a pure substance is fixed, regardless of it’s size!


Floating and Sinking

A denser (heavier) substance will sink in liquid which is less denser (lighter).
A less dense substance will float in a liquid which is denser.
If substance has the same density as the liquid, it will be suspended in the liquid.


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Conversion of m/s to km/h and vice versa – Usain Bolt’s world record for 100m

Usain Bolt smashed the 100m and 200m World record with a time of 9.58 s and 19.19s respectively in Berlin 2009! Can’t embed the 100m clip here. So just how fast is he? Is he much faster than your dad’s car on the expressway 90 km/h???

100m in 9.58s means approx. 10.44 m/s = 37.58 km/h. So well, 2 legs are slower than 4 wheels =) How do you convert?  

View the step by step conversion from m/s to km/h. (in short, multiply by 3.6) 

Below is another record set by him. 200m in 19.19s. You can try to do the conversion!