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# Form 4 Physics: Floating and sinking online lessons

Applications of Archimedes principle and relative density

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Applications of Archimedes principle and relative density:
1. Ships – steel which is used to make ships is 6-7 times dense than water but a ship is able to float on water because it is designed to displace more
water than its volume.
2. Submarines – they are made of steel and consists of ballast tanks which contain water when theyhave to sink and filled with air when they
have to float. This makes the submarines to balance their weight and be able to rise upwards.
3. Balloons – when they are filled with helium gas balloons become lighter and the upthrust on the balloon becomes greater than their weight therefore becoming able to rise upwards.
4. Hydrometers – they are used to measure the relative densities of liquids quickly and conveniently. Various types of hydrometers are made to
measure different ranges of different densities i.e. lactometer – for measuring milk water (range 1.015 – 1.045), battery acid tester – used to test the charge in a lead-acid battery.
Examples
1. A solid of mass 1.0 kg is suspended using a thread and then submerged in water. If the tension on the thread is
5.0 N, determine the relative density of the solid.
Solution
Mass of solid = 1.0 kg Weight of solid W = mg = 10 N
Tension on the string (T) = 5 N
Upthrust on solid (u) = W – T = 10 – 5 = 5
Relative density (d) = W / u =10 / 5 = 2.

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