Physics Kenya
Floating and Sinking
Floating and Sinking
Floating and Sinking
Archimedes' principle, Law of floatation
If a heavy object is lifted while immersed in water, it may be raised more easily than when outside the water. This is due to the reason that when anything is placed in liquid, it receives an upward force called upthrust. A body appears to have lost some weight when immersed in water due to the upthrust exerted on the body by the water.
- By definition upthrust is referred as an upward force exerted by the body when it’s partially or totally immersed in water.
- Consider the experiment below to verity the concept of upthrust.
From this experiment, it will be observed that W1>W2. This is because:
When a body is partially or totally immersed in any liquid, the liquid exerts an upward force.
A weight recorded on the spring balance of a body that is totally or partially immersed in any liquid is called apparent weight. E.g. W2 and the force, which temporally reduces the weight of the body, are called upthrust (u).
Relative density
Applications of Archimedes' principle and relative density
Problems on Archimedes' principle
Relative density (R.D) of a substance can be defined as the ratio of the mass of a substance to the mass of an equal volume of water.
Relative density can also be defined as the ratio of the weight of the substance to the weight of an equal volume of water
According to Archimedes’ principle mass of equal volume of water is the mass of the water displaced when the substance is immersed in water.
Note that, the volume of water displaced after immersing a substance in water is equal to the volume of the substance.
That means,
Or,
Example 5
A piece of glass weigh 1.2N in air and 0.7N when completely immersed in water. Calculate its:
- Relative density
- Density of a glass
Given that density of water = 1000kg/cm3
And acceleration due to gravity = 10N/kg
Solution
Weight of the glass in air (W1) = 1.2N
Weight of the glass in water (W2) = 0.7N
R.D = W1/W1 – W2
1.2N/(1.2 – 0.7)
1.2/0.5
R.D = 2.4
R.D = Density of glass/Density of water
Density of a glass = R.D x Density of water
= 2.4 x 1000kg/cm
= 2400kg/m3
NB: Relative density has no SI unit
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