FORM 1-PHYSICS-ARCHIMEDES' PRINCIPLE AND LAW OF FLOTATION
Difference between Floating and Sinking of Objects
Distinguish floating and sinking of objects
As we have discussed in upthrust, different objects with different density can sink or float. The object with higher density than water density will sink while that object with a density lower than water’s density will float. For example, a coin sinks in water and a large ship floats on water.
The Conditions for a Substance to Float in Fluids
Explain the conditions for a substance to float in fluids
When an object is completely or partially immersed in fluids, there are two forces acting on it, the weight (W) acting downwards and the upstrust (u) acting upwards. Refer to the figure below:
Conditions
Conditions for a body to float include:
- If W>U, there is downward movement of the body which is termed as sinking.
- If W<U, there is upward movement of the body.
- If W=U, the body is equilibrium under the action of two equal and opposite force. Thus, the body floats.
Relationship between Upthrust and Weight of Floating Body
Relate upthrust and weight of floating body
The relationship can be determined by considering the following experiment.
Upthrust and Weight of Floating Body
Principle of Floatation
State the law of flotation
The above experiment shows that, the upthrust is equal to the weight of the liquid displaced and therefore the upthrust is equal to the weight of the floating body as the two forces are equal.
The mass of the floating body is equal to the mass of fluid displaced; in the above experiment is the same as the weight of the piece of wood. This result is agreement with the principle of floatation.
The principle of floatation states “A floating body displaces a weight of the fluid which is equal to its own weight.”
The Law of Floatation in Everyday Life
Apply the law of flotation in everyday life
Floating of ship
Iron is much denser than water and a block of iron sinks immediately in water. Ships are made with hollowness such that their total densities are less than that of water. Therefore, a ship displaces water equal to its weights.
The upthrust of the water is sufficient to support the weight of the ship. When the ship is loaded with cargo it sinks lower in the water. The volume of water displaced by the ship and its cargo depends upon whether it is floating in fresh water or in seawater. It floats lower in fresh water (R.D= 1.0) than in seawater (R.D=1.025) the mass of fresh water displaced.
For example, if a ship weighs 20,000 tons, then it must displace 20,000 tones of water to float. If 2,000 tones of cargo is added, the ship lowers in water until an extra 1,000 tons of water have been displaced
Plimsoll line;This line indicates the safe limit of loading. Many plimsoll lines may be marked on a ship to show minimum heights above different types of water in different seasons.
Balloons
Figure below shows the type of balloons used to carry instruments to a high altitude for recording meteorological measurements when filled with gas. E.g. helium, it displaces a volume of air equal to its volume.
Air has greater density compared to the density of a gas in the balloon. Therefore, the weight of air displaced is greater than that of balloon. The balloon drifts up by a force, which is equal to the difference between the upthrust and the total weight of the balloon (W).
Submarine
The buoyancy of a submarine depends on the quantity of water in its ballast tanks. When it is required to drive, water is admitted to special tanks. When the water is ejected from the tanks by means of compressed air, the submarine raises to the surface and floats just like any other ship
The mode of Action of a Hydrometer
Describe the mode of action of a Hydrometer
A hydrometer is an instrument used for measuring the densities of liquids such as milk, alcohol and acids. The greater the density of the liquid the shorter the stem of hydrometer immersed. Hydrometer sinks lower in kerosene and floats higher in water.
Construction of a Simple Hydrometer
Construct a simple Hydrometer
Single hydrometer can be made using pieces of straws or test tubes weighed down with wax.
Hydrometer in Determining the Relative Density of Different Liquids
Use Hydrometer to determine the relative density of different liquids
The relative density of liquid is measured directly by using a suitable hydrometer, it contains:
- A heavy sinker, containing mercury or lead shots that keep the hydrometer upright when it floats.
- An air bulb to increase the volume of displaced liquid, and overcomes the weight of the sinker.
- The stem is thin so that small changes in density give large differences in readings.
- The hydrometer is made up of glass so it does not soak up liquids
