Lab Manual: Archimedes’ Principle - Extra Documents & Tests for Class 9

Objective

• tap water
• strongly salty water, with the weight of water displaced by it by taking at least two different solids.

Theory

• Fluids: Gases and liquids flow and are thus called fluids.
• Buoyancy: The upward force exerted by fluids on any body is called the buoyant force and this phenomenon is known as buoyancy.
• Thrust: The force acting on a body perpendicular to its surface is called thrust. S.I. unit is Newton.
• Pressure: The thrust per unit area is called pressure.
  Pressure = Thrust/Area
  S.I. Unit = N/m² or Nm^-2 This unit (Nm^-2)is also called  Pascal,
  1 Pascal (Pa) = 1  Nm^-2
• Weight of a body = Mass x acceleration due to gravity W = mg
• When a body is immersed in water or liquid, the body displaces some liquid.
• The volume of liquid displaced = total volume of the solid.
• The mass of liquid displaced can be measured as:
  Mass of liquid displaced = Volume x Density
  M = V x D
• Weight of liquid displaced = Volume x Density x g (acceleration due to gravity)
  W = V x D x g
• The body loses some weight when immersed in fluid, it can be found as follows:
  Weight of the body in air = W₁
  Weight of the body when immersed in liquid = W₂
• Hence loss in weight = W₂ - W₁
  How much will be this upward force/buoyant force depends on the density of liquid in which it is immersed. The upthrust is more by denser liquids.
• Archimedes’ Principle: When a body is immersed fully or partially in a fluid, it experiences an upward force that is equal to the weight of the fluid displaced by it.

Materials Required
A spring balance, a metal bob, a cotton thread, an overflow can, a glass beaker, tap water, salty water, an iron stand.

Procedure

• Take an iron stand and suspend a spring balance to it.
• Study the spring balance, its scale and its least count.
• Record your observations. If any error, record it as ‘x’ gf.

2. Find the weight of solid metal bob in air:

• Take a metal bob, tie thread to it and suspend on the hook of the spring balance.
• Record the weight of the metal bob in air. Let this weight be W_r

3. Find the weight of the metal bob immersed in tap water and record the apparent loss in weight

• Take an overflow can, fill it with water such that its water level touches the spout of the overflow can.
• Keep an overflow can under the spring balance such that the metal bob gets fully immersed in the water of the overflow can.
• Keep a beaker whose weight P₁ is recorded, at the mouth of the spout of overflow can.
• As soon as the metal bob is suspended in water the weight on spring balance scale is recorded. This loss in weight is due to buoyancy (W₂).
• Collect the water that has overflown in a beaker till the last drop that comes out of the spout.
• Weigh the beaker with water (P₂).
• Calculate the weight of the displaced water (P₂ - P₁).
• Calculate the loss in weight of the metal bob when immersed in water.
  

4. Find the weight of metal bob immersed in salty water and record the apparent loss in weight.

• Prepare salty water by taking a 500 mL beaker and adding 300 mL of water in it and dissolving common salt till the saturated solution is obtained.
• Take the same metal bob and repeat the steps from 1 to 8 as given in procedure ‘C’
• Tabulate your observations.

Observations and Calculations

• Zero error in spring balance = (x) = 0 gf.
• Least count of the spring balance = 2 gf.
• Density of water = 1 g/cm³
• Weight of empty beaker P₁= 100 gf.

Table A : Verification of Apparent Loss  in Weight of Solid  Body in Tap Water

Table B : Verification of Apparent Loss  in Weight of Solid  Body in Salty Water

(i)  Table A → W₁ - W₂ ≅ P₂ - P₁
(ii) Table A → W'₁ - W'₂ ≅ P'₂ - P'₁

Conclusion

• Hence it is proved that the weight of the water displaced by the metal bob is approximately equal to the apparent loss of weight of the metal bob in water. Archimedes’ principle is verified.
• The weight of water displaced by a given solid when immersed fully in strongly salty water is more than the weight of the water displaced when fully immersed in tap water.

Precautions

• Carefully study the spring balance used for the experiment.
• Fill the overflow can above the mark of the spout, allow extra water to overflow through the spout without disturbing it. Use this overflow can for the experiment.
• Do not allow the suspended solid mass i.e. metal bob/stone to touch the base and sides of the overflow beaker.