Lab Manual: Pressure - Class 9 PDF Download

Objective

Theory

• Thrust: The net force exerted by a body in a particular direction. (Thrust = weight of body)
• This force is perpendicular to the surface. When you stand on loose sand or lie down on the sand, the thrust acting on the surface is same, but the effects are different. Pressure increases with decrease in the area of contact. S.I. unit of Thrust is newton (N).
• Pressure: The thrust acting on unit area is called pressure.
• S.I. unit of pressure = (N m^-2) or pascal (Pa)
  1 Pa = 1 N/l m²

Cuboid and its surface area

In the given cuboid (figure)

Area of face ABCD
i. e. Surface Area = length x breadth
= 15 cm x 10 cm
=150 cm²
Area of face BCGF
Surface Area A2 = length x breadth
= 10 cm x 5 cm
= 50 cm²

Area of face ABFE
Surface Area A3 = length x breadth
= 15 cm x 5 cm
= 75 cm²

Materials Required

1. Determination of weight of the given cuboid

• Tie the string to the cuboid.
• Note the zero error of the spring balance.
• Suspend the cuboid on the hook of the spring balance that is hung on the clamp stand.
• Note the reading of the spring balance. If the spring balance shows the reading in newtons, record the reading shown as the weight of the cuboid.
  If the spring balance shows the reading in g, then convert the reading into newtons, as follows:
  Convert the ‘g’ into kg. Multiply it with (g = 9.8 m/s²)

Observations

• Number of divisions on spring balance between 0 to first (W0) reading………………. (n)
• Least reading of the spring balance = W₀/n =……….
• Reading of spring balance suspended freely in air (zero error) = ‘0’
• Weight of the Cuboid =………….. N.

2. Determination of the pressure exerted by different faces of cuboid

• On a plain table keep a tray as shown in the figure.
• Fill in the tray with fine sand.
• Place the solid cuboid on the tray with fine sand as follows:
  Case (a) Face 5 cm x 10 cm should be in contact with the sand.
  Case (b) Face 10 cm x 15 cm should be in contact with the sand.
  Case (c) Face 5 cm  x  15 cm should be in contact with the sand.
• Carefully observe the depressions made by all the three cases and record the maximum and the minimum depression caused by the cuboid using a scale.
• Do the calculations to find the pressure exerted by each dimension i.e. case a, b and c.

Observation and calculations

1. The maximum depression in the sand is seen in case (a) .
2. The minimum depression in the sand is seen in case (b) .

(i) The weight of the cuboid = 1 kg
(ii) Thrust exerted by the cuboid = (Force) = m x a
e.g. = 1 x 9.8 m/s² = 9.8 N
Thrust exerted by any surface of the cuboid remains the same = 9.8 N.
(iii) Surface area
Case (a) 5 cm x 10 cm = 50 cm² = 5 x10^-3 m²
Case (b) 10 cm x 15 cm = 150 cm² = 15 x10^-3 m²
Case (c) 5 cm x 15 cm = 75 cm² = 75 x10^-4 m²

Observation Table

Conclusion

• Smaller the surface area of contact, greater is the pressure exerted.
• The pressure exerted by the body depends on the surface area that comes in contact with the sand.
• The depression on the sand is directly proportional to the pressure exerted by the body.

Precautions

• Carefully and gently place the cuboid on the sand.
• The spring balance should be checked for proper range and zero error.
• Weight of the cuboid and measuring of the height of depression on sand should be taken accurately.
• Sand should be wet.