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introduction:Density and Pressure in Solids and Fluids Key unit competence The learner should be able to define pressure and explain factors affecting it.

My goals By the end of this unit, I will be able to:

࿤ define and explain the pressure as a relationship of force acting on a surface area.

࿤ identify force and area as factors affecting pressure in solids.

࿤ give the relationship between force, pressure and area.

࿤ explain how pressure varies with force and the area of contact.

࿤ describe liquid (mercury) in glass barometer.

࿤ explain floating and phenomena.

࿤ describe how to measure atmospheric pressure.

࿤ carry out calculations using the equation. Pressure = force/area and P = ρgh.

࿤ explain the variation in atmospheric pressure with altitude.

࿤ explain the change in pressure by reducing or increasing area of contact and vice versa.

࿤ measure atmospheric pressure using a barometer, liquid in glass barometer. 70 Physics for Rwanda Secondary Schools Learner’s Book Senior Two

࿤ explain the functioning of aneroid barometer.

࿤ describe and explain pressure transmission in hydraulic systems.

࿤ explain functioning of a hydraulic press and hydraulic brakes. Key concepts

1. How experimentally can one determine the effect of force exerted on a solid?

2. How can one define pressure in a fluid?

3. How can pressure in liquids be measured?

4. What instrument should be used in measuring pressure?

5. Where can pressure in solids and liquids be applied in real life?

 Vocabulary Pressure, atmospheric pressure, fluids, hydrostatic pressure, barometer, manometer. y Reading strategy As you read this unit – mark the paragraphs that contain definitions of key terms.

Use all the information to write a definition of each key term in your own words.

Perform calculations related to pressure in solids and fluids.

4.1 Force exerted by solids Activity 4

.1: Investigation pressure of a solid Materials:

⚫ One concrete brick

⚫ Balance

⚫ A pile of sand

⚫ A ruler

⚫ A long beam of wood

1 Unit 4: Density and Pressure in Solids and Fluids Procedures:

Measure the mass (m) of the brick and calculate its weight (w = mg).

Pour two bucketfuls of sand outside your laboratory such that it forms a pile as shown in (i). Use the long wooden beam to spread the sand such that you have a fairly large plain surface on top of the sand pile, as shown in (ii).

⚫ Take measurement of dimensions of one of the large surface side and calculate its area A1.

⚫ Take measurement of the dimensions of the small side and calculate its area A2.\

⚫ Gently place the brick in the sand on its big side and let it rest on the sand for 15s. Carefully remove the brick from the sand. Note the depression formed on the sand and carefully measure its depth. Measure the depth at four different places and determine the average depth of the depression on the sand left by the brick. Calculate pressure exerted by the brick using P1 = W A2

⚫ Gently place the brick on the sand on its smaller side but at a point away from the first experiment. Calculate the pressure exerted by the brick P2 = W A2 Compare and discuss the result obtained. (i) Sand pile (ii) Levelled sand pile Fig. 4.1: Force exerted by a solid 72 Physics for Rwanda Secondary Schools Learner’s Book Senior Two If the force is concentrated on a small area, it will exert higher pressure than if the same force is distributed over a larger surface area. As an example of varying pressures,

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