Under pressure

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PEPPS College

Scientific inquiry and problem solving skills based on Pressure Grade 91

1 Entire text is adapted from L. Pieterse’s Grade 9 P.H.S. Textbook notes and selected slides taken from SlideShare

PEPPS Vision: Ensuring that each learner assumes individual

responsibility towards creating a brighter tomorrow.

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ASSIGNMENT 1:

1. A woman, mass 75 kg, wears shoes with heels of a size 1,2 cm by 1,2 cm form at the bottom. Calculate

the pressure on the ground in N.m-2 if her full weight rests on one heel.

2. Calculate the pressure of a large container with a base size 4m x 6m exerted on the floor if the mass of

the container is 1 800 kg.

3. A piece of cloth can resist a pressure of 4 000 kPa before it rips. What is the maximum force (weight)

that the cloth can carry on a surface of 0,001 m2?

4. What pressure is exerted by a man with a weight of 800 N if he:

(a) wears shoes with a contact area of 0,0003 m2 each?

(b) wears ice skates with a length of 2,5 m and width of 0,1 m each?

5. A brick, of length 22 cm, breadth 10 cm and height 6 cm, is consecutively placed on a table as shown

in the diagrams below:

(a) (b) (c)

The mass of the brick is 2 kg. Complete the table below and answer the questions that follow:

Position Weight of the brick

(N)

Area against the

table (m2)

Pressure exerted

against the table

(N/m2)

(a)

(b)

(c)

Questions:

5.1 In which position (a/b/c) does the brick exert the greatest force on the table? Provide a reason for your

answer.

5.2 In which position does the brick exert the least force on the table? Provide a reason for your answer.

PEPPS Vision: Ensuring that each learner assumes individual

responsibility towards creating a brighter tomorrow.

Page 3 of 9

PRACTICAL 1.

Write a complete report of the following case study.

Learners have to investigate the relationship between area and pressure.

The learners were provided with a metallic block with a mass of 6 kg with dimensions 1 m x 0,5 m x 0,2m.

1 m

0,5 m

0,25 m

The block is placed on its different sides on a piece of sponge and it is observed that the sponge press in

deeper as the surface area of the metallic block decreases in size. The sponge presses in 1 cm, 2 cm and 3 cm

respectively.

Determine all the variables: independent, dependent and controlled variables. Write a complete report and

draw the following graphs: graph 1 – amount of centimeter of sponge pressed in versus the area, graph 2 –

pressure exerted versus area. (Tip: for graph 2, multiply by 102 on the y-axis in order to work with smaller values.

Have your teacher explain.)

Refer in your conclusion to the relationship between the depth that the metallic block presses in to the sponge

and the size of the pressure exerted. Conclude from the graphs what is the relationship between pressure and

area.

___________________________________________________________________________________________________________

PRACTICAL 2.

The learners are told to determine the relationship between weight (thus force exerted) and the pressure. They

are provided with two more metallic blocks of 2 kg and 4 kg respectively that have the same dimensions as the

6 kg metallic block. They first place the greatest surface of the three metallic blocks on a piece of sponge and

see that the sponge respectively presses in 0,33 cm, 0,67 cm and 1 cm as the mass of the metallic blocks

increase.

You will be expected to write a complete experimental report and to draw graphs of (1) force versus amount

that the sponge presses in (2) force versus pressure for the greatest surface area. Under the topic results, also

draw a table for force exerted by the smallest surface area. Use these values and draw another graph on the

previous set of axes of force versus pressure. Draw a conclusion by comparing the slopes of the graphs.

___________________________________________________________________________________________________________

PEPPS Vision: Ensuring that each learner assumes individual

responsibility towards creating a brighter tomorrow.

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PEPPS Vision: Ensuring that each learner assumes individual

responsibility towards creating a brighter tomorrow.

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PRACTICAL 3.

Set up the apparatus as in the second diagram on the previous page. Hold the funnel on the same depth, but

turn it in different directions. Record the pressure reading every time by measuring the pressure in mm on the

manometer. Write a complete report about the investigation. No graph is necessary.

__________________________________________________________________________________________________________

You will observe that pressure remains the same at a specific depth, it does not matter in which direction you

hold the funnel.

PEPPS Vision: Ensuring that each learner assumes individual

responsibility towards creating a brighter tomorrow.

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ASSIGNMENT 2:

1. The following table shows the density of a pair of fluids as well as the pressure (in

difference in height) at the same depth. The pressure is determined with the help of a

manometer.

Fluid Density (g/cm3) Difference in height / pressure (mm)

Water 1 12,0

Spirits 0,8 9,6

Alcohol 0,81 9,7

Glycerin 1,26 15,1

Olive oil 0,92 11,0

1.1 Use the table and draw a graph of density of the different fluids at the same height versus the pressure.

1.2 What will the density of the liquid be that causes a pressure (difference in height) of 14mm at the same

depth?

1.3 Complete: The greater the density of a fluid, the … is the pressure at the same height. Pressure is

therefore … proportional to the density of a fluid on a specific depth.

2. In an investigation to the pressure in fluids, the following experimental observations was made.

A cork that is left under water, immediately move to the top when it is released.

If a tin, with a little hole in the side, is filled with water, the water flow out of the hole.

If a tin, with a little hole in the bottom, is filled with water, the water will flow out of the hole.

If little holes are made around the circumference of the tin at the same height and filled with water, the

water will flow equally out of all the holes.

If little holes are made on different heights in a tin, the water flows furthest out of the bottom hole.

2.1 Which conclusions can you make from the observations above regarding fluid pressure?

3. A car with a mass of 550 kg, rests on a hydraulic lift. The area on which the car rests, is 0,25 m2. If the

area of the smaller area is 0,1 m2, calculate the force that have to applied to this surface to lift the car.

PEPPS Vision: Ensuring that each learner assumes individual

responsibility towards creating a brighter tomorrow.

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PEPPS Vision: Ensuring that each learner assumes individual

responsibility towards creating a brighter tomorrow.

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A balloon filled with air, will expand because the air inside the balloon exerts pressure on the walls of the

balloon. The higher the balloon moves up in the air, the smaller the atmospheric pressure and thus the balloon

increases in size, because the pressure in the balloon remains the same. The pressure that the air in the balloon

exerts increases in relation to the atmospheric pressure.

Try and explain the following:

Why does liquid in a straw move up when you drink from it?

PEPPS Vision: Ensuring that each learner assumes individual

responsibility towards creating a brighter tomorrow.

Page 9 of 9

ASSIGNMENT 3

1. In terms of the particle model of gases, explain

1.1 why gases exert pressure.

1.2 why atmospheric pressure decrease as height decrease.

2. Which instrument is generally used to measure atmospheric pressure?

3. What is the value of standard pressure?