Chapter 3 Forces & Pressure Student Copy

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Physics Module Form 4 Chapter 3: Forces and Pressure

CHAPTER 3: FORCES AND PRESSURE 3.1 UNDERSTANDING PRESSURE

1. The press pressure ure actin acting g on a surfac surfacee is defin defined ed as …….. per unit ………. on the surface.

2. Pres ressur sure, P =

3. Unit Unit for for pre press ssur uree is ……. or ……………… or ……………… 4. Example 1 : A wooden block is placed at different position on the surface of a piece of plasticine. At what position is the pressure higher?

Wooden block A

Plasticine

B

Answer: …….

5. Example 2 : Which shoe will exert a greater pressure on the when it is worn by the same women?

Answer: ………

1

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6. Example 3 : The diagram below shows a wooden block of dimensions 8 cm × 10 cm × 12 cm. Its weight is 12 N. On which side should the wooden block be placed to produce a maximum pressure exerted on the table. What is value of this pressure ? Weight (F) P

=

Minimum Area (A)

= 12 / (0.08)(0.10) = ……………

Application of Pressure

1. Tools like like knives, chisels, chisels, axes axes and saws have have sharp cutting cutting edges. The The surface surface area of contact is ……. ……... When a force is applied on the tool, the small area of contact will produce a …….. pressure to cut the material. 2. The flat flat base base of of each each metal metal pole pole of a tent tent has a …… surface area to ……………….. the pressure exerted on the ground. The poles will not sink into the ground because b ecause of the flat bases.

Exercise 3.1

1. A table table of mass mass 50kg has 4 legs is placed placed on a floor floor.. Each legs has has a cross sectional sectional area of 25cm2. Find the pressure pressure exerting on the floor (g=10ms-2) : Solution:











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2. The diagram diagram below shows shows a concrete concrete block block of dimension dimension 1.5m 1.5m x 2.0m x 3.0m. 3.0m. Its weight weight is 60N. Calculate (a)maximum pressure, (b)minimum pressure:

1.5m

2.0m 3.0m a) maxi maximu mum m pres pressu sure re Solution:

b) b) minim minimum um pres pressu sure re











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3.2 UNDERSTANDING PRESSURE IN LIQUIDS Density

1. Density ( ρ ) is defined as …….. per unit ……… ……….. The SI unit for density is ……….. ρ) = Density ( ρ)

mass

ρ

volume

=

m V

2. Chang Changee of of unit unit exam exampl ple: e: 800 kg m-3 =

800 kg m

3

=

(800)(1000) (100)

3

=

(800)(1000) 1000000

=

800 1000

= ……g cm-3

3. Example 1: 1: Calculate the density of a stone of mass 250 g if its volume is 100 cm3.

ρ =

m V

=

250 g 100 cm

3

= 2.5 g cm-3 = …………….

4. Example 2: 2: Abu’s Abu’s weight is 60 kg, when he is totally immersed in a tank of water, the water level raise by b y 55 liter. Density, ρ Density, ρ =

m V

=

60,000 g 55,000 cm

3

= ……….. g cm-3 = ……….. kg m-3

The Pressure Formula

1. Pres Pressu sure re in in liqui liquids ds act actss in …….. directions.











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3. The above above formula formula can can be derived derived from the followi following ng steps: steps: Mass of a cylinder c ylinder of water, m = ρV ρV = ……………… Weight of the cylinder of water, W = …….. = ρ A ρ A h g A cylinder of water A h

Volume

V = Ah = Ah

The pressure of water at the base of the cylinder of water is, Water pressure, P =

F A

=

W A

=

m g A

=

ρ A

h g

A

= ρ g h

4. Example 1 : A balloon is situated at 10 m below sea level, what is the total pressure experience by the balloon ? [ The density of sea water is 1100 kg m-3 ]

Total Pressure, P = Atmospheric pressure + Liquid pressureAtmospheric pressure at sea = 100,000 N m-2 + ρ g ρ g h h level : = 100,000 + …………………….. Patm = Pa = 100,000 + ……………….. = …………… N m-2 = ……………. Pa











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5. Example 2 : Water Water with density of 1 g cm-3 and oil are filled into a U-tube. What is the density of the oil ?

A

Pressure at A = Pressure at B Patm + h1 ρ 1 g = Patm + h2 ρ 2 g h1 ρ 1 g = h2 ρ 2 g

Oil 12 cm

h1 ρ 1 = h2 ρ 2 ρ 1

h2 ρ 2 =

h1

B

10 cm

20 cm

= (10)(1) ÷ (12) = …………. Water

Exercise 3.2

1. Given Given that that the dens density ity of of mercury mercury is 13600 13600kgm kgm-3. Calculate the pressure of mercury at a point 25cm from the mercury surface (g=10ms-2) Solution:

2. The figure figure shows shows a glass tube tube filled filled with 50cm 50cm height of of liquid liquid M and 30cm height height of liquid N. The densities of liquid M and N are 1000kgm-3and 2500kgm-3 respectively. respectively. By giving g=10ms-2, what is the pressure of a) liqu liquid id M at at poi point nt x b) b) liqui liquid d M and and N at poi point nt y Liquid M

50cm













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Solution:

3.3 UNDERSTANDING GAS PRESSURE AND ATMOSPHERIC ATMOSPHERIC PRESSURE Gas Pressure

1. The gas gas press pressure ure in in a contain container er is caus caused ed by the the ………... of gas molecules with the …… of the container.

2. Gas pressur pressuree can be measure measured d by using using 2 types types of instrum instrument ent known known as : (a)………….

gauge (consists of a semi-circular or C-shaped copper tube that tends

to straighten if more and more gas is pu mped (compressed) into it).











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Altitude and the Magnitude of Atmospheric Pressure

The greater …………… greater …………… from the sea level, the smaller will the atmospheric pressure.

Instruments for Measuring Atmospheric Pressure

1. Barometer is an instrument to measure measure atmospheric pressure. pressure. There are 2 types of barometer: (a) ................................ (is made of a partially vacuum sealed metal box). (b) …………………. (is made of a long glass tube about 1 meter in length fully filled with mercury and then inverted (turned upside down) into a bowl of mercury).











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Exercise 3.3

1. Figure Figure 3.3 shows shows apparatus apparatus set up which which is used used to measure measure atmospheric atmospheric pressur pressure. e.

10 cm

Vacuum

75 cm

Mercury

15 cm Q

Figure 3.3

(a) Calculate Calculate the pressur pressuree at point Q in in Pa unit. [Mercury density = 1.36 x 104 kg m –3]

Solution:











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2.

Figure shows a manometer connected to a gas tank whose valve is then turned on. What is the pressure of the gas , in unit N m-2 , in the tank? [Density of water = 1 000 kg m-3]

Solution:

3. If the atmospheric pressure is 76 cm Hg, what is the pressure of the the trapped air P?











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3.4 APPLYING PASCAL’S PRINCIPLE

Fill in the blanks with appropriate word. 1. ………… Principle state that pressure exerted on an ………… liquid is transmitted …… ……………………………. ………………………… ….

to every part of the liquid.

Exercise 3.4 Pascal’s Principle

1. By applying the Pascal’s Pascal’s Principle, draw the direction of water when the piston is pushed.

Push

Hydraulic Systems

water













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Solution:

Applications of Pascal’s Principle 1.












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3.5 APPLYING ARCHIMEDES’ PRINCIPLE

Fill in the blanks with appropriate word. 1. Archimedes’ Archimedes’ Principle Principle states states that when when an object object is ………. or ………… or ………… immersed in a fluid, it experiences a ………………. equal to the weight of the fluid displaced. 2. For a free floati floating ng object, object, the the buoyant buoyant force force is equal to the………. the ………. of the object.

Exercise 3.5 Archimedes’ Principle

1. An obje object ct of den densi sity ty,, 40gcm 40gcm-3 and mass 500g is immersed in a liquid of density 2 gcm-3. Calculate a) the volume volume of liquid liquid displa displaced ced b) the mass mass of the the liquid liquid disp displac laced ed c) the buoyant buoyant force experienced experienced by the the object object (g=10ms-2)

Solution:











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3.6 UNDERSTANDING BERNOULLI’S PRINCIPLE

1. Bernoulli’ Bernoulli’ss Principle Principle states states that for uniform uniform flow flow of a fluid, fluid, region of high high velocity velocity corresponds to ……. pressure whereas region of ……... velocity corresponds to high pressure.

Bernoulli’s Principle

1. P

Q

R Vertical lass tube

Moderate pressure Low pressure

Glass tube of uniform diameter Water











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2.


Figu Figure re bel below ow sho shows ws an an ………… ………….. The The uppe upperr regi region on of of the the aero aerofo foil il has has hig highe herr air air velo veloci city ty than the lower region of the aerofoil. By Bernoulli’s principle, the lower region has ………. pressure ………. pressure than the upper region of the aerofoil. This causes a ……………

on the aerofoil.

Exercise 3.6

1.











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Answer: A Velocity Pressure

2.

B High

C

Moderate

Figure below shows a Bunsen burner and a carburetor. carburetor. Mark with X for low pressure area.

Bunsen burner













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Part A: Objective Question

1. A cylind cylinder er has has a mass mass of of 12kg 12kg and and a cross-sectional area of 200cm2. What is the pressure acting at its base? A. 6 kPa

D. 15 kPa

B. 9 kPa

E. 18 kPa

C. 12 kPa 2. Wind blows blows norm normally ally on a wall wall at a

A. 1.1 1.19 x 10 105 Pa B. 1.90 x 105 Pa C 2 19 x 105 Pa











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10. Figure Figure 10 shows metal metal tube tube is blowed hardly at the opening. It is observed that the polystyrene ball is lifted to the opening of metal tube. Figure 7

Blow hard

A. 300N

D. 900N

B. 600N

E. 1000N

C. 800N

Polystyrene ball

Metal tube











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Part B: Structured Question

1.












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2. Diagram shows a set up of apparatus for measuring atmospheric atmospheric pressure.











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Part C: Essay Question

1. (a) A fisherm fisherman an finds that that his boat is at diffe different rent levels levels in the sea and in the river river,, although the boat carries the same load. The density of sea water is 1 025 kg m-3 and of river water is 1 000 kg m-3.













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…………………………………………………………………… ……… …… ……… ………… …… ………………

……… ………… ………… ……… ……… ………… ………… ……… ……… ………… ………… ……… ……… ………… ……











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Chapter 3 Forces & Pressure Student Copy

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