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Objectives

By the end of the lesson you should be able to:

  1. Describe experiments to illustrate cohesion, adhesion and surfaces tension.
  2. Define cohesion, adhesion and surface tension


Cohesion and Adhesion

When a finger is dipped into water and then removed, the finger becomes wet and usually some drops remain attached to it. This shows that there is attraction between the finger and water. The force of attraction beween particles of different substances (e.g. water and finger) is called adhesion.

When two drops of the same liquid are placed very close to each other, they are observed to fuse up (come together), an indication that particles of the same substance also experience attraction. The force of attraction beween particles of the same substance (e.g. water particles) is called cohesion.

Cohesion and adhesion determine the shape of liquid meniscus. The meniscus curves downwards whenever adhesion is greater than cohesion.

Water and kerosene, for example, experience stronger adhesion to glass than the cohesion between their particles; so their menisci curve downwards.


Surface Tension

In the set-up below, a pin is gently placed horizontally on the surface of water. What do you observe?


The pin remains supported on the surface. It also forms a shallow depression where it rests and this indicates that the liquid surface behaves like an elastic skin. The force that holds the pin at the surface is the surface tension. The elastic behaviour of liquid surface can be amplified as shown in the next animation.


Surface tension



Drag the pin and drop it from high above the water. what happens?

Surface tension

Drag the pin and drop it from high above the water. what happens?

Experiment on Surface Tension



Click within the loop of the thread and observe what happens

Apparatus


Mass

What is mass?

The term mass is often confused with weight, which is an entirely different quantity. Although these quantities are related, a clear distinction exists between them. Mass is defined as the quantity of matter in a body. It is measured in kilogrammes (kg) as the SI unit (Weight is dealt with under the topic, Force). In this section, we discuss the measurement of mass.


Measurement of Mass

Observe the movements on the beam balance as different objects are placed on the stage. Which of the objects: stone, wooden board and bottle has the largest mass?

Did you get it right? The stone happens to be the heaviest, followed by the wooden board.

View the next video clip to observe the adjustments normally made before taking a reading.



 

TEMPERATURE

Drag the Bunsen burner and place it below the tripod stand.

Scalar and Vector Quantities

All physical quantities such as length, volume, mass, density and force can be classified as scalar and vector quantities. What are scalar and vector quantities? Think of other physical quantities that are not mentioned in this list.

Temperature

Drag the Bunsen burner and place it below the tripod stand.


Apparatus


When water is heated its temperature increases. There is increase of degrees in the thermometer.

The increase in temperature of the water is measured in degrees.

Volume


Volume is the amount of space occupied by an object. There different methods that can be used to obtain the volume of an object or substance. Normally, the method applied depends on the shape of the object, or of the container, in the case of liquids and gases. In the section that follows, we will learn how to determine the volume of regular and irregular solids.

Meaning of Force


Force is a push or a pull.

 

Weight

Observe the demonstration below. What is the effect of adding loads to the hook of a spring balance?

Discussion

The objects added to the hook pull and stretch the spring downwards. That is, they experience a downward pull towards the earth. This pull is called weight. Weight is the earth's gravitational pull on an object.


Upthrust

Observe what happens when a cork is forced into water and then released.

The cork is observed to buoy up when released under water. This shows that there is a force, due to water, that pushes the cork upwards. This upward force is called upthrust. Upthrust is the upward force exerted on an object when it is immersed in a liquid or gas.


Defining Scalar and Vector Quantities

The sketch below shows three directions; north, east and west. Some physical quantities have direction, while others do not.

A quantity that has both magnitude and direction is called a vector quantity. An example is force.

Quantities which have only magnitude but no direction are called scalar quantities. Examples of scalar quantities are length, volume, mass and densiy.


Exercises



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