When the rain drops fall freely under gravity, their motion is opposed by the viscous drag of air. They attain terminal velocity when the viscous force due to air becomes equal to the weight of the drops. Terminal velocity, v ∝ r². As radii of rain drops are small, so they do not acquire very high velocity.

Variable factors refer to those factors, which can be changed in the short run. They vary directly with the output. For example, Labour, raw material, etc.

The painting is of an oxen which is painted with some sort of paste and the man who published this discovery in his writings was initially not accepted by the other European archaeologists.

he main organs of the alimentary canal are:

• The Mouth and Oral cavity.
• Esophagus.: It is a muscular tube through which small bolus of food passes from the mouth to the stomach. It is sharply bent at the point at which it meets the stomach. This prevents the backward movement of food from the stomach into it. Food in the digestive tract moves by an involuntary movement of alternate contraction and relaxation of muscles called peristalsis.
• Stomach:

  It is a muscular bag which has three parts- cardiac part, fundus, and body.

• Cardiac part– It is so called because it is present close to the heart. The opening of the esophagus to the stomach is regulated by the cardiac sphincter.
• Fundus– It is dome-shaped and is usually filled with air.
• Body– This is the main part of the stomach. The opening of the stomach into the small intestine is regulated by the pyloric sphincter.
• Largeintestine.The small intestine leads into the large intestine which has three parts- Caecum, Colon, and Rectum.
• Caecum– It is a small sac-like structure at the point where the small intestine meets the large intestine.
• Colon– It is divided into four regions-  transverse colon,  sigmoid colon ascending colon and descending colon.
• Rectum– It opens into the ****.
• Small intestine.:It is the longest part of the alimentary canal and comprises three parts- Duodenum, Jejunum, and Ileum.
• Duodenum– It is C- shaped and receives the hepatopancreatic duct formed by the union of bile and pancreatic duct.
• Jejunum–  Middle part of small intestine with thick walls and more vasculature.
• Ileum– The lower part of the small intestine with thin walls and less vasculature.

• Mouth

The mouth is the opening through which the intake of food occurs.

• Oral cavity

The oral cavity has three parts –the palate, tongue, and teeth.

Palate -It forms the roof of the oral cavity. The anterior part is called hard palate and the posterior part is called soft palate.

Tongue -Tongue is a muscular and glandular structure attached to the base of the oral cavity. On the upper surface, it has a V-shaped furrow called sulcus terminalis. It divides the tongue into the anterior oral part and posterior pharyngeal part. Its upper surface also consists of tiny projections called lingual papillae. Lingual papillae are of three kinds in humans-

(i)Vallate or circumvallate papillae -have gustatory receptors to sense taste

(ii) Filiform papillae- smallest and contain touch receptors.

(iii) Fungi-form papillae- Rounded and mostly present at the tip of the tongue.

• Teeth

Humans are diphyodont i.e. they have two sets of teeth- milk or deciduous and permanent teeth. Here is the list of different types of teeth in humans with their functions.

The human digestive system consists of the alimentary canal and its associated glands. Food is taken into the body through the mouth.

Digestion in human beings starts in the mouth. The mouth contains teeth, tongue and salivary glands. The teeth cut the food into small pieces, chew and grind it. The tongue mixes the food with saliva. The salivary glands secrete a watery liquid called saliva.

The slightly digested food goes into the food pipe or oesophagus by peristalsis. No digestion of food takes place in the oesophagus.

The slightly digested food reaches the stomach through oesophagus for further digestion where it is broken down into smaller pieces. The inner lining of the stomach secretes gastric juice which includes mucus, hydrochloric acid and digestive juices. Mucus protects the lining of the stomach from hydrochloric acid. Hydrochloric acid makes the medium in the stomach acidic to facilitate the digestion of proteins. The digestive juices break down the protein into simpler substances. Then the partially digested food goes into the small intestine.

The small intestine is a very long and narrow tube where complete digestion of food takes place. It receives secretions of liver, pancreas and its own walls. Liver secretes liquid called bile which helps in the digestion of fats and converts them into small droplets. Pancreas secretes pancreatic juice which breaks down fats into fatty acids and glycerol. Pancreatic juice also breaks down carbohydrates and proteins into simpler forms. The intestinal juice secreted by the walls of small intestine breaks down the carbohydrates into glucose, proteins into amino acids. Thus, the food gets completely digested in small intestine.

Absorption: The digested food material passes into the blood vessels in the walls of small intestine. This process is called absorption. The inner walls of small intestine have millions of tiny, finger-like outgrowths called villi which increase the surface area for rapid absorption of digested food. Each villus has a network of blood vessels which absorbs the digested food materials into the blood flowing through them.

Assimilation: The blood carries the absorbed food materials to the cells in all the parts of the body where it is used for growth, energy and repair. This is called assimilation. Glucose breaks down utilizing oxygen to form carbon dioxide and water, and releases energy.

The undigested and unabsorbed food passes from the small intestine into large intestine. Most of the water is absorbed from the undigested food materials in the large intestine.

Egestion: The undigested food is temporarily stored in the last part of the large intestine called rectum and finally the semi-solid waste is passed out from the body through **** in the form of faeces. This is called egestion.

Teeth: Teeth break down the food into smaller pieces. There are four types of teeth in our mouth. They are:

1. Incisors: These are chisel shaped teeth at the front of the mouth and used for biting and cutting the food.
2. Canines: These are large and pointed teeth just behind the incisors and are used for piercing and tearing the food. 
3. Premolars: These are large teeth just behind the canines on each side and are used for chewing and grinding the food.
4. Molars: These are very large teeth present just behind the premolars and help for chewing and grinding the food.

• The human digestive system consists of the alimentary canal and the associated glands.

Alimentary canal 

• The alimentary canal begins with an anterior opening – the mouth, and it opens out posteriorly through the ****.
• The mouth leads to the buccal cavity or oral cavity, which has a number of teeth and a muscular tongue and each tooth is embedded in a socket of jaw bone with a type of attachment called as thecodont.
• A set of temporary milk or deciduous teeth is replaced by a set of permanent or adult teeth, and the type of dentition is called diphyodont.
• An adult human has 32 permanent teeth, which are of four different types
• incisors (I)
• canine (C)
• premolars (PM)
• and molars (M).
• Arrangement of teeth in each half of the upper and lower jaw in the order I, C, PM, M is represented by a dental formula which in human is 2123/2123.
• The tongue is a freely movable muscular organ attached to the floor of the oral cavity by the frenulum, and the upper surface of the tongue has small projections called papillae, some of which bear taste buds.
• The oesophagus and the trachea open into the pharynx.
• A cartilaginous flap called epiglottis prevents the entry of food into the glottis, which is the opening of the wind pipe.
• The oesophagus is a thin, long tube which extends posteriorly passing through the neck, thorax and diaphragm and leads to a ‘J’ shaped bag like structure called stomach.
• The stomach has three major parts
• a cardiac portion into which the oesophagus opens
• a fundic region
• a pyloric portion which opens into the first part of small intestine.
• Small intestine is distinguishable into three regions
• a ‘U’ shaped duodenum
• a long coiled middle portion jejunum
• a highly coiled ileum.
• The opening of the stomach into the duodenum is guarded by the pyloric sphincter.
• Ileum consists of
• caecum
• colon
•  
• Caecum is a small blind sac from which a vestigial organ called vermiform appendix
• The colon is divided into three parts
• an ascending
• a transverse
• a descending part.
• The descending part opens into the rectum which opens out through the ****.
• The wall of alimentary canal from oesophagus to rectum possesses four layers
• Serosa
• muscularis
• sub-mucosa
•  
• Serosa is the outermost layer and is made up of a thin mesothelium and some connective tissues.
• Muscularis is formed by smooth muscles usually arranged into an inner circular and an outer longitudinal layer.
• The submucosal layer is formed of loose connective tissues.
• Mucosa is the innermost layer which forms rugae in the stomach and small finger-like foldings called villi in the small intestine.
• The cells lining the villi produce numerous microscopic projections called microvilli giving a brush border appearance.
• Villi are supplied with a network of capillaries and a large lymph vessel called the lacteal.
• Mucosal epithelium has goblet cells which secrete mucus that help in lubrication.
• Mucosa forms gastric glands in the stomach crypts of Lieberkuhn in different parts of the alimentary canal.

An electrochemical cell is a device that can generate electrical energy from the chemical reactions occurring in it, or use the electrical energy supplied to it to facilitate chemical reactions in it. These devices are capable of converting chemical energy into electrical energy, or vice versa. A common example of an electrochemical cell is a standard 1.5-volt cell which is used to power many electrical appliances such as TV remotes and clocks.