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       CLASS – 4    SCIENCE   EXERCISE D SECTION QUESTION AND ANSWER Que 1 – Name the organ through which food enters the stomach. Ans 1- Food pipe ( Oesophagus) Que2 – How many sets of teeth are formed during one’s lifetime ? Name them. Ans 2- There are two sets of teeth- (a) Milk teeth or temporary teeth (20) (b) Permanent teeth (32) Que 3 – Write any one difference between milk teeth and permanent teeth. Ans 3- (a)Milk teeth is the temporary teeth, which arises first after the birth whereas permanent teeth arises after the milk teeth. (b)   Milk teeth are of 20 in number whereas permanent teeth are 32 in number. Que 4 – What do you mean by the term ‘digestion’? Ans 4- Digestion is a process to break down complex material of food into simpler forms. Que 5 – What happens to the food when it enters the stomach ? Ans 5- In stomach the food mixes with digestive juices secreted by the walls of the stomach a...

A conservative force is a force done in moving a particle from one point to another, such that the force is independent of the path taken by the particle. It depends only on the initial and final position of the particle. Gravitational force and elastic spring forces are two such examples of conservation forces. What is Conservative Force? As the name suggests, conservative force conserves energy. It follows the law of conservation of energy. Many forces in nature that we know of like the magnetic force, electrostatic force, gravitational force, etc. are a few examples of a conservative force . Let us understand the concept better with the help of the following example. Gravitational force acting on a particle In the given image, the gravitational force acting on the particle has a magnitude equal to mg , where m is the mass of the substance and g is the acceleration due to gravity. The particle moves from point A to point B , and its vertical displacement is given by...

 सार्थक अंक क्या होते हैं?  साधारणतः  मापन के परिणामों को एक संख्या के रूप में प्रस्तुत करते हैं, जिसमें वे सभी अंक सम्मिलित होते हैं जो विश्वसनीय (निश्चित) है तथा वह प्रथम अंक भी सम्मिलित होता है जो अनिश्चित है। किसी माप के मान में वे सभी अंक जो निश्चित हैं तथा इनके साथ का प्रथम अनिश्चित अंक, सार्थक अंक कहलाते हैं। सार्थक अंकों की धारणा बहुत महत्वपूर्ण है। सार्थक अंक मापन की परिशुद्धता इंगित करते हैं। इनसे यह संकेत मिलता है कि कोई माप किस सीमा तक विश्वसनीय है। सार्थक अंक किसे कहते हैं? किसी माप के उन अंको, जिन तक हम प्रमाणिक एवं यथार्थ जानकारी कर सकते हैं, को सार्थक अंक कहते हैं। The digits that reflect the precision of the measurement are called significant figure. सार्थक अंक कैसे पहचाने जाते हैं ? यदि मापन के बाद किसी वस्तु की लंबाई 479.5 व्यक्त की जाती है, तो इसमें अंक 4,7,9 तो निश्चित एवं विश्वसनीय हैं जबकि दशमलव बिंदु के बाद का अंक 5 अनिश्चित है, इस प्रकार मांपे  गए मान में चार सार्थक अंक हैं।   सार्थक अंक ज्ञात करने के नियम कि...

Percent Error Percent Difference What is Percent Error? Percent errors tells you how big your errors are when you measure something in an experiment. Smaller percent errors mean that you are close to the accepted or real value. For example, a 1% error means that you got very close to the accepted value, while 45% means that you were quite a long way off from the true value. Measurement errors are mostly unavoidable: equipment can be imprecise, hands can shake, or your instruments just might not have the capability to measure accurately. Percent error will let you know how badly these unavoidable errors affected your results. Formula The formula for percent error is: PE = (|accepted value – experimental value| \ accepted value) x 100%. Example question: The accepted distance to the moon is 238,855 miles.* You measure the distance as 249,200 miles. What is the percent error? Solution: Step 1: Insert your data into the formula: PE = (|accepted value – ex...

The Four Fundamental Forces of Nature   The Four Fundamental Forces of Nature are Gravitational force, Weak Nuclear force, Electromagnetic force and Strong Nuclear force. The weak and strong forces are effective only over a very short range and dominate only at the level of subatomic particles. Gravity and Electromagnetic force have infinite range. Let’s see each of them in detail. Gravitational Force The gravitational force is weak, but very long ranged. Furthermore, it is always attractive. It acts between any two pieces of matter in the Universe since mass is its source. Weak Nuclear Force The weak force is responsible for radioactive decay and neutrino interactions. It has a very short range and. As its name indicates, it is very weak. The weak force causes Beta decay ie. the conversion of a neutron into a proton, an electron and an antineutrino. Electromagnetic Force The electromagnetic force causes electric and magnetic effects suc...

Scalars and Vector Quantities Scalar Quantities: The physical quantities which are specified with the magnitude or size alone are scalar quantities. For example,  length, speed, work, mass, density, etc. Vector Quantities: Vector quantities refer to the physical quantities characterized by the presence of both magnitude as well as direction. For example, displacement, force, torque, momentum, acceleration, velocity, etc. Comparison between Scalars and Vectors Criteria Scalar Vector Definition A scalar is a quantity with magnitude only. A vector is a quantity with the magnitude as well as direction. Direction No direction Yes there is the direction Specified by A number (Magnitude) and a Unit A number (magnitude), direction and a unit. Represented by Quantity symbol Quantity symbol in bold or an arrow sign above Example Mass and Temperature Velocity and Acceleration Characteristics of Vectors The characteristics of vectors are as follo...

Unit and Dimensions The comparison of any physical quantity with its standard unit is called measurement. Physical Quantities All the quantities in terms of which laws of physics are described, and whose measurement is necessary are called physical quantities. Units A definite amount of a physical quantity is taken as its standard unit. The standard unit should be easily reproducible, internationally accepted. Fundamental Units Those physical quantities which are independent to each other are called fundamental quantities and their units are called fundamental units. S.No. Fundamental Quantities Fundamental Units  Symbol 1. Length metre m 2. Mass kilogram kg 3. Time second S 4. Temperature kelvin kg 5 Electric current ampere A 6 Luminous intensity candela cd 7 Amount of substance mole mol Supplementary Fundamental Units Radian and steradian are two supplementary fundamental units. It measures plane angle and solid angle respectively. S.No. Suppleme...