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Mass of Earth

How is the Mass of Earth Calculated?

Asked Raj


There are multiple ways to calculate the mass of earth. Assuming that your question is from the chapter Gravitation, I explain that here.

Knowing the radius of earth and the value of acceleration due to gravity, we can calculate the mass of earth.



If we put g = 9.8 ms-2 R = 6400000 m and G = 6.67 x 10-11 Nm2 kg-2

We will get M=6.0 x 1024 kg

Physics in Irrigation

Somnath asked:

“water in a canal 6m wide n1.5m deep is flowing with a speed of 10km/hr.How much area will it irrigate in 30min if 8cm of standing water is need”


Answer (to be posted soon)

CBSE Class X Physics Questions selected from Previous Question Papers

  1. Name the part of the human eye that helps in changing the focal length of the eye lens.( 1 mark)
  2. Write the  value of near point of distinct vision for normal eye (1 mark)
  3. What is the cause of dispersion of white light passing through a prism? Which colour of light deviates the (1) most      (2) least? (2 marks)
  4. Why does the clear sky appear blue? How would the sky appear in the absence of earth’s atmosphere? (2 marks)
  5. The refractive indices of alcohol and turpentine oil with respect to air are 1.36 and 1.47 respectively. Find the refractive index of turpentine oil with respect to alcohol .Which one of these will permit the light to travel faster? (2 marks)
  6. Why do we observe difference in colours of the sun during sunrise sunset and noon? (2 marks)
  7. Describe the formation of rainbow in the sky with the help of diagram? (2 marks)
  8. What is a rainbow? Explain in brief the formation of rainbow? (3 marks)
  9. A person cannot see objects farther than 12 m from the eye clearly. Name the , defect of vision he I suffering from and the lens that should be used for correction of this defect. Illustrate with the help of a diagram, how this lens will correct the defective vision. (3 marks)

A point object is placed at a distance of 12 cm from the convex lens on its principal axis. Its image is formed on the other side of the lens at a distance of 18 cm from the lens. Find the focal length of the lens. Is the image magnified? Justify your answer. (3 marks)


  1. A compound lens is made of two lenses in contact having powers +12.5 D and _2.5 D. Find the focal length and power of the combination.
  2. The magnification produced by a mirror is +1.What does this mean ? (3 marks)

12.    A person is not able to see distinctly the object placed at large distances but is able to read the matter written on a book comfortably. Name the defect of vision he is suffering from? State two causes for this defect? With the help of a ray diagram show how this defect can be corrected    (3marks)


13. Name the type of mirror used in the following:   (a) Solar furnace (b) Side/rear- view mirror of a vehicle. Draw labeled ray diagrams to show the formation of image in each of the above two cases. Which of these mirrors could also form a magnified and virtual image if an object? Illustrate with the help of a ray diagram.(5 marks)


  1. Draw a ray diagram in each of the following cases to show the formation of image, when the object is placed:
    1. Between optical centre and principal focus of a convex lens
    2. Between F and 2F of a concave lens
    3. At 2F of a convex lens. What can you say about sign and value of linear magnification ratio in (1) and (2) above (5 marks)
      1. Define principal focus of a concave mirror.
      2. Why do we prefer a convex mirror as a rear view mirror in vehicles?
      3. We wish to obtain an erect image of an object using a convex lens. Draw a ray diagram to show the image formation in this case (5 marks)




Space Time

Prejith asked:

What is space time?



In order to describe an event, we need to specify its position in space and the time of occurrence. A four-dimensional continuum with the three co ordinates of space and time as the fourth co ordinate is called space-time.

The members and visitors may add more related information via comments


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Human Eye and the Colorful World – An online test

Class X students can practice this online Fill in the Blanks test and get evaluated online instantly. The sentences are taken from NCERT Text Book. This is an excellent way of revising fast.

Click the link below to start

Fill in the Blanks Exercise from Human Eye and the Colorful World

Hyper Nova Explosion

Anoop Krishna Asked:


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January 2012

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