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A question from Gravitation.

Prince Patel Asked:

“Sun have high gravitational energy ,then why planets of solar system doesn’t colloid with sun?”


“The planets are moving around the sun and the gravitational force is used in providing the necessary centripetal force (The force required by a body to move in a circular path). As long as the planets are in motion, they will not move towards the sun. A similar explanation hold good for moon going around earth also.”

Starry Night : Why do stars twinkle? Can you answer?

We have been hearing the rhyme “twinkle twinkle little star” since we were kids.

Why stars twinkle? Can anyone give a fitting explanation?

See below a world famous painting “Starry Nights” by Van Gogh.

Optics MCQ – image formation by concave mirror

John Abraham asks:

I am having trouble with a problem.

Q. In an experiment to determine the focal length (f) of a concave mirror by the u–v method, a student places the object pin A on the principal axis at a distance x from the pole P. The student looks at the pin and its inverted image from a

(A) x < f

(B) f < x < 2f

(C) x = 2f

(D) x > 2f


The answer is B

(Try giving your explanation as comment. If not satisfied, ask again)


The image and the object will move together at 2f

The one closer to the eye will move faster

In the question, the situation occurs when the image is closer to the eye and this happens when the object is between f and 2f

Hence the answer is B

In which direction should we lie to sleep?

Haritha asked:

it is heard that we should not lie in north-south direction.is there any explanation for it?


Answer to be posted soon.

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