# 495 - Physics Questions Answers

Why earth moves in orbital motion around the sun ...? why not in circular motion...?

**Asked By: HARISH**

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**Joshi sir comment**

Since every heavenly body like sun, earth, moon are rotating freely in free space, there is no body which is placed on a base, so rotation of every body depends on all the surrounding bodies. It means its track will also depend on the attraction forces given by all the surrounding bodies.

Two cylinders are twisted by the same external torque.If the ratios of their modulous of rigidity , radii & lengths are 1:2, 2:3 & 3:4 respectively. Then find the ratio of angle of twist ??

**Asked By: AMIT DAS**

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**Joshi sir comment**

Let the angle rotated due to torque = θ and angle of twist (shear) = δ

then arc covered = rθ (here r is radius)

now for twist of wire δ = rθ/l (here l is length)

according to formula η = F/Aδ and torque ζ = kFr (here k is constant of proportion)

we get ζ = kηAδr

Use the formula and get the answer, you have to calculate ratio of δ's so k is not required

how far from earth we can see earth as a whole elliptical?{we can clearly see the earth

**Asked By: SAHIL KUMAR**

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**Joshi sir comment**

for getting the complete vision we should draw tangents at the extreme of major and minor axis of the earth, this will give a point of intersection for which height from centre of earth will be R√2 so height from earth surface will be R√2 - R = R(√2-1)

here R is used for radius of earth

This is approximate calculation only

A 20 cm long test tube (cylindrical) is inverted & pushed vertically down into the water . when the closed end is at water surface, how high has the water risen inside the tube? [p atm =10^5 N/m²]

**Asked By: AMIT DAS**

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**Joshi sir comment**

Let the height of water in the tube is x cm. then according to PV = constant, calculate the pressure by (20-x) cm. column

then P_{0} + pressure by 20 cm water = pressure by (20-x) cm. column + pressure by x cm water column

A body is moving towards north with initial velocity of 13m/s. Its is subjected to a retardation of 2 m/s^{2} towards south.The distance travelled in 7th sec is ??

**Asked By: NIKHIL VARSHNEY**

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**Joshi sir comment**

displacement of any particular sec. = u+at-(a/2)

here direction of motion is opposite to the direction of acceleration so formula will be u-at+(a/2)

displacement in 7th sec = 0

now we know that direction of motion will become south after 6.5 sec. so distance for last half sec = ut+(1/2)at^{2 }= 1/4

same will be the distance for the first half sec of 7th. so total distance for 7th sec = 1/4 + 1/4 = 1/2