# 514 - Physics Questions Answers

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