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# Category Archives: KINEMATICS

## Displacement – A Numerical Problem

A fly is sitting in the middle of a wall of a cubic room of side ’a’.If it flies and sit to one of opposite corner of the wall. Find its displacement?

**Answer:**

**A to B**in the diagram) the displacement is “

**a**“

**A to C**in the diagram) the displacement is “

**√2 a**“

**A to H**in the diagram) the displacement is “

**√3 a**“

## Problem from Kinematics

**Devika** posted:

There are two particles A and B moving with same velocity u such that initially there velocity are perpendicular to each other and distance between them is L.the velocity of A is always directed towards B.find the distance between them after a long time.

Ans will be posted soon

## Ball falls to earth, and earth moves to ball?!!

Sandeepan Dhar Choudhary asked:

“When the ball falls on the earth, the earth also moves up to meet it. But the motion of the earth is not noticeable. Explain why?”

Answer:

The force exerted by the ball on earth is same as the force exerted by the earth on the ball. The acceleration of the ball towards earth is approximately 10 m/s^{2}.

The acceleration of earth towards the ball = force/mass of earth ≈ 10^{-23} m/s^{2}

When the earth moves towards the ball with this acceleration, the time required to travel the distance will be very large. If we assume a distance of 1m, and from the equation

we get **t ≈ 10 ^{12} seconds whereas the ball **

**reaches the earth in 0.4 seconds**

**[warning]In a nut shell, we can say that we cannot notice the motion of earth towards the ball due to the extremely low value of acceleration of earth towards the ball.[/warning]
**

## Another problem from kinematics

Zeen asks:

A boy jumps from rest, straight down from the top of a cliff. He falls halfway down to the water below in 0.800 s. How much time passes during his entire trip from the top down to the water? Ignore air resistance.

**Answer **:

let the total height be h

So, **for first case**

S = h/2

a = g=10 m/s^2

u=0 m/s

t = 0.8 sec

using the relation

h/2 = 0.5 x 10×0.8 ^2

h=6.4 m

**In second case (considering the full motion)**

S=h=6.4m

t=?

a=g=10m/s^2

u=o m/s

using the relation

6.4 = 0.5 x 10 x t^2

or

t = 2 x 6.4/10 = **1.28 s**

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