Entrance Question Answers: Motion in Stright line

Q1.

Water drops from the roof of a building 10 m high. The drops fall at regular intervals of time, the first drop striking the floor at the instant the 4th drop begins to fall. Find positions of individual drops,when a drop strikes the floor?

Let t is the time interval between A,B,C and D positions
That means, 3t second taken to travel from A to D.
That is, 10 m traveled in 3t seconds.

S = ut + 1/2 a t^2 

10 = 0 + 1/2 x 10 (3t)^2

10 = 5 x 9 x t^2

t^2 = 2/9




 Now,  

S1 = ut + 1/2 g t^2 

S1 = 0 + 1/2 x 10 x  2/9 = 10/9 m (AB length)

 S1 +S2 = 0+1/2x10x(2t)^2
 S1 +S2 = 0+1/2x10x(2x2/9)^2 = 40/9 m (AC length)

S1+S2+S3 = 0+1/2x10x(3t)^2
S1+S2+S3 = 0+1/2x10x(32/9)^2 = 10m  (AD length)
 

Electric charge density

Suman m.s said...

sir...one doubt in problem follows

The earth has on electric charge that causes an electric field near its surface of the order of 100 vm-1.What is the surface density of charge on earth of earth is a conducting sphere? pls help me 2 solve it

Anish V Rajan said...

Take earth as a conducting sphere of surface charge density (sigma)

Assume total charge that is spread on the surface is Now concentrated on the center. Find Electric field at the surface by usual equation

Work Power and Energy

Q1 Certain physical quantities are listed below. Select any three pairs of physical quantity with same dimension.
(1) Stress (2) Energy (3) Displacement (4) Force (5) Work
(6) Pressure (7) Torque (8) Weight (9) Velocity

A1. Stress & Force, Energy & Work, Force & Weight




Q2 In the above question, Select the odd one ? Write the uses of dimensional analysis ?

A2. odd one is Torque because it is related to circular motion.

Uses of dimensional analysis are
I. Check the correctness of an equation
II. Derive an equation relating to two or three physical quantities
III. Find dimension of constants in an equation
IV. Convert unit from one system to another.

Motion in one dimension

Q1. A balloon raises with constant velocity 10 m/s. A load tied by a rope to the balloon is cut when it is at a height of 500 m.
(a) How long will it take to reach the ground ?

A1.

Here Initial velocity u = 10 m/s ( Positive , since direction of initial motion is positive)

Displacement , S = - 500 m (towards downward - against initial motion direction)

Acceleration (due to gravity) , a = - 9.8 m/s2 (also downwards - against initial motion direction)

Use equations: S = ut + 1/2 a t2

See answer as : t = 11.12 seconds

Vector sum

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