In this section, the following topics are discussed with examples

  • Basic Formulae
  • Unit Conversion
  • Relative Speed
  • Problems on Trains
  • Example

Basic Formulae

Distance=Speed * Time

Speed = Distance / Time

Time = Distance / Speed

Unit Conversion

•  To convert m/s into km/hr:

x m/s = x * (18/5) km/hr

•  To convert km/hr into m/s:

x km/hr = x * (5/18) m/s

•  If a man covers a certain distance at two different speed x km/hr and y km/hr. Then,

Average speed= 2xy / (x + y)

Example 1:

If a person covers a distance of 75m in 2 hrs.What is his speed in km/hr?

Sol:

Distance=75m

Time = 2 hrs

Speed=x km/hr

Speed= (75 * 18) / ( 5 * 2 )km/hr

=135 km/hr.

 

Example 2:

If a person covers a distance at a speed of 8/10 of his usual speed he reaches his office 5 minutes late?What is the time taken by him to reach his office?

Sol:

s = (8/10) *s

=8/10(d/t)

T=t+5

s=d/t

(8/10) * (d/t) = d / (t + 5)

t=20


Relative Speed

•  If two objects A and B are moving in the same direction with the speed of x km/hr and y km/hr. Then, their relative speed is,

Relative Speed=(x-y)km/hr

•  If two objects A and B are moving in the opposite directions with the speed of x km/hr and y km/hr.Then, the relative speed is,

Relative speed=(x+y)km/hr

•  The ratio of the speeds of two objects is inversely proportional to the root of their time taken 

S1/S2 = root (T2/T1)


The ability to simplify seems to eliminate the unnecessary so that the necessary may speak.

Hans Hoffman

Problems on Trains

Types of Problems 

  1. Train crossing a man/pole/lamp post
  2. Train crossing a running man(Same direction)
  3. Train crossing a running man(Opp direction)
  4. Train crossing an another train/bridge/platform
  5. Train crossing an another running train(Same direction)
  6. Train crossing an another running train (Opp direction)

Formula to calculate time taken by train to cross,

1.   A man/pole/lamp post,                              Time = Ltrain / Strain

2.   A running man(Same direction),                  Time = Ltrain / (Strain-Sman)

3.   A running man(Opp direction) ,                   Time = Ltrain / (Strain + Sman)

4.   An another train/bridge/platform,              Time = Ltrain+Lobj / Strain

5.   An another running train(Same direction,    Time = Ltrain +Lobj / (Strain-Sobj)

6.   An another running train (Opp direction),    Time = Ltrain +Lobj / (Strain + Sobj)

Where,

train     –    Length of train

Strain       –     Speed of train

Sman        –    Speed of man

Lobj           –     Length of object/train 2

Sobj           –     Speed of object /train 2

Examples:

A train is 600 meter long and is running at the speed of 72 km per hour. Find the time taken to cross

1)a man

2)a man running at 5 m/s in same direction

3)a man running at 5 m/s in opposite direction

4)a 200 meter long platform

5)a 200 meter long train running at 10 m/s in same direction

6)a 200 meter long train running at 10 m/s in opposite direction

Answers:

  1. 30 sec              2. 40 sec             3. 24 sec             4. 40 sec            5.80 sec            6.80/3 sec

Explanation :

  1. 600/20 =30 sec
  2. 600/20-5 = 40 sec
  3. 600/20+5=24 sec
  4. 600+200/ 20 = 40 sec
  5. 600+200/20-10=80 sec 
  6. 600+200/20+10= 80/3 sec

Example 1:

Train A travels at a speed of 90km/hr crosses a man at 8 seconds. what is the length of the train.

Soln:

Speed=90 km/hr

=90*(5/18)m/s

=25 m/s

Time=8 s

Length=25*8

=200 m

 


Example 2:

Chennai express of length 1050 m crosses a waiting man in 45 sec while it takes 90 sec to cross a platform.What is the length of the platform?

Soln:

Length of the train=1050 m

Time for man=45 s

Speed=(1050/45)m/s

Let, the length of the platform=x

(x+1050)=(1050/45)*90 (S*T=D)

x+1050=2100

x=1050

Length of the platform=1050 m