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Speed.

posted Jul 31, 2019, 3:30 PM by Upali Salpadoru   [ updated Aug 7, 2019, 12:08 PM ]


          All three are speeding.  
         They have different speeds. Some move slowly ,the falcon moves fastest. We can describe a speed as slow or fast but in science we have to give them in numbers.
They have to be properly measured. 
         It is not easy to measure the speed of a moving object on sight without some proper instrument.
         The cars have speedometers to give speed at any time. The police have an instrument to get the speed of an oncoming vehicle.
 Practical:-   How can you find out how fast you can run?
           Measure a definite distance in your lawn.         Say 50 meters.
         Then start a timer and start running from one end to the other.
                                                                             Say you took 20 seconds.
         Calculation:-
            In 20 seconds you have done  50 m.
         ∴  in  1 s.      you have run.........= 50 / 20 m.
                                 That is  ..................= 2.5 m.    Then the speed we say is 2.5 m/ second.


  Is it correct to say that this was your speed throughout the run?      No !
  
   Speed is distance changing with time. Consider the graph below.

   

Fig. Distance against time.
   
Starting from zero a cyclist speeds

As he did 60 m in 5 seconds his speed was 60/5 = 12 ms-1.

At the start, that means during the first second he covered only about 3 m.
His speed was only  3 ms-1



    In 2 seconds  he has covered 8 m.         Then the speed is  8/2  = 4 ms-1
   
 Time. second. 1st. 2 nd. 3 rd. 4 th. 5 th. 6th.
 Speed during
that time.   ms-1
 

3
 d2 - d1
 8 - 3
 =
5
 d2 - d1  
18 -8
10
 d2 - d1
32-18
14
 d2 - d1
60 - 32
28
 
                                                 Highlight to check the answers.
              The speed canges; Yet we say the speed is 12 ms-1.
               This is the Average speed.       Not the speed at any one time.
             
   A Speed time graph with an upward curve shows increasing of speed.
   What will a linear horizontal graph show?
   What will a downward curve show?

    Here the speed remains the same:- 
celeration.
   
 What is the acceleration shown by the above graph?
        It is 0 ms-2                      Highlight o see the answer.  
       


The graph is the case of an object traveling at the same speed.

       Do you know the speed?       Yes !  It has gone 25 m  in every second.
       As it has traveled  for  5 seconds  he would have gone 5 x 25  meters.
       Have you noted the  distance is the same as the AREA BELOW THE GRAPH.

    






              What is the difference between SPEED  and VELOCITY.

             The moon speeds round the earth with almost the same speed. Once in about 28 days.  As the direction changes moons velocity is not the same.

Changing of speed.

         The figure shows the two pedals in a car that can do this.



         Pressing the accelerator will accelerate;
 That means the speed will increase.
        Releasing the accelerator will decelerate. Speed will reduce.  The brake pedal will do the same thing very quickly.




Acceleration.

   Here is a case of increasing speed in a regular way.
       
  
  When there is acceleration the graph is inclined upward,
  The steepness shows whether the acceleration us high or low.
   Acceleration =  (high velocity - low velocity) /  Time
  

   That is  (25 - 0 ) / 5  = 5 meters per second per second.
   This may be written as:-  6 ms-2
   
 Formulae.
1.  Distance = Average Speed x  Time. 
2.  Average speed =    Distance / time.
3.  Velocity = Displacement / time.
4.  Acceleration  =  Change in speed / time.       A = (V2 - V1) / t.
5.  Under uniform acceleration:-   Average. speed.    Va =   ( V1 + V2) / 2.
6.  Under uniform acceleration Highest velocity  .  V2 = 2 x Va.
         

    Multiple choice questions. 
  1.  Arthur scoots 100 m. in 10 seconds. Which is correct.
   A. His movement is uniform.
   B. He showed  uniform acceleration.  
   C. He has an average speed of 10 ms-1
   D. He has a top speed of 10 ms-1

    From  2 to 4 .  Use this graph to get the answers.
   
    2.   What does the graph indicate ?
          A.  Increasing speed.    B.  Uniform speed.  C. Acceleration. D.   Deceleration.
    3.   What is the total distance traveled in 5 seconds.
            A.   75 m.   B. 37.5 m    C.    12.5 m.  D. 25 m.
     4.   What is the speed for the first 3 seconds?
            A.  15 ms-1.   B.  7.5 ms-1.    C.  5 ms-1.     D,  3 ms-1
   


   From question 5 to  7.


5What does the graph show?
  A.  Total distance to be 25 m.                        B.  Uniform speed.  
  C.  Acceleration. 
  D.  Deceleration.

 6.  What is the average speed?
   A. 17 .5 Ms-1.   
   B.  17.5 ms-2.
   C.  7.5 ms-1.  
   D. 7.5 ms-2.

 7.  What is the total distance?
 A.  Area of the red triangle.   B. 10 x 6 m ,    C.25 X 6 m.   D.  17.5 x 6. m 

  For  the questions 8 to 10.
  Martin cycles 100 m  in  5. seconds.  Assuming the acceleration to be uniform .
  8.  What is the average speed ?   A. 100 / 5 ms-1.  B. 100/5 ms-2
                       C.  100 /2 x 5 ms-1.      D.  100 /2 x 5  ms-2.
  9.  What was the highest speed achieved?
                A.  20 ms-1.   B. 40 ms-1 .   C.  80 ms-1.  D.  100 ms-1.
  10. What was the acceleration?
       5    A. 20/5 ms-2.  B.  20 x 5 ms-2.  C.  40/ 5 ms-2   D.  100 / 20 ms-2.   
   
 

   ANSWERS .
     1.-   C,    2.- B,   3.-     D,   4.-   C,   5.- C,    6.- A,   7.-   D,   8.-  A,   9.-  B,     10.- C.

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