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Speed.

posted Jul 31, 2019, 3:30 PM by Upali Salpadoru   [ updated Jun 16, 2021, 10:50 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. 

  Animal                                                        Distance it can go in 1 hour. Time taken to travel 
10 kilo meters.
 



 0. 3 km.
 
 0.3 km in   1 hour 
I km in        .   .  1/ 0.3
                         = 1x 10
                                 3
                         = 3.33 hours.
 



  
 



  

 
Can you complete the values for the Hare and the Falcon ? 

 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.
                                                       You can write this as 2.5  ms-1


   Is it correct to say that this was your speed throughout the run?  No !
  Why?   You start from rest. Then the speed is 0 m/s
   You change your speed as you run.
   So What you may find is an average speed.
   Speed is distance changing with time. Consider the graph below.

  Graph 1.

Fig. Distance against time.

An object has moved 50 meters in 5 seconds.  
The values for the movement .
 Time 1 2 3 4 5
 Distance 10 20 30 40 50

 What does the graph show?
1. Start to finish the speed has been the same.           (Uniform speed)
2. The gradient gives the speed. That is the distance divided by time.
                             
                   Speed =  Distance / Time.            V =  d/t

  
It follows that  
 
Distance =  Speed x Time
         d = V x t 
 
Time =  Distance / speed.
       t = d / t



  Graph 2.
 
    Starting from rest a cyclist speeds. So the starting speed is zero.
   As he did 60 m in 5 seconds his speed was 60/5 = 12 ms-1.
   Distance Time graph showing acceleration.
     Fig.Distance time graph.
Starting from zero a cyclist speeds
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
each second.ms-1
 

3
ms-1
 d2 - d1
 8 - 3
 =
5
ms-1
 d2- d
 
18 -8
10
 d2- d1
32-18
14
 d2- d1
60 - 32
28
 
      Can you copy and complete the chart?                                         
      The speed changes; Yet we say the speed is 12 ms-1.
     This is the Average speed.   
      Please note that this is not the speed through out.
             To verify your answers highlight the cages.

      What does the graph show?
       1. Non linear graph shows changing of speed..  Not uniform.
       2.   The upward curve shows increasing speed. This is  acceleration.  

  Graph 3.

The graph is the case of an object traveling at the same speed.
 Do you know the speed?  
 Here the speed remains the same:- 
 Has the distance moved in every second the same?
    Fig. Speed Time gaph.



 What is the acceleration shown by the above graph?
        It is 0 ms-2                      .  
       
Yes !  It has moved 25 m  in every second.
As it has traveled  for  5 seconds  he would have gone 5 x 25  meters.

What does the graph show?
1. Uniform  speed.
2.the distance is the same as the 
      Area below the graph.

       
      

    

    What is the difference between SPEED  and VELOCITY.
    Example:-
     The moon speeds round the earth with almost the same speed. Once in about       28 days it completes an orbit..   Though the speed remains the same 
     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.

Fig. Speed time graph.

   
When there is acceleration the graph is inclined upward,

The steepness shows whether the acceleration us high or low.
Acceleration =  (Early velocity - final velocity) /  Time

This is a case of increasing speed in a regular way.


    The change in speed  =  25 - 10   = 15 m/s
     Iime taken to change  = 6 seconds
     Rate of change =  15 / 6 m/s  per second
     Acceleration     = 2. 5 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 starting from rest 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 starting from rest.  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?
        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.

    Method of working from Q, 8 - 10.
  8. Average speed =  distance / time.
                            = 100m /5s    = 20 ms-1

  9. When there is uniform acceleration  Maximum speed = 2x Average speed.
                                                        That is  "       "     = 2x 20  = 40  ms-1

  10. Acceleration = change in speed / time.
                          =  40 -0           ms-1            =  8 ms-2
                                        5    S




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