Q91 / 180 · NEET 2018 · Physics

A toy car with charge q moves on a frictionless horizontal plane surface under the influence of a uniform electric field E→. Due to the force  qE→, its velocity increases from 0 to 6 m s-1 in one second duration. At that instant the direction of the field is reversed. The car continues to move for two more seconds under the influence of this field. The average  velocity and the average speed of the toy car between 0 to 3 seconds are respectively

  • A
    1 m s-1, 3.5 m s-1
  • Correct answer
    B
    1 m s-1, 3 m s-1
  • C
    2 m s-1, 4 m s-1
  • D
    1.5 m s-1, 3 m s-1

Solution

0<t≤1 s  velocity increases from 0 to 6 m/s

1 s≤t≤2 s : velocity decreases from  6 m/s to  0 m/s

But car continues to move forward and then it reverses its direction at  t=2 s and after 2 s accelerates backward

Figure

Distance undefined

| 1 2 ×6×2 |+| 1 2 ×( −6 )×1 |

=6+3

=9  m

Displacement = Total area under v−t graph

=6−3=3 m

∴ Average velocity =DisplacementTime=33=1m/s

∴ Average speed =Total distanceTime=93=3m/s

Hence, the correct answer is option: 'B'.

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