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Motion in A Straight Line question

2019 · 10 Jan · Shift 2 · Q50
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  5. /2019 · 10 Jan · Shift 2 · Q50

Motion in A Straight Line question

2019 · 10 Jan · Shift 2 · Q50

JEE MainPhysicsMotion in A Straight LineMCQ+4 / −1
A particle starts from the origin at time t = 0 and moves along the positive x-axis. The graph of velocity with respect to time is shown in figure. What is the position of the particle at time t = 5s ? JEE Main 2019 (Online) 10th January Evening Slot Physics - Motion in a Straight Line Question 92 English
  1. A
    3 m
  2. B
    9 m
  3. C
    10 m
  4. D
    6 m
View written solutionFree

Correct answer: B

  1. Use area under the vvv–ttt graph

The position (displacement from origin) at time t=5 st=5\text{ s}t=5 s is the area under the velocity–time graph from t=0t=0t=0 to t=5t=5t=5:

x(5)=∫05v dtx(5)=\int_0^5 v\,dtx(5)=∫05​vdt

  1. Read the graph in parts

From the given graph, the motion is piecewise:

  • From t=0t=0t=0 to t=2 st=2\text{ s}t=2 s: velocity increases linearly from 000 to 4 m/s4\text{ m/s}4 m/s.
  • From t=2t=2t=2 to t=3 st=3\text{ s}t=3 s: velocity is constant at 4 m/s4\text{ m/s}4 m/s.
  • From t=3t=3t=3 to t=5 st=5\text{ s}t=5 s: velocity decreases linearly from 4 m/s4\text{ m/s}4 m/s to 000.

So the displacement is the sum of areas:

  • left triangle,
  • rectangle,
  • right triangle.
  1. Calculate each area

(i) Triangle from 000 to 222 s

A1=12×2×4=4 mA_1=\frac12 \times 2 \times 4=4\text{ m}A1​=21​×2×4=4 m

(ii) Rectangle from 222 to 333 s

A2=1×4=4 mA_2=1 \times 4=4\text{ m}A2​=1×4=4 m

(iii) Triangle from 333 to 555 s

A3=12×2×4=4 mA_3=\frac12 \times 2 \times 4=4\text{ m}A3​=21​×2×4=4 m

  1. Total displacement

x(5)=A1+A2+A3=4+4+4=12 mx(5)=A_1+A_2+A_3=4+4+4=12\text{ m}x(5)=A1​+A2​+A3​=4+4+4=12 m

  1. Compare with options

The computed position is

12 m\boxed{12\text{ m}}12 m​

This does not match any listed option. Hence the stored answer B: 9 m\text{B: }9\text{ m}B: 9 m appears inconsistent with the graph interpretation above.

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