Motion in a Straight Line

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Question : 23
Total: 27
A three-wheeler starts from rest, accelerates uniformly with 1 m s2on a straight road for 10 s, and then moves with uniform velocity. Plot the distance covered by the vehicle during the nth second (n = 1, 2, 3..) versus n. What do you expect this plot to be during accelerated motion : a straight line or a parabola?
Solution:  
Initial velocity, u=0,a=1ms2,t=10s , If Sn be the distance covered by the three-wheeler in nth second. Then
Sn=u+
a
2
(2n1)
,

Sn=
a
2
(2n1)
=
1
2
(2n1)

For n=1,
S1=
1
2
(2×11)
=0.5 m

n=2,
S2=
1
2
(2×21)
=1.5 m

n=3,
S3=
1
2
(2×31)
=2.5 m
and so on
The following is the table for the distance Snth travelled in nth second.
  n(s)  1  2  3  4  5  6  7  8  9  10
  Sn(m)  0.5  1.5  2.5  3.5  4.5  5.5   6.5  7.5  8.5  9.5
Also Dn= distance covered by the particle in 10 sec.
Dn=un+
1
2
a
n2

=0×10+ 12×1×102
(n=10)
=50 m.
As the equation describing the relation between Dn and n is of seconddegree, so the graph of the vehicle from start to 10thsecond is expectedto be a parabola. Velocity of the vehicle at the end of 10th second isv=0+1×10=10ms1 and it will move with this velocity after 10 seconds.Thus the graph will be a straight line inclined to time axis for uniformlyaccelerated motion. The plot of the distance covered by the three wheelerwith time is shown in the graph.
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