Question

A body travels 200 cm in the first two seconds and 220 cm in the next four seconds. The velocity at the end of seventh second from start is

• A. 15 cm/s
• B. 10 cm/s
• C. 5 cm/s
• D. 20 cm/s

Answer 1

The correct option is B

10 cm/s

We have, $s_1$ = 200 cm; $s_2$ = 420 cm;
$t_1$ = 2s and $t_2$ = 6s.
Case I :
$s_1=u{t}_1+\frac{1}{2}a{t}_1^2$
$\Rightarrow 200=2u+\frac{1}{2}a\times 4$
$\Rightarrow$ u+a=100 …(1)
Case II :
$s_2=u{t}_2+\frac{1}{2}a{t}_2^2$
$\Rightarrow 420=u\times 6+\frac{1}{2}a\times 6^2$
$\Rightarrow$ 420=6u+18a
$\Rightarrow$ u+3a=70 …(2)
Solving (1) and (2), we get
$a=-15m/s^2$
and u = 115 cm/s.
Since, v = u + at
V = 115 - 15 $\times$ 7 = 10 cm/s.