A ball is thrown from ground at an angle with horizontal and with… - Vidyadip

MCQ

A ball is thrown from ground at an angle $\theta$ with horizontal and with an initial speed $u_0$. For the resulting projectile motion, the magnitude of average velocity of the ball up to the point when it hits the ground for the first time is $V _1$. After hitting the ground, ball rebounds at the same angle $\theta$ but with a reduced speed of $u_0 / \alpha$. Its motion continues for a long time as shown in figure. If the magnitude of average velocity of the ball for entire duration of motion is $0.8 V _1$, the value of $\alpha$ is. . . . . .

A
$2$
B
$3$
✓
$4$
D
$5$

✓

Answer

Correct option: C.

$4$

c
Average velocity $=\frac{\text { Total displacement }}{\text { Total time }}$

$\text { Total time taken }=t_1+t_2+t_3+\ldots . . . . . . . .$

$=t_1+\frac{t_1}{\alpha}+\frac{t_1}{\alpha^2}+\ldots \ldots . .$

$\text { Total time }=\frac{t_1}{1-\frac{1}{\alpha}}$

$\text { Total displacement }=v_1 t_1+v_2 t_2+\ldots \ldots . . .$

$=v_1 t_1+\frac{v_1}{\alpha} \cdot \frac{t_1}{\alpha}+\ldots \ldots . .$

$=\frac{v_1 t_1}{1-\frac{1}{\alpha^2}}$

On solving

$< v \rangle=\frac{ v _1 \alpha}{\alpha+1}=0.8 v _1$

$\alpha=4.00$

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