MCQ
A ball after falling from a height of $10\,\,m$ strikes the roof of a lift which is descending down with a velocity of $1\,\,m/s$ . The recoil velocity of the ball will be ............. $\mathrm{m}/ \mathrm{s}$
- A$8$
- B$11$
- ✓$12$
- D$15$
✓
Answer
Correct option: C.
$12$
c
Speed of ball just before striking with lift is-
Speed of ball just before striking with lift is-
$\mathrm{u}_{1}=\sqrt{2 \mathrm{g}} \mathrm{h}=\sqrt{2 \times 9.8 \times 10}=14 \mathrm{m} / \mathrm{s}$
$\mathrm{As} \mathrm{M}_{\mathrm{lift}}>>>\mathrm{M}_{\mathrm{ball}}$
velocity of ball after collision with lift is
$\mathrm{v}_{1}=-\mathrm{u}_{1}+2 \mathrm{u}_{2}=-(-14)+2(-1)=12 \mathrm{m} / \mathrm{s}$
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