IIT-JEE 2010 · Paper 2, Physics question 47

Convex mirror, $f=+10$ m. From $\frac1u=\frac1f-\frac1v$:

• $v=\frac{25}3$: $\frac1u=\frac1{10}-\frac3{25}=-\frac1{50}$, $u=-50$ m.
• $v=\frac{50}7$: $\frac1u=\frac1{10}-\frac7{50}=-\frac1{25}$, $u=-25$ m.

Speed. The object moves 25 m in 30 s: $\frac56$ m s⁻¹ $=3$ km h⁻¹.

Answer: 3