A spring stores 18 J energy when stretched 3 cm. Force constant k is:
A. 2 N/m
B. 200 N/m
C. 6 N/m
D. 40000 N/m
Answer: Option D
Solution (By JKSSB Mock Tests)
Elastic PE: U = ½kx² ⇒ k = 2U/x². x = 3 cm = 0.03 m. k = 2×18/(0.03)² = 36/0.0009 = 40000 N/m. Unit conversion critical: cm to m. Memory aid: 'k = 2U/x²; always use SI units'. Tests spring energy formula application with unit conversion, frequently appearing in competitive exams.
Explanation:
Total internal reflection (TIR) occurs only when light travels from denser medium to rarer medium (e.g., glass to air, water to air) and angle of incidence exceeds critical angle. Air to glass (A) causes refraction toward normal; water to glass (C) may or may not cause TIR depending on refractive indices; vacuum is rarer than any medium but TIR requires denser→rarer. Memory aid: 'TIR condition: denser→rarer AND i > θ_c'. This optics condition question is frequently tested in competitive exams. Always verify both conditions (medium order and angle) for TIR; competitive exams often test common misconceptions about direction of travel.
Explanation:
Tension is towards centre, displacement is tangential (velocity). Force perpendicular to displacement, cos90°=0, work zero. Centripetal force does no work, doesn't change speed.
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