Work and Energy — Maharashtra (SSC) Class 9 Science Solutions (Free)
Free step-by-step Maharashtra (SSC) Class 9 Science solutions for "Work and Energy" — important questions with detailed answers, download PDF for board exam preparation.
TL;DR: Free step-by-step Maharashtra (SSC) Class 9 Science solutions for "Work and Energy" — important questions with detailed answers, download PDF for boar…
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Q1: Define work and write its SI unit. When is work said to be zero?
Work (W) = Force × Displacement × cos(θ), where θ is angle between F and s. SI unit: Joule (J). Work is zero when: (1) Force = 0, (2) Displacement = 0, or (3) F ⊥ s (θ = 90°). Example: Carrying a book horizontally—gravity does zero work.
Q2: A force of 50 N is applied on a body at 30° to the horizontal. Calculate work if displacement is 10 m.
Step 1: F = 50 N, s = 10 m, θ = 30°
Step 2: W = F × s × cos(θ) = 50 × 10 × cos(30°)
Step 3: W = 500 × (√3/2) = 500 × 0.866 = 433 J
Q3: Define kinetic energy and potential energy. Give one example each.
Kinetic energy (KE) = ½mv², energy due to motion. Example: A rolling ball. Potential energy (PE) = mgh, energy due to position/height. Example: A book on a shelf. Both measured in Joules and depend on mass.
Q4: A 5 kg object is raised to a height of 4 m. Calculate gravitational PE (g = 10 m/s²).
Step 1: m = 5 kg, h = 4 m, g = 10 m/s²
Step 2: PE = mgh = 5 × 10 × 4 = 200 J
Potential energy = 200 J
Q5: State the law of conservation of mechanical energy with an example.
In the absence of friction/air resistance, total mechanical energy (KE + PE) of a system remains constant. Example: A ball dropped from height h: At top, E = mgh (all PE); at bottom, E = ½mv² (all KE). Both equal if no energy loss.
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