A cyclist moving at 18 km/h stops pedalling and comes to rest after 10 m due to friction. Deceleration is MCQ with Answer and Explanation

Explanation:
72 km/h = 72 × (5/18) = 20 m/s. Initial velocity u = 20 m/s, final v = 0, t = 10 s. Using v = u + at => 0 = 20 + a×10 => a = -2 m/s², so deceleration magnitude is 2 m/s². Conversion factor 5/18 is essential: multiply km/h by 5/18 to get m/s.

Explanation:
PE = mgh = 2×10×10 = 200 J.

Explanation:
Carnot efficiency η = 1 - T₂/T₁, where T₁ is source temperature, T₂ sink temperature (in Kelvin). Here T₁=400 K, T₂=300 K, so η = 1 - 300/400 = 1 - 0.75 = 0.25 = 25%. This maximum possible efficiency for given temperatures is a fundamental thermodynamics result. Memory tip: 'η_Carnot = 1 - T_cold/T_hot'. Competitive exams frequently test this formula with varying temperatures. Always use absolute temperatures (Kelvin); Celsius would yield incorrect efficiency. This problem assesses understanding of heat engine limitations.