Q14 - The potential energy-displacement graph of

Last updated at September 16, 2026 by

  • šŸ“‹ Revise, Reflect, Refine, Q14 (Page 138)
    The potential energy-displacement graph of a 0.5 kg ball moving along a frictionless track is shown. At O, the velocity of the ball is 0 m sā»Ā¹ and potential energy is 30 J. Calculate the velocity of the ball at P, Q and R. (From the graph ā€” PE at P = 20 J, at Q = 30 J, at R = 40 J.)
    GIF Watch: why does the pendulum eventually stop in real life Animation for "Why does the pendulum eventually stop in real life". A short looping GIF, three or four beats, drawn in the flat classroom style of the chapter. Beat 1 ā€” In real life, the pendulum slows down and eventually stops. Beat 2 ā€” This is because of energy loss due to friction at the support and air resistance. Beat 3 ā€” The mechanical energy slowly converts into heat and sound. Label every arrow and quantity, name the direction each force or motion acts in, and hold the last frame for a moment before the loop starts again.
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    Answer
    • At O ā€” velocity = 0, so kinetic energy = 0.
    • Total mechanical energy = PE + KE = 30 + 0 = 30 J
    • The track is frictionless ā€” mechanical energy stays 30 J everywhere.
    Position Potential energy Kinetic energy = 30 āˆ’ PE Velocity (from Ā½ Ɨ 0.5 Ɨ vĀ² = KE)
    P 20 J 10 J āˆš40 ā‰ˆ 6.3 m sā»Ā¹
    Q 30 J 0 J 0 m sā»Ā¹
    R 40 J āˆ’10 J ā€” impossible! Ball can never reach R
    • Kinetic energy can never be negative ā€” so the ball does not have enough mechanical energy to reach R.
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