Potential Energy Curves, Turning Points and Stability · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Reading motion off an energy hill

Science · Energy · ages 18-20
Name ______________________   Date ____________

  1. Regions where the curve lies above the total energy line are forbidden.

    Circle one:   True   False

  2. On an energy diagram, what is the gap between the total energy line and the curve?

    • The kinetic energy at that position
    • The potential energy stored at that position
    • The total energy the body started with

  3. What is a turning point on a potential energy curve?

    • Where the body moves fastest, at the bottom of a valley
    • Where the energy line meets the curve and the body turns
    • Where the curve is highest, deep inside the forbidden region

  4. At some point the curve slopes steeply uphill. Which way does the force point?

    • Forward uphill, with the slope
    • Nowhere, since force is zero on slopes
    • Back downhill, against the slope

  5. A valley and a peak both have zero slope. Which is the stable equilibrium?

    • The peak, since higher means more stored energy
    • The valley, since nudges roll back
    • Both, since each has zero slope

  6. A ball tossed upward slows to a stop at its top and falls back. What is the top in energy language?

    • A stable equilibrium, since the ball stops there
    • A turning point, where the line meets the curve
    • A forbidden region the ball cannot enter

  7. Total energy is 10 joules and potential is 6 joules at some spot. How many joules of kinetic energy are there?

    Answer: ______________

  8. A student points at a spot where the curve is above the energy line and says the body moves fastest there. What is the error?

    • The body moves fastest exactly on the energy line
    • Speed is the same at every point on the curve
    • The body cannot be there at all, since that region is forbidden
LightMySky · lightmysky.comW1-mt_ZgRbrDaQzh-s1

Answer key

For grown-ups. Fold this page away before handing over the rest.

Reading motion off an energy hill W1-mt_ZgRbrDaQzh-s1

  1. True · Motion there would need negative kinetic energy, which is impossible.
  2. The kinetic energy at that position · Total energy splits into motion plus stored, so the gap left over is motion energy.
  3. Where the energy line meets the curve and the body turns · At a meeting point the kinetic gap is zero, so the body stops and reverses.
  4. Back downhill, against the slope · Force is minus the slope, so an uphill slope pushes back downhill.
  5. The valley, since nudges roll back · A nudge in a valley rolls back toward rest, while a nudge off a peak runs away.
  6. A turning point, where the line meets the curve · Speed hits zero exactly where the potential curve reaches the total energy.
  7. 4 · The kinetic gap is total minus potential, so 10 minus 6 gives 4 joules.
  8. The body cannot be there at all, since that region is forbidden · Above the line means negative kinetic gap, which no real motion can have.
Worksheet · LightMySky