Investigate energy changes with a rolling object

Learning goal: Explain kinetic energy in terms of motion and gravitational potential energy in terms of relative position. Use a repeated ramp investigation to connect height with observed motion without pretending to measure energy from a picture.

An object in motion has kinetic energy. A rolling ball, flowing water and a rotating wheel are examples. For the same object, greater speed means greater kinetic energy. The lesson compares changes qualitatively; you do not need a formula that calculates energy. Record what moves and how you know its motion changed.

Gravitational potential energy belongs to the interaction of an object and Earth and depends on their relative position. For the same object near Earth's surface, lifting it higher above a chosen reference increases that gravitational potential energy. State the reference, such as the bottom of a ramp. The label “high” is incomplete unless the comparison position is clear.

There are many named forms of energy, but each can be placed in one of two categories. Kinetic energy is energy of motion, whether the motion is of a whole object or of the particles inside it. Potential energy is energy of relative position between interacting objects, such as a ball and Earth. Ohio's grade 6 model describes thermal energy as the total kinetic energy that comes from the random motion of a substance's atoms and molecules. That random particle motion has kinetic energy, and it is the part of thermal energy this grade studies. It is not the whole account: interactions between particles also contribute to a substance's thermal energy, as the changes-of-state lessons note, and later courses study that contribution. Sound involves particles of the material it travels through vibrating back and forth, and that particle motion has kinetic energy. (The squeezing and stretching of the material also stores some energy, which later grades study.) Gravitational potential energy is the potential energy investigated at this grade.

A ball held at the top of a ramp has gravitational potential energy relative to the lower position, even though it is not yet rolling. As it rolls downward after release, its height decreases and its motion increases. Gravitational potential energy is transformed into kinetic energy. The ball can have both kinds at an intermediate position: it is already moving and is still above the reference.

The diagram shows three positions of one ball at different times. They are not three balls colliding on one track. Position A is held before release, B is moving partway down and C is moving near the bottom. The drawing is schematic and does not provide energy quantities. Longer motion arrows suggest a qualitative speed comparison under the modeled conditions; their lengths are not measured meters per second.

Avoid defining potential energy simply as energy that is stored and kinetic energy as energy that is used up. A moving object can continue to have kinetic energy without immediately giving it to another object: a rocket coasting at constant speed through empty space keeps its kinetic energy and gives none of it away, so “stored” would describe it too. The relevant distinction is motion versus relative position in the examples studied here. Energy is not a material that leaks out whenever a ball moves.

In a real ramp system, friction and other interactions matter. Some energy is transferred into thermal motion in the ball, track and surroundings; sound can also be produced. A ball that rolls onto a rough surface and stops has not made energy disappear. A complete numerical accounting is beyond this investigation, but the explanation should recognize changes beyond the visible forward motion.

Thermal motion and sound provide further connections. Even a track that looks stationary contains particles in random motion. A vibrating object can set nearby material into back-and-forth motion associated with sound. Do not confuse the speed of the whole rolling object with the random microscopic motion inside its material. Both involve motion, but at different scales.

To investigate release height, use the same ball, track and release method across trials. Measure vertical height above the same reference. Release without a push. One method measures the time to cross a marked short level section after the ramp; with the same measured distance, a shorter time indicates greater average speed over that section. Another method compares travel distance on the same stopping surface, while recognizing that stopping distance depends on surface interactions too.

A fictional class records level-section times for a ten-centimeter release as 0.82, 0.79 and 0.84 seconds, and for a twenty-centimeter release as 0.61, 0.64 and 0.60 seconds. For the same section length, the second set indicates faster passage. This supports the proposed height–motion relationship for that setup, but it does not directly measure joules or establish an exact proportional rule.

A fair explanation names conditions and limitations. Changing the ball and the release height together would make the comparison harder to interpret. Hand timing over a very short interval can introduce large uncertainty; slow-motion recording with a visible distance scale can improve the observation if available. Preserve all trials and describe inconsistent results rather than reporting only the fastest one.

Worked energy account: At the held starting position, the ball has no rolling kinetic energy but has gravitational potential energy relative to the bottom. Halfway down it has both. Near the bottom it has less gravitational potential energy relative to that reference and more kinetic energy than at release, with real interactions also transferring energy to the surroundings.

Worked evidence critique: “Twice the release height gave exactly twice the speed” is not established by the six fictional times. The observations support a faster crossing for the higher release; neither the diagram nor these data justify that exact speed rule.

Actual investigation: An adult helps secure a low ramp, provides a catch tray and marks a clear observation zone. Keep the ball on the controlled track and away from faces and walkways. Choose two or three safe release heights. Repeat at least three times at each height, using the same release method and measurement section. Record conditions, distance and times.

Design performance: Set a practical target such as reaching a marked zone without leaving the catch tray. Adjust one design feature, predict its effect and retest. Submit a before-and-after diagram, raw observations and an explanation of gravitational and kinetic energy changes. Explain why the design's success is evidence about this device rather than a measurement of every energy transfer. Because calculating energy from measurements is beyond this grade, an adult-selected simulation such as the linked Energy Skate Park: Basics can display energy bars for a model skater; compare its kinetic and gravitational potential energy displays with your ramp explanation.