Moving an object across the table
Practical Activity for 14-16
To move along a horizontal plane there is no net change to the way energy is stored.
Apparatus and Materials
- Dynamics trolley
- Pulley, single, on clamp
- Bosses, 8
- Retort rods, 3
- Retort stands, 2
- Hanger, 10 g mass
- Platform, wooden, approx 5 cm x 25 cm
Health & Safety and Technical Notes
Long runways or heavy shorter ones should be handled by two persons. Ensure that a string is tied across the bottom of the runway, to prevent the trolley falling onto anyone. At least one retort stand should be clamped to the bench.
- Compensate the runway for friction. See the experiment:
- Half-way along the runway, place a retort stand (preferably 100 cm high) on each side. Connect a single rod between the tops of the retort stands. Suspend a pulley on a clamp at the centre, so that the pulley runs freely, with the plane of the pulley parallel to the runway. Fix a simple wooden platform between the retort stands, about 30 cm below the pulley.
- Lower down the retort stands, fix two rods parallel to each other and close together across the runway as illustrated. These must be at such a height that a trolley moving along the runway will just clear them. At least one of the retort stands should be clamped firmly to the bench with a G-clamp.
- Join a length of thread to the 10 g mass hanger on the wooden platform and over the pulley, down between the double rods, and fasten it to the trolley. The thread should be just long enough for the trolley to be about 30 cm from the parallel rods.
- Pull the trolley back so that the weight hanger rises to the pulley. Release the trolley. It is pulled by the descending mass for the first 30 cm of its travel and then proceeds at constant velocity. Towards the end of its run, the thread tightens and the mass is hauled up again. Ideally, it should rise to its original position.
- It is essential to arrange this so that energy dissipated is reduced to a minimum. The mass of the load should be quite small by comparison with the mass of the trolley, so that the energy dissipated when the load hits the platform is small.
- Living in a world where there is plenty of friction, students are likely to think that work must always be done to move an object from one place to another at the same horizontal level. A force is needed to keep an object moving with a steady speed to overcome the frictional force. Demonstrations with CO2 pucks on a glass plate refute that, as the contact is almost friction-free.
- Other experiments should support the story of no work being needed to move across a table. Students can accept that the energy of a brick stored gravitationally at one end of the table is the same as its energy of a brick stored gravitationally near the other end. They will agree that, if the brick is at rest in the first position and again at rest in the second position there is no energy in the stored kinetically, and that the energy stored gravitationally has not changed. Yet they will feel uneasy if you say that there has been no change to the way or ways that energy is stored due to the journey from one position to the other.
- In fact, unless you are prepared to allow infinite time for it, the journey does require the shifting of some energy, temporarily. To get across, in reasonable time, the brick must move quite fast and energy must be stored kinetically in the brick. So to transport it so energy has been shifted from somewhere; but at the end of the trip that energy can be shifted back.
- Ideally the load would be hauled up just as far at the end as it fell at the beginning. This would demonstrate that the same initial energy stored gravitationally is transferred to be stored kinetically, and then is stored gravitationally at the end. Unfortunately there is not only friction but an unavoidable inelastic collision when the thread pulls taut. Also, unless the mass of the falling load is a very small fraction of the mass of the trolley, energy is stored kinetically in the load itself, which will be stored thermally in the platform when it hits it. The platform heats up. Altogether there will be a much smaller rise than the original fall of the load.
- If the load rises to a height that is as low as 60 per cent of the original height, the experiment is not worth doing.
This experiment was safety-tested in November 2005