Moving an object across the table

Demonstration

To move along a horizontal plane there is no net transfer of energy from potential energy to kinetic (motion) energy.

Apparatus and materials

Runway

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

G-clamp

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.

Procedure

a Compensate the runway for friction. (See the experiment Compensating for friction).

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.

b 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.

Teaching notes

1 It is essential to arrange this so that energy losses are 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 loss on hitting the platform is small.

2 Living in a world where there is plenty of friction, students are likely to think that energy must always be supplied to move an object from one place to another at the same horizontal level. Demonstrations with CO2 pucks on a glass plate refute that, as the contact is almost friction-free.

3 Other experiments should support the story of no energy being needed to move across a table. Students can accept that the gravitational potential energy of a brick at one end of the table is the same as its gravitational potential energy 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 it has gained neither kinetic energy nor gravitational potential energy. Yet they will feel uneasy if you say the journey from one position to the other requires no energy.

In fact, unless you are prepared to allow infinite time for it, the journey does require some energy, temporarily, on loan from some store. To get across, in reasonable time, the brick must move quite fast and it must have some kinetic energy. So to transport it some kinetic energy must be gained from somewhere; but at the end of the trip that energy can be given back.

4 Ideally the load would be hauled up just as far at the end as it fell at the beginning. This would demonstrate that the initial gravitational potential energy is transferred to kinetic energy, and that is transferred back to gravitational energy at the end. Unfortunately there is not only friction but an unavoidable inelastic impact when the thread pulls taut. Also, unless the mass of the falling load is a very small fraction of the mass of the trolley, the load itself will gain energy which will warm up the platform when it hits it. Altogether there will be a much smaller rise than the original fall of the load.

5 If the efficiency of energy transfer from potential energy to kinetic energy and back again is as low as 60 per cent, the experiment is not worth doing.

This experiment was safety-checked in November 2005