Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds

Electrical measurement of velocity of a large trolley motion


This experiment illustrates a fundamental point about the nature of measurement, as well as providing a way of measuring the speed of a trolley.

Apparatus and materials

Demonstration trolley

Meter attachment, including wheel contact, small DC dynamo, and millivoltmeter

Health & Safety and Technical notes

Clearly there are dangers of collisions. The activity should be done in a reasonably large, clear space, on a level floor or surface.

The meter attachment is a special device, recommended not for general use but for the learning involved in setting it up and calibrating it. 

Demonstration trolley


a One wheel of the trolley drives a small DC dynamo. Connect the dynamo to a millivoltmeter which will show a reading that depends on velocity. 

b Calibrate the system. Move the trolley at constant velocity, as closely as you can. Drop cards at 1-second intervals and use the distances between them to calculate the velocity of the trolley. Do this at several speeds. Match these measured velocities with the readings on the millivoltmeter. 

Teaching notes

1 Once calibrated, you can now use the system for further investigations on velocity and hence also on acceleration. Measurements taken from the meter at 1-second intervals can, for example, substitute for the cards used in the previous experiment. 

2 Measuring acceleration: Instead of connecting directly to the millivoltmeter, you can feed the output to the primary coil of a small transformer; the secondary of the transformer is connected to the millivoltmeter. The voltage across the secondary coil will be roughly proportional to the rate-of-change of the primary current, so that the meter gives a measure of acceleration. Once again, students could test whether the meter measures this quantity and can attempt a rough calibration. 
3 Since this activity is linked with work on acceleration, we refer to velocity rather than speed. It would, however, be correct to say that the meter measures speed. 
This experiment was safety-checked in December 2004