This weeks lab is on work and energy. Students measure the speed of a mass as it is pulled along a track by a falling mass. By comparing the kinetic energy of the system to the work done by gravity on the falling mass they should be able to verify the work energy theorem. An alternative interpretation is that by summing the kinetic energy and the gravitational potential energy of the falling mass they can show mechanical energy is conserved in this system.
There are two steps the students need to take to ensure their data has no significant influence from friction. First they need to tilt the track that the sliding mass sits on so that the mass slides without acceleration (i.e. tilt it down a bit to overcome friction). Secondly they need to ensure the string that connects the masses touches only the pulley, and not the mechanical stopper at the end of the track. If the string is routed improperly it will slide along the bottom of the mechanical stopper dissipating energy.
Since the measurement of the speed of the falling mass involves the timing of the photocells and measurement of the width of the flags which pass through them, both of which will need to be measured in future labs as well, this would be a good opportunity to go over how the photocells work and how to measure flag width using the vernier calipers. This slide that is also on transparency in the lab rooms that may be of assistance in doing this. There is also an excellent online tutorial through hyperphysics.