light gates and switches

These are digital switches that record a time when triggered by say, a dynamics trolley or a card. When you enter parameters like distance or mass into the software, velocity, acceleration or momentum are instantly calculated. This and the accuracy and precision of the timing offer huge potential for physics. Measuring ‘g’ becomes a two-minute experiment from start to finish. Measuring reaction time and karate chops are neat ways of introducing pupils to them. 

You can find various designs of ‘gate’ – there are U shapes with large or small gaps between their infra-red emitter and detector. Some have a gap of a metre or so and can time a person. Some are not gates as such, but a single sensor – probably a photo transistor which responds to a change in light intensity – you really need to clamp them still and illuminate them across a gap where the trolley might pass. These can be much cheaper than a gate, but more of a fiddle to set up and get working – in a permanent set up they would be fine. Look out for switches that find special uses. A push switch can be used to initiate timing manually; a peg switch is used to start timing when you drop a card. A pressure mat switch can be placed on the floor to time people cycling over it, or measure how long they can jump and stay in the air. Rest assured light gates and switches have their uses, but do compare them with sonar distance sensors or dynamics pulleys because these give a more analogue picture of motion. 

I’ve rated these sensors but your assessment will be different. I give a score, out of five, for the sensor’s intrigue, interest, and learning potential. I also give a score for how often a science department might use it.