Photogate – Tech Tips with Vernier


Measure. Analyze. Learn. This is the Vernier Photogate. It’s used in motion studies, like for the physics book. The way it operates, is that is has an LED that sends a, it’s an infrared LED, and there’s a detector on the other side. And, if an object breaks the beam, it can measure how long the beam is broken. And so, there are a variety of different timing regimes that you can use with this. And that’s part of what we’re going to go over in this video. So, to use it, again, you’re going have to have the motion go through this gap in the device there. Another way to use this is to shine a laser into this hole, where there is a detector that can detect the light coming from the laser. This allows you to run a larger object through the photogate. And, we sell a laser and stand that you can use. And, you can shine a laser into here, and you’re measuring the time that the laser beam is broken. So, the distance goes from here to here, to here to wherever your laser is. And so, you could put a large object through that. You could walk through it if you wanted to. So, that’s what that’s about. If you’re using that one with the laser, then there is a slide here that covers one of the detectors. And so the, you want to make sure that if you’re using the laser part you have this covered up. One problem sometimes that people have when they’re using this, is that they don’t realize that this is here, and they may cover it up when they don’t intend to, when they’re trying to time something in here. So, you need to be a little careful about that. Now, I’m going to use this with the default collection mode to start with. And, that is the mode we call motion timing. And, it’s designed for me to be able to drop this object, this is the picket fence, and I’m going to drop it through the photogate. And, I could use it to measure the acceleration due to gravity. So, that’s where I’ll start. And, then we’ll go over some of the other timing regimes that we could use. This is a digital sensor, and so it needs to plug into one of the digital ports on the device. And so, I’m going to plug it in here at the top. And, digital channel one, it doesn’t really mater which one. And, right now it comes up here, and it’s using what’s called “Gate State”. So, it’s tells me whether or not the beam is broken or unbroken. So, right now it says that it is “Unblocked”. If I come here, I block it. If I unblock it, we end up with that. And so really what the device is, is a timer. And, it times how long it takes to block and unblock the beam. So, for this lab, I’m just going to hold the photogate up above the tabletop. And, I’ve put something here that’s a little bit soft, so that when the picket fence falls, it wont fall off the table there and break. And so, I’m ready to collect. And, I’m going to use the default mode for this. To begin collection, press our collect button. It’s going to scoot over to the distance and velocity graph. And, what’s happening is, is that it’s waiting for us to drop the picket fence through the photogate. And, when I drop it through the photogate, at that point it will actually collect the data. One of the things that’s important, is to use good technique when dropping the picket fence. I’m holding it towards the top middle there, so it wont fall out of the beam when it drops. And, I’m also holding it not too far above the photogate. So, it’s not going too fast when I drop it through. [Thud] So, we have our data on the screen. And, now we’re ready to do some analysis of the data. So, the upper graph is a position time graph. And, the bottom graph is the velocity graph. And we might be interested in looking at the slope of the velocity graph to determine the acceleration due to gravity. So, to do this, we could run a curve fit on our lower graph. So, I’m going to go to “Analyze”, and I’ll do “Curve Fit”, and I’ll do “Velocity”. Looks fairly linear to me, so I’m going to come up and say “Linear”. And, when I do that, I get negative 9.7 meters per second squared. Not to shabby for being able to do this. So, it’s very easy to do an experiment where you’re measuring the acceleration due to gravity using the picket fence for acceleration. So, in addition to the motion timing, there are other timing regimes that you might use. One would be the gate timing. And, what happens with gate timing: it times from when the beam is unblocked, to being unblocked again. And so, it just times that. So, it would time going through and being unblocked. One of the things that you can do there is, maybe give it a defined distance, and it could calculate and instantaneous velocity with that. So, that’s what we call gate timing. The other is pulse timing. And, pulse timing, it times from the time that it blocked, until the time that it is blocked again. And so, blocked, blocked again. And so, there are times that you might want to do that. In addition to having it just be one together, you could have multiples of these. And so, maybe you block it, and then when it goes through and blocks the next one, you might use that pulse timing. The next one is the pendulum timing. It’s great for studying, say, periodic motion. Maybe you have a pendulum. And, it times the period. And so it goes from here, to here. So, one two three four. And so, that’s what’s called pendulum timing. And another one is what we call gate and pulse. And, what happens with gate an pulse, is a combination of the gate and pulse. So, what it will do, is it will allow it to measure the instantaneous velocity as the object goes through here, so that’s a known distance. And then as it comes back through again, it will do it again. And, so it’s allowing us to do gate and pulse. And, it’s also timing from the time it broke it here, till the time it came back to there. So that would be the pulse part of that. So, the gate is this part coming through, and the pulse is from when it’s unblocked back to being blocked again. And so, that gives you that pulse timing. There are additional accessories that can be used with the photogate, in addition to the picket fence that we used. Such as, the Photogate Bracket. This allows you to attach the photogate to the Vernier Dynamics System. So, that’s one of the options there. Another accessory that you might use is the Pulley Bracket. This will allow you to attach the pulley. And, one of the pulleys that you might want to use would be the Ultra Pulley. And, the Ultra Pulley is slotted so that it can actually time the pulley as it moves around. In addition to that, we have the Laser Pointer. So, if you want to use the laser pointer to shine into that extra port for the detector, you can do that. And if you’re doing that, it is handy to actually have the stand that will hold the laser pointer. So, you can have the two things together. Then the Photogate Bar Tape Kit. Now this allows us to attach a tape, that can move along and you can set it where it can measure the tape as it slides through the bar there. And the idea is to use something like a the old ticker tape timer that you might be used to using. So, it allows you to use that mode with a photogate. Then, we also have the Cart Picket Fence. And so, it has a different spacing there. It makes it easier to measure the velocity of carts on the Vernier Dynamics System. For additional information about the photogate, and how to use it, we’ve produced this “Introduction to Vernier Photogate using LabQuest App”. It’s available from our website. And, it details some of the timing regimes that you can use. How you might use some of those accessories that we mentioned. And so, it’s definitely worth a download, because it will help you in setting up the experiments using the photogate device.

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Reader Comments

  1. tim nguyen

    I use this.  and we have 2 graphsthe top one is distance vs time graphthe lower one is velocity vs  time graph I use the distance vs time graph to find out the velocity by velocity/ timebut this velocity doe not match with the velocity in the velocity vs time graph( below)Anyone know, Please helpThanks

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