Measuring speed in the laboratory

A human thumb on a stopwatch reacts late, by about a fifth of a second. That is why sprint world records only count when a machine did the timing. Now picture a trolley rolling down a school ramp in about half a second. A fifth of a second of thumb is a huge share of that, and the thumb is the piece of kit you trusted most.

Video six of six in Units and describing motion, for combined science or triple physics, Foundation or Higher. If the equation here feels unfamiliar, go back to Speed, typical speeds and s equals v t first.

Every speed measurement needs exactly two readings, a distance and a time. Name those two first, and only then choose the equipment. The simplest kit is a metre rule for the distance and a stopwatch for the time. Mark a start line and a finish line, and measure the gap between them. The weak point is the thumb. Someone starts the clock as the trolley crosses the first line and stops it at the second. Each press comes late, and never by the same amount twice, so the reading is not the real time. A light gate takes the thumb out. It is a beam of light across a gap, wired to a timer, and a card fixed on top of the trolley breaks the beam as it passes through. The timer starts the instant the card's front edge cuts the beam. It stops the instant the card's back edge clears it. While the gate was timing, how far did the trolley go: the ramp's length, or the card's? The card's length. The timer only ran while the card was passing through, so that is the distance that goes with the time. The ramp is scenery. However long it is, the gate never measured it.

A trolley carries a five point nought centimetre card through a light gate, and the gate reads nought point nought four nought seconds. The CONVERT line comes before anything else. Card length: five point nought centimetres, arrow, nought point nought five metres. The time is already in seconds. Now the three-line stack. Line one: s equals v t. Line two: divide both sides by t, so v equals s over t. Line three: v equals nought point nought five metres divided by nought point nought four seconds. That is one point two five metres per second. One run can be a fluke, so you repeat it and take a mean. Three runs give nought point nought four two, nought point nought three nine, and nought point nought four one seconds. Add them, and the total is nought point one two two seconds. Divide by three for the mean, which is about nought point nought four nought seven seconds. Keep the full value on your calculator, and round only at the end. The speed is nought point nought five metres divided by nought point nought four nought seven seconds. That is one point two metres per second, to two significant figures.

The same trolley run is timed two ways. A stopwatch reads nought point six two seconds, and the light gate reads nought point nought four one seconds. Why can the stopwatch time and the gate time not be compared? They are timing different distances. The stopwatch covered the whole run between two lines, while the gate covered only the card's own length. For an object this fast, trust the light gate. Half a second of motion is too short for a thumb that reacts a fifth of a second late.

Method questions are marked on the answer form, so here is the form. Every line has two halves: the measurement on the left, and the instrument on the right. Take a toy car along a bench. Line one: measure the distance between two marked lines, using a metre rule. Line two: time the car from the first line to the second, using a stopwatch. Line three: repeat the run three times with the same stopwatch, and use the mean time. Then read the method back and ask one thing. Does it give a distance and a time, in units the equation can take? This one does, so the method works. Now, which method line is better: time the car, or time the car with a stopwatch? Time the car with a stopwatch. It names the instrument, and a method line needs both halves. One examiner's report on a Higher paper gives this instruction. With all practical method questions students should always state the equipment and describe what they are measuring and with which piece of equipment. The same report adds this. Only about ten per cent of students described a method that would lead to a valid outcome. In plain words, each measurement needs its instrument beside it, and the whole method has to end in a speed. The fix is the two-column sheet, read back at the end. Your handle for this video: the two-column method sheet, measurement on the left, instrument on the right.

Three to test yourself with, all on kit and numbers this video has not used. A ten centimetre card blocks a gate for nought point two five seconds. Speed? Nought point four metres per second. Ten centimetres is nought point one metres, and nought point one divided by nought point two five is nought point four. Another one. A ball rolls across a floor: name both readings, and an instrument for each. The distance, with a metre rule or a tape measure. The time, with a stopwatch, or with light gates. Now the last one. A method times the car, then finds its speed. What is missing? A distance. With nothing measured along the bench, and no instrument named for it, the method cannot produce a speed. And the thumb on the stopwatch: it is the reason sprint records need a machine, and the reason a light gate earns its place in your method. Every speed you measure this way sits on the scalar side of the two-by-two picture from the Velocity video, until you give it a direction.

When you have got this down, thumb it up, and your unthumbed videos in this chapter become a short, honest revision list. If it has not landed, save the playlist and come back later in the week; method questions often land on a second viewing.

That completes Units and describing motion. Next chapter: Motion graphs and acceleration.

For more, visit scholafly.com, or watch the next video.