Classroom · Physics
Everyone times the same swing. Do they agree?
Every student times the same pendulum, swinging on the big screen, with the stopwatch on their phone: three trials over a single cycle, then three over twenty. The projector sets each trial's periods side by side and against the pendulum's known period, so the room sees what the longer count did to the spread, and what it cannot prove.
- Up to 600 people
A practice run with fictional students. No real classroom or join code is created.
Try an answer on the phone. Lead the lesson from the screen.
On their phone
You are presenting
Setting up the room…
Lesson deck
Teach it as a ready-made lesson
Hook, how it works, the live activity and takeaways. Edit anything.
Can you trust a stopwatch?
A ready-to-teach lesson deck: a hook, how it works, the live activity and takeaways for the debrief.
6 slides · 1 activity
What it teaches
Students turn a raw stopwatch reading into a period by dividing by the cycles they counted, see how much a room's readings of one event vary, and compare two timing methods trial by trial. They learn what a tighter spread shows, and that it cannot rule out an error the whole room shares.
- Measurement and uncertainty
- Repeated measurement
Debrief, while the boards are still up
- 01
What did you predict before any numbers went up, and what did the first look show?
- 02
Why divide by the cycles you actually counted, not the planned number?
- 03
In each trial, did the twenty-cycle spread come out narrower, similar or wider? What explains a trial that disagreed?
- 04
If everyone started their stopwatch late in the same way, would the spread show it? What would?
- 05
Which missing or off-count readings would you keep, and why?
How it works
- 01
Add it to your library
Sign up free and pull the activity from Decka's built-in classroom catalog — one click copies it into your own library, ready to run from Decka or inside PowerPoint.
- 02
Students join by QR
Everyone scans from the projector and plays in their phone's browser. No app, no account, nothing to install.
- 03
You run the room
You move the activity phase by phase: collection closes, the boards resolve on the big screen, and the debrief happens while the results are still up.
A pendulum lab built on the room's own readings
The big screen draws the pendulum everyone times, and each student's phone is their stopwatch. A real pendulum works too: switch the screen pendulum off in the lesson settings and set one up, safely secured, where everyone can see it. Start before anyone times a swing: will everyone record the same time? Agree what one complete cycle is, out and back to the same turning point; the screen counts down and releases its pendulum once, and it swings from then on. Then run three trials timing a single cycle. Each student taps the phone at a turning point to start and again when the bob is back there to stop, then confirms the cycles they actually counted. A student can also type a reading from a stopwatch of their own, or record that they have no reading and why. After a first look at each trial, change the method: the same pendulum and start–stop convention, timed over twenty cycles, three more times, with the start tap counted as zero.
Decka turns each reading into a period and compares the two methods one trial at a time, using only the students with a reading at the planned count in both. Missing readings stay missing, and a reading timed over a different count is kept as entered and counted as off-protocol rather than plotted. The projector shows individual periods and paired lines without names, with the missing and off-protocol totals; raw stopwatch text and cycle counts stay on each phone. With the screen pendulum, each comparison also draws the period the pendulum is known to have, so the room sees where its readings sat against it.
The debrief asks what a tighter spread actually shows. It can come out narrower, similar or wider, and even a tight cluster does not rule out an error the whole room shares: only a known true period can show one, and the screen pendulum has one. To try numbers before class, use Decka's free paired period calculator.
FAQ
Pendulum lab questions, answered
What do I need besides phones?
A projector or big screen, which draws the pendulum everyone times. A real pendulum is optional: to time one, safely secured where everyone can see it, switch the screen pendulum off in the lesson settings. Each timing card is a stopwatch built into the student's phone: one tap anywhere on the phone's screen starts it and one tap stops it. Each phone times its own two taps, so network speed never touches a reading. A student timing with a stopwatch of their own types the reading instead, as a decimal number of seconds or in minutes and seconds: 40.12 or 00:40.12, and past a minute 65.20 or 1:05.20.
Will twenty cycles always agree more closely?
Nothing guarantees it. Dividing by more cycles shrinks each person's start–stop error per period, but a timing bias the whole room shares does not shrink, and a miscount adds error, so the spread may narrow, stay similar or widen.
What happens to a missed trial or a miscount?
A student with no reading chooses “No reading recorded” and a reason, and the reading stays missing; nothing is filled in. A reading timed over a different number of cycles than planned is kept as entered, counted as off-protocol and left out of the plotted periods. The projector shows both totals.
Can students see each other's readings?
The room sees individual periods and paired lines without names, and someone may recognise a reading in a small group. Raw stopwatch text and actual cycle counts stay on each student's own phone.
Can I change the counts?
Yes. The planned cycle counts, the stopwatch unit each timing card starts on, the allowed reading range and whether students tap only to start and stop or at every cycle are lesson settings. So are the screen pendulum's length and whether the class times it or a real one.
How long does it take?
No step has a timer: you advance each one yourself, so the lesson runs at the pace of your six trials and the discussion between them. The whole run closes itself after an hour.
Is there a tool to try numbers first?
Yes. Decka's free paired period calculator normalises elapsed times by cycle count and compares the spread of paired readings, so you can rehearse the arithmetic before class.
Do students need an app or an account?
No. Students scan a QR code from the projector (or open a link) and play in their phone's browser. Nothing to install and nothing to sign up for.
Does it work in PowerPoint or Google Slides?
In PowerPoint, yes: open the Decka add-in on a slide and choose the Classroom tile. Google Slides is not supported yet; there you present from Decka itself. Students play from their phones either way.
How many students can play at once?
Up to 50 participants per live activity on the free plan, 150 on Pro, and 300 on Badass.
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