To make an hourglass you will need to make sure the sand is dry. The bottles also need to be dry and light. Make sure the mouths of each pair of bottles are the same size. Place the materials for the hourglass (bottles, pencils, pieces of card, scissors, sticky tape, and sand) ready at the front of the classroom.
Activity 1: How long does a minute last?
Ask the question: What is time? Can you tell what time it is without agreeing on some things beforehand?
Organise the students into pairs. Explain that they are going to see how long 1 minute takes. Give student 1 a stopwatch. Student 1 gives a signal when he/she starts the stopwatch. When student 2 thinks that 1 minute has passed, he/she gives a signal and student 1 stops the stopwatch. How much time does the stopwatch show?
Explain that they are going to repeat the experiment. Now they have to think of a way to help them make a more accurate estimate of when 1 minute has passed. They could count, or draw lines. Encourage the students to try out their suggestions; then discuss how successful the experiment was. Were they able to make a better estimate the second time? What did they use to help them?
The students complete Task 1 on the worksheet.
Ask why they think it is important to know how much time has passed. And why is it important to make agreements about time? Come to the conclusion that it is important that everyone uses the same definition when they refer to time. This makes sure we arrive on time for an appointment, for example.
Ask the question: ‘What ways of measuring time do you know?’ If necessary, add the following to their ideas: stopwatch, looking at the position of the sun, or using an hourglass (egg-timer).
Activity 2: Make an hourglass
Explain that an hourglass works by gravity pulling on the sand so that it falls downwards. Gravity pulls objects towards the centre of the Earth. This happens at a constant speed when we consider a small distance like in an hourglass, so an hourglass is a reliable way to measure time.
Good to know. Long ago sailors used an hourglass to ring the ship's bell every hour and half hour. Doctors used a 15-second hourglass to measure their patients' pulse.
Organise the students into pairs. The students examine the materials you have prepared at the front of the classroom and decide how they will use these to make their hourglass. Give each pair of students a pencil. Assist them by asking what an hourglass looks like. What is inside it? And how can this ‘flow’? The students complete Task 2 on the worksheet, up to step 5.
An example of how you could make an hourglass: cut out a cardboard circle that fits between the mouths of the two bottles. Make a small hole in the centre of the card. Pour some sand in one of the bottles. Place the card on the mouth of the bottle, and place the second bottle upside down on the first. Stick them together at the middle using sticky tape. Make sure that the bottles used for the hourglass are light to make a stable assembly.
Look at the drawings showing the students' ideas. Do they look like they will work? Ask the students how much sand they will use, how large will they make the hole in the card, and how will they fix the bottles together. Note: the larger the hole, the less time it takes for all the sand to run through it. The more sand there is in the bottle, the more time it takes to all pass through the hole. This experiment will only work if the bottles and the sand are perfectly dry.
Once the students have created a good picture of their design, they can make their hourglass.
The students test their hourglasses to see if they work properly. The worksheet contains some tips on how they can improve their design. Now they complete step 5 of Task 2 on the worksheet. Each hourglass will take a different length of time to empty. Explain that this has to do with the size of their hole and the amount of sand they used.
Activity 3: Use your hourglass
Take the students to the playground and encourage them to use their own hourglass to measure how long it takes them to run a certain distance. Mark a start and finish line. One student from each pair gets ready to run. The other student waits for the starting signal, and then turns over their hourglass.
How long did it take for the student to cross the finish line? Could they measure it with their hourglass? Was there enough sand to measure the time? Ask the students to swap places and repeat the task.
The students return to the classroom to complete Task 3 on the worksheet.
Discuss these tasks. Come to the conclusion that it is difficult to measure the exact time with an hourglass. If the hourglass is only partially empty, you can only guess how much time has passed.