الأسبوع 14 · المحور الثاني «المادة والطاقة»، الوحدة الثانية «حركة الجسيمات»، المفهوم 2.3 «مقارنة تغيرات المادة» (د) ومشروع الوحدة «الرمال الزلقة» · 3 دروس
3
The slippery sand investigation
Discovery Education Science P5: Term 1, Unit 2, pp. 214–217
GoalPlan and run a fair test about how water changes the way sand behaves when a block is pulled across it, and explain the result with friction.
New words
friction · a force that slows things down when two surfaces rub togethertheory · an idea that tries to explain something, which tests can supportvariable · something in a test that can change
Explanation
Long ago, people in ancient Egypt moved huge stone statues across the desert on wooden sledges. A wall painting shows a person pouring liquid on the ground in front of a sledge that carries a statue. Some people thought this was part of a ceremony. Some scientists think the Egyptians wet the sand in front of the sledge, and tests show that damp sand is easier to pull over than dry sand. This is a theory and a clue, not proof. When a sledge slides over dry sand, the sand piles up in front of it and the rubbing, called friction, slows it down. Damp sand is firmer and smoother, so there is less friction. To test this, we plan a fair test. We change only one thing, the amount of water, and keep everything else the same: the sand, the tray, the block and the way we pull. We measure how many coins are needed to make the block start to slide.
Part of the test
In the sand test
question
Does water make sand easier to pull over?
change
dry sand or damp sand
keep the same
same sand, tray, block and string
measure
coins needed to start the block moving
Worked examples
Sand
Test 1
Test 2
Test 3
Average
dry
12
16
14
14
damp
5
7
6
6
Tarek repeats each test three times. On dry sand he needed 12, 16 and 14 coins. On damp sand he needed 5, 7 and 6 coins. He finds the average of each: (12 + 16 + 14) ÷ 3 = 14 and (5 + 7 + 6) ÷ 3 = 6. Damp sand needed 14 − 6 = 8 fewer coins.
Two plans for the test. Plan A uses a big block on dry sand and a small block on damp sand. Plan B uses the same block on both. Only Plan B is fair, because only the water changes.
Dry sand
Damp sand
sand in front of the block
piles up
stays flat
friction
more
less
pull needed
more coins
fewer coins
The table compares what happens to the block on dry and damp sand. It explains the result with friction.
Class activity
Slippery sand test (a grown-up helps, outdoors or on newspaper, a light wooden block or a book only, no heavy bricks, no throwing sand, wash your hands). Tie a string to the block and let it run over the edge of a table to a small cup. Put the block on a tray of dry sand and add coins to the cup one at a time until the block starts to slide. Repeat three times, then do the same with a measured amount of water mixed into the sand. Record your results and find the averages.
Practice
1
Why do some scientists think the painting shows water being poured on sand?
Tests show that damp sand is easier to pull a sledge over. It is a theory and a clue, not proof.
2
Name three things that must stay the same in the sand test.
Any three: the kind of sand, the tray, the block, the string, the way the coins are added.
3
Farida needed 5, 8 and 5 coins on damp sand. What is the average?
Test
Coins
1
5
2
8
3
5
(5 + 8 + 5) ÷ 3 = 18 ÷ 3 = 6 coins.
4
Use the word friction to explain why damp sand is easier to pull a block over.
Damp sand does not pile up in front of the block, so there is less friction and less pull is needed.
