Why do we see lightning before hearing thunder?

Light reaches us almost at once while sound moves only about 340 m each second, so the thunder arrives seconds later.

Why does a stone sink?

At 3 g/cm³ it is three times denser than water, and the upthrust is too small to hold it up.

Primary General Studies, Science and Technology, Paper 2: forces, light, sound and states of matter

2026-10-31

Tutor Prep Guide · Primary General Studies

Ten items on force and motion, shadow formation, sound from vibration, fixed shapes of solids, sound speed and distance, resultant force and density.

Focus　Forces change motion; shadows come from light travelling in straight lines; sound starts with vibration; solids keep a fixed shape; distance is speed times time; opposite forces subtract; denser than water means sinking.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	25
Difficulty mix	Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1
Chapter	Chapter: Primary General Studies: Science and Technology
Level	P4 to P6

02　Questions (10 items)

FoundationWhat happens when you push a toy car that is standing still?
FoundationWhat property of light explains shadows?
FoundationWhen you hit a drum, what is the drum skin doing?
FoundationWhich state, ice, water or steam, has a fixed shape?
IntermediateThunder is heard 3 seconds after the lightning. Sound travels 340 m each second in air. How far away was it?
IntermediateA pushes a box with 20 N to the right and B pushes with 8 N to the left. What is the resultant force?
IntermediateA stone of 60 g has a volume of 20 cm³. Will it float or sink in water of 1 g/cm³?
ChallengeA 2 kg mass sits 3 m from the pivot of a see-saw. Where must a 3 kg mass go to balance it?
Challenge200 mL of water at 80 °C is mixed with 200 mL at 20 °C. What is the final temperature?
Error redoError fix: heavy objects always fall faster than light ones. What is correct?

03　Answers and one-line notes

#	Answer	One-line note
1	It starts to move and its speed increases.	A force can change the state of motion of an object, from rest to moving or from slow to fast.
2	Light travels in straight lines, so an opaque object blocks it and casts a shadow.	Because light travels in straight lines an opaque object leaves a dark shape behind it.
3	It is vibrating.	Sound comes from vibration, and the sound stops when the vibration stops.
4	Ice, the solid state.	Solids keep their own shape and volume; a liquid keeps its volume but takes the shape of its container.
5	340 × 3 = 1020 m, about one kilometre.	Distance is speed times time, so 340 m/s times 3 s gives 1020 m.
6	12 N to the right, because 20 − 8 = 12.	Opposite forces subtract, and the resultant points the way the larger force pushes.
7	It sinks, because its density is 60 ÷ 20 = 3 g/cm³, which is more than water.	Anything denser than water sinks, and 3 g/cm³ is three times the density of water.
8	2 × 3 = 3 × d, so it must go 2 m from the pivot.	The see-saw balances when weight times distance is the same on both sides.
9	50 °C, from (200×80 + 200×20) ÷ 400.	With equal amounts the temperature lands halfway, as heat flows from the hot water to the cold.
10	Without air resistance all objects fall at the same rate, whatever their weight.	In a vacuum a feather and an iron ball land together; everyday differences come from air resistance.

04　What to do after marking

After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
Repeat the same item two days later; move to the next paper only when both attempts are correct.
Parents only need the answer and note columns; there is no need to explain every item.

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