Science notes, unit 6

Force

Textbook Science Year 6 (DLP, Semakan 2017), pages 87–112

What this chapter covers

You can state that force is a push or a pull, and give examples (6.1.1)
You can explain the five effects of force on an object (6.1.2)
You can state what frictional force is and that it acts against movement (6.2.1)
You can describe the advantages and disadvantages of frictional force (6.2.2)
You can carry out fair tests to show how mass and type of surface affect frictional force (6.2.3)
You can suggest ways to increase or reduce frictional force in daily life (6.2.4)
You can explain that air pressure comes from air particles colliding with surfaces (6.3.1)
You can describe how air pressure changes with height above sea level (6.3.2)
You can explain everyday uses of air pressure such as the siphon, syringe and plunger (6.3.3)

Force is all around you. You open a door, kick a ball, brake a bicycle and sip a drink. In this unit you will learn what force is, how frictional force works against movement, and how air pressure comes from tiny air particles.

6.1Force and its effects

Force is a push or a pull that acts on an object.

Action	What it does	Examples
Push	Moves an object away from us	Closing a door, pressing keyboard keys, pressing a clipboard
Pull	Moves an object closer to us	Opening a door, opening a laptop lid, pulling a zip

School library scene with six circled pictures of pushing and pulling: opening and closing a door, pressing a clipboard, pressing keyboard keys, pulling a zip and opening a laptop lid — Each activity in the library is a push or a pull.

Some forces need contact, such as pushing a pencil case. Other forces act without touching: a magnet pulls paper clips, and the Earth's gravity pulls a ball back down. Gravity is a pulling force, because it pulls every object towards the centre of the Earth.

Did you know? A plastic comb rubbed with a cloth can attract tiny pieces of paper. That is electrostatic force.

Effects of force

We cannot see a force, but we can see and feel what it does. A force can:

Effect	Example
Change the shape of an object	Squeezing a toothpaste tube; pressing clay into a flowerpot
Change the direction of movement	Hitting a shuttlecock back; dribbling a ball
Change the speed of an object	Pedalling harder to go faster; waves speeding up or slowing a kayak
Move a stationary object	Pulling a trolley of boxes; a bowling ball hitting pins
Stop a moving object	A goalkeeper stopping a ball; stopping a toy car

A squeezed toothpaste tube and clay being shaped on a potter's wheel — Force changes the shape of an object.

A badminton player hitting a shuttlecock and a footballer dribbling a ball — Force changes the direction of movement.

A force is not an object. Say "the force of the kick" or "the push", not "a force that is a ball".

A goalkeeper catches a fast ball. State the effect of force.

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The force from the goalkeeper's hands stops a moving object (the ball).

6.2Frictional force

Frictional force is the force produced when two surfaces are in contact and rub against each other. It always acts in the opposite direction to the movement.

A boy pushes a crate; the magnified surface is rough; arrows show movement forward and friction backward — Movement goes forward, so the frictional force acts backward. This is why a moving scooter slowly stops.

Effects of frictional force

Advantages	Disadvantages
Rubber can erase writing on paper.	Shoe soles wear out, so we may slip.
Brake pads slow down and stop a bicycle.	Drilling makes loud noise (sound pollution).
Sandpaper smoothens rough wood.	Friction in a car engine damages it over time.
Tyre treads grip the ground.	Rusty screws are harder to loosen.

Friction also produces heat. A match lights when the head rubs the rough side of the box, because the heat makes the coating burn.

Four photos: rubber erasing, bicycle brake pads, sandpaper on wood, tyre treads on soil — Advantages of frictional force.

Four photos: worn shoe soles, drilling, car engine, rusty screw — Disadvantages of frictional force.

Factors affecting frictional force

Two factors are tested in the textbook's fair tests.

Factor	Change	Result
Mass of the object	Heavier object	Frictional force is larger. More weight is needed to move it.
Type of surface	Rougher surface	Frictional force is larger. A rolling cell stops sooner and goes a shorter distance.

Fair test: type of surface

Manipulated: the surface (paper, cloth, sandpaper, carpet).
Responding: distance travelled by the dry cell.
Constant: the same dry cell, same slope, same starting point.
Conclusion: the rougher the surface, the larger the friction and the shorter the distance.

Bar chart: paper 85 cm, cloth 50 cm, sandpaper 20 cm, carpet 35 cm — Sample results: the dry cell goes farthest on paper (smoothest) and shortest on sandpaper (roughest).

In a fair test, change only one thing. To test the type of surface, use the same object every time; to test mass, use the same surface every time.

Why does a dry cell travel a shorter distance on a carpet than on paper?

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The carpet is rougher, so the frictional force is larger and slows the dry cell more.

Frictional force in daily life

Reduce friction	Increase friction
Oil a bicycle chain.	Use a cloth to grip a tight jar lid.
Sprinkle talcum powder on a carrom board.	Scrub a stain with a brush.
Put a heavy box on a trolley with wheels.	Wrap a grip on a racquet handle.

Three situations: oiling a bicycle chain, powder on a carrom board, a box moved on a wheeled trolley — Ways to reduce frictional force.

Did you know? A maglev train floats above the track using magnets, so it overcomes friction and can travel very fast.

A pupil cannot open a tight bottle cap because her hands slip. Suggest one way to solve it.

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Increase friction: hold the cap with a cloth or rubber glove.

6.3Air pressure

Air pressure is the force produced when air particles collide with the surface of an object. The air around us pushes on everything. We call it atmospheric pressure.

Air particles moving randomly inside a balloon and colliding with its surface, causing air pressure — More air particles in a balloon mean more collisions, more pressure, and the balloon expands.

Presence of air pressure

Sipping with a straw: sucking removes air particles from the straw, so pressure inside is lower. The higher air pressure outside pushes the water up the straw.
Inverted glass of water with a card: the air pressure from below pushes up on the card, so the water stays in the glass.
Bottle with holes: water spurts from the lower holes only when the top hole is opened, because air pressure then pushes on the water.

The straw does not "pull" the water. The water is pushed up by the higher air pressure outside.

Air pressure and height

Earth is surrounded by a layer of air called the atmosphere. The higher you go above sea level, the fewer air particles there are, so there are fewer collisions and the air pressure is lower. Climbers on Mount Kinabalu find it hard to breathe for this reason.

Place	Air particles	Air pressure
Peak of a mountain	Fewer	Lower
Base of a mountain / sea level	More	Higher

An empty closed plastic bottle brought down a mountain gets crushed — A closed bottle filled with air at the peak is crushed at the base: the pressure outside is higher.

Mountain diagram: fewer air particles and lower pressure at the top; more particles and higher pressure at sea level — Elevation up, air particles down, air pressure down.

Why is a sealed packet of crisps puffed up when brought to a high mountain?

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The air pressure outside is lower at the top, so the air inside pushes the packet outwards and it swells.

Applications of air pressure

Two holes in a milk can: with only one hole, the air pressure outside stops the milk flowing out. With two holes, the air pressure outside pushes the milk out.
Siphon: a tube lets water flow from a higher container to a lower one. A lower pressure forms in the tube, and the higher pressure outside pushes the water into it.
Syringe: pulling the piston lowers the pressure inside, and the higher pressure outside pushes liquid in.
Plunger: pressing it down gives a lower pressure inside it. When it is pulled back up, the higher pressure in the sinkhole pushes the blocked waste out.

Milk will not flow from a can with one hole but flows with two holes — Two holes are needed so that air pressure can push the milk out.

A syringe drawing liquid in and a plunger clearing a blocked sink — Syringe and plunger use air pressure.

Why must you pierce two holes in a can of condensed milk?

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With two holes, the air pressure outside can push the milk out. With one hole, it stops the milk flowing.

Pictures: Science Year 6 (DLP) textbook, Unit 6: library scene p. 88, effects p. 90, frictional force p. 93, advantages p. 95, disadvantages p. 96, reducing friction p. 99, balloon p. 101, bottle p. 105, air pressure and height p. 106, milk can p. 107, syringe and plunger p. 108. The bar chart is drawn for this site.

Summary

Idea	Remember
Force	A push or a pull.
Effects	Change shape, change direction, change speed, start movement, stop movement.
Friction	Two surfaces in contact; opposes movement; larger for heavier objects and rougher surfaces.
Reduce friction	Oil, powder, wheels.
Increase friction	Cloth, brush, grip.
Air pressure	Particles colliding with a surface; lower at a higher place.
Uses	Two holes in a can, siphon, syringe, plunger.