Science notes, unit 12

Stability and strength

Textbook Science Year 6 (DLP, Semakan 2017), pages 193–210

What this chapter covers

You can explain what the stability of objects and structures means (12.1.1)
You can explain what the strength of objects and structures means (12.1.2)
You can state how height and base area affect stability, and test them in a fair experiment (12.1.3)
You can state how the type of material and the shape of a structure affect strength, and test them (12.1.4)
You can explain why strong and stable structures are important for a sustainable life (12.1.5)
You can define stability and strength operationally by building and testing a structure (12.1.6)
You can design and make a model of a strong and stable structure from recycled materials (12.1.7)

Why does a tall pile of cups topple over so easily, while a wide pyramid of blocks stays put? Why do builders put triangles in bridges and roofs? In this unit you will learn what makes a structure stable and what makes it strong.

12.1Stability and strength of objects and structures

Stability

The stability of an object or structure is its ability to remain stable, or to return to its original position after it is pushed or moved.

A stable structure does not collapse when a movement or force is applied to it. A tower of blocks that falls down when a fan blows on it is not stable.

Science info: The centre of gravity is the point near the middle of an object where its weight seems to act. The lower the centre of gravity, the more stable the object is.

Factors that affect stability

Two factors affect stability: height and base area.

Factor	Change	Effect on stability
Height	Increases (taller)	Stability decreases
Height	Decreases (shorter)	Stability increases
Base area	Increases (wider)	Stability increases
Base area	Decreases (narrower)	Stability decreases

Two block structures. Adam's is low and wide: height 20 cm, length 28 cm, width 28 cm. Raju's is tall and narrow: height 32 cm, length 16 cm, width 2 cm. — Adam's low, wide structure is more stable than Raju's tall, narrow one.

Figure: Science Year 6 (KPM), p.195.

Example: working out base area

Base area of a rectangle = length × width.
Adam: 28 cm × 28 cm = 784 cm². Raju: 16 cm × 2 cm = 32 cm².
Adam's structure has the bigger base area and is also shorter (20 cm against 32 cm), so it is more stable.

Giraffes spread their front legs when they drink. This makes the base area bigger and the height lower, so they do not topple over.

Careful! The base area is the area that touches the ground, not the biggest area of the object. A box standing on its small end has a smaller base area than the same box lying flat.

Investigating height and base area

In a fair test only one variable is changed. The rest are kept the same.

	Height of structure	Base area of structure
Aim	To find out how height affects stability	To find out how base area affects stability
Manipulated variable	Number of paper cups stacked (height)	Base area of the cans stacked
Responding variable	Time taken for a cup to fall	Time taken for a can to fall
Constant variable	Size of cups, cardboard, how it is moved	Size of cans, cardboard, how it is moved
Method	Move the cardboard back and forth, time how long until a cup falls	Same, using the different arrangements of cans
Result	Taller stack falls sooner	Smaller base falls sooner

Structure P is 10 cm tall with a base of 8 cm × 8 cm. Structure Q is 25 cm tall with a base of 4 cm × 4 cm. Which is more stable? Give two reasons.

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P. It is shorter (10 cm against 25 cm) and its base area is bigger (64 cm² against 16 cm²).

Strength

The strength of an object or structure is its ability to withstand a force that could damage it or change its shape.

Chua and his father used steel rods for the pillars of a rabbit cage so that the cage is stronger and lasts longer. Two factors affect strength: the type of material and the shape of the structure.

Type of material

Material	Properties
Concrete	Strong, not flammable, does not rust or rot
Steel	Strong and durable metal
Plastic	Light and easy to shape
Wood	Strong, hard and easy to cut to size

Different materials have different strength. In the test, paper, plastic and steel straws are held between supports and weights are added. The straw that carries the most weight before it bends is the strongest.

Shape of the structure

Truss: poles joined to make triangles. Triangles resist heavy loads. Trusses are used in bridges and roofs.
Arch: a curved structure that supports loads, used on bridges.
Dome: a half-sphere shape that is strong and needs no pillars to hold it up. It is used for stadium roofs.
Cylinder: strong and durable, and it reduces wind resistance. Lighthouses are cylinders.

A bridge made of triangles and a house roof frame with the truss triangle outlined in red — Trusses in a bridge and in a roof frame.

A bridge supported by an arch, and a dome-shaped tent — An arch under a bridge, and a dome.

Figures: Science Year 6 (KPM), p.200.

Investigating shape

Four beams are made from the same size of manila card (24 cm × 12 cm): A is folded into a shape with four 3 cm lines, B a triangular beam, C a cylinder (rolled) and D a cuboid. Each beam is laid between two desks and weights are added until it bends or collapses. The beam that holds the most weight has the strongest shape.

Variable	What it is
Manipulated	Shape of the beam
Responding	Mass of weights the beam holds before it changes
Constant	Size of the manila card, distance between the desks, type of card

Careful! Stability and strength are not the same. Stability is about not toppling (height, base area). Strength is about not breaking or bending (material, shape).

A student says, "A taller tower is stronger because it is bigger." Is she right?

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No. Height affects stability, not strength. A taller tower is less stable. Strength depends on the material and shape.

Operational definitions

An operational definition says what you do and what you observe. For example:

Stability and strength of a straw structure

Stability: a structure is more stable if it stays standing for a longer time when the table is shaken.
Strength: a structure is stronger if it can hold more weights before it bends or collapses.

Why strong and stable structures are important

Situation 1: a wooden village bridge in 2010 and broken in 2021. Situation 2: a steel bridge in 2010 and still sturdy in 2021. — The wooden bridge broke after 11 years. The steel bridge was still sturdy.

Figure: Science Year 6 (KPM), p.206.

They are not easily damaged.
They save maintenance cost.
They are safe to use.
They can be used for a long time, which helps a sustainable life.

Make a model: a bottle chair uses 18 recycled plastic bottles joined with wide tape. Reusing bottles cuts waste, and the cylindrical bottles give strength. To make a model stable, keep it low with a wide base.

Name two ways to make a tall bookshelf more stable.

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Make the base wider (bigger base area). Make it shorter or put heavy things on the bottom shelves (lower centre of gravity).