Strength of Shapes! The shape of a structure

affects how strong it is. Rectangles, arches, and triangles are the most common shapes used to build big structures.

To understand which shapes should be used it is important to know what forces act on them.

A force is a push or pull exerted by one object on another.

A tensile force expands or lengthens the object it is acting on. A compressive force

compresses or shortens the object it is acting on.

Forces in Tension and CompressionHow do we define a force?

The rope is in tension

The stand is in compression.

Forces in Tension and Compression

Triangles are the only shapes that cannot be deformed without changing the length of one of its sidesArches are strong because the force resulting from a heavy load placed at the top spreads down the sides to the foundation.A group of arches placed around a central point creates a dome, which distributes loads down through curving sides to the foundation.

Engineers consider shapes under heavy loads when building large structures.

Forces on Shapes

Shapes Lab (click here)Use the slider to apply weight to each shape and see how they react

Stansted AirportCentre Pompidou

Eiffel Tower Bank of China Tower

Roman Colosseum

Iron Bridge at Coalbrookdale

Musee d'OrsaySan Lorenzo, Florence

Florence CathedralSuleyman Mosque

US Pavilion at Expo '67 - spherical structure made of trianglesDome tent -poles bent into arches

with nylon fabric in tension.

Psyclone Rollercoaster- triangles in structure

Campo Volantin Footbridge - arch with cables in tension

When an object is at rest or moving with a constant velocity, it will stay at rest or moving with a constant velocity until is is subjected to an unbalanced force.

Newton's Laws

To understand these forces we must first define force.

How do we define force?

If an unbalanced force, F, is applied to a mass, m, then the mass will experience an acceleration, a. F = ma

Newton's Laws

Every force of action between objects has an equal, opposite, and collinear reaction. Basically, what is pushing down must be pushing up if the object is not accelerating!If the object is not accelerating the sum of all the forces must equal zero.

Newton's Laws

Static Equilibrium is the state of an object when it is at rest or moving with a constant velocity. There may be several forces acting on the object. If they are canceling each other out the object is not accelerating, then it is in a state of static equilibrium.

Static Equilibrium