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.

Forces in Tension and Compression How do we define a force? 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 Compression The rope is in tension The stand is in compression.

Forces on Shapes Engineers consider shapes under heavy loads when building large structures. Triangles are the only shapes that cannot be deformed without changing the length of one of its sides Arches 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. Shapes Lab (click here) Use the slider to apply weight to each shape and see how they react

Triangles Bank of China Tower Eiffel Tower Stansted Airport Centre Pompidou

Iron Bridge at Coalbrookdale Arches San Lorenzo, Florence Musee d'Orsay Roman Colosseum Iron Bridge at Coalbrookdale

Domes Suleyman Mosque Florence Cathedral

Applications Psyclone Rollercoaster- triangles in structure US Pavilion at Expo '67 - spherical structure made of triangles Dome tent -poles bent into arches with nylon fabric in tension. Campo Volantin Footbridge - arch with cables in tension

Newton's Laws 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. To understand these forces we must first define force. How do we define force?

Newton's Laws 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.

Static Equilibrium 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.