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Name: Jonh
Status: student
Grade: K-3
Country: USA
Date: Spring 2014


Question:
Why cant you change the shape of a brick by squeezing it?



Replies:
Jonh,

Let me introduce some terms related to materials:

Strong versus weak - a strong material is something that requires a lot of energy applied to the material in order for it to change shape or to break. The opposite, a weak material, requires only a little applied energy or force for the material to change shape or break. Strong materials are usually held together well, like steel and wood. Weak materials do not hold onto itself well, like some paper products or things like chalk and crayons.

Brittle versus tough - a brittle material breaks into pieces when energy is applied to it. Tough materials tend to change shape, rather than break, when energy or force is applied to it. Brittle materials usually are made of particles that do not slide around each other like chalk or hard candy. Tough materials are made of particles that can move around like bubble gum or erasers.

A brick is both strong and brittle - the particles in the brick are held together well (strong), but it does not allow the particles to move (brittle), so when a force is applied to a brick it will usually be able to survive the force, until the force becomes to strong, at which point the brick breaks. Compare this to most metals (like iron or copper), on the other hand, which are strong but tough - the particles will resist an applied force and then when the force becomes strong enough, the particles move and causes the metal to stretch or change shape.

Can you think of a material that is weak and tough?

Greg (Roberto Gregorius) Canisius College


You can, in principle. However, the compressibility [(V2 –V1) / V2] /(P2 –P1) (that is, change in volume per change in pressure) is small compared to the strength of the brick is small. Put another way, the brick is not very flexible so it tends to break before it is distorted appreciably. Regular glass behaves similarly. With appropriate equipment these are measurable mechanical properties, but they require specialized equipment.

Vince Calder


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