Materials

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Solids

A lot of materials are solid, e.g., paper, rock, wood, alloy, and ice. The atoms, or molecules, in solids are very close together, therefore they cannot usually be reduced in size, compressed. Forces of attraction between the particles hold them together and keep them vibrating about a fixed position. The atoms in solids are arranged in a regular way. This gives solids a fixed shape and means that they cannot flow like fluids. The hotter a solid gets, the faster its particles vibrateabout their fixed positions. This means that solids expand when they are heated.

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Hook's Law

$$F=k<mi>x</mi>$$

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Prerequisites

Forces

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Describing Hook's Law

Word equation:

The algebra:

$$F=k<mi>x</mi>$$

• where
• $F$ is the Force on the spring in newtons, $N$,
• $k$ is the Spring Stiffness in newtons per metre, $N/m$ or $N{m}^{-1}$,
• and
• $x$is extension of the spring in metres, $m$.

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The rearrangements

$$x=\frac{F}{k}$$ $$k=\frac{F}{x}$$

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Related equations:

$$E=\mathrm{½}k{x}^2$$

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Questions:

AQA Combined Science Questions
(Care of scienceworksheets.co.uk)

Questions
(Care of gcserevisionbuddy.co.uk)

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Further reading:

AQA Required Practical: Force and Extension

Spring Combinations

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Mass

The mass of an object is a measure of the amount of matter the object has. Mass is different from weight. Weight is the vector (directional) the product of mass and gravitational field strength, e.g. $9.81m{s}^{-2}$. Weight is sometimes called the force of gravity (see Mass verses Weight for more (Care of gcsephysicsninja.com.

The mass of an object is measurable when a force is exerted on the object. If the mass is greater, the object will have less acceleration. Mass is sometimes called inertial mass as it measures the inertia of an object.

The unit of mass in the International System of Units is the kilogram, which is represented by the symbol '$kg$'.

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Conservation of mass and relativity

Mass is an intrinsic property of the object: it is independant of its volume and position. The sum of the masses of objects that interact or of the chemicals that react remain conserved throughout these processes. This remains an excellent approximation for everyday life and even most laboratory work.

However, the special theory of relativity shows that the mass $m$ of an object moving at speed $v$ with respect to an observer must be higher than the mass of the same object observed at rest $m_0$ with respect to the observer.

$$m=\frac{m_0}{\sqrt{1-(v^2/c^2)}}$$

This change in mass is only important when the speed of the object with respect to the observer becomes a large fraction of $c$.

In nuclear processes mass can be converted in to energy and visa versa. The following equation is used to calculate these changes

$$E=m{c}^2$$

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Temperature

Temperature is a physical quantity which describes how quickly molecules within the material are moving. In solids and liquids the molecules are vibrating around a fixed point, but in gases they are in free flight and bouncing off each other, and the walls of any container, as they travel. In a gas the temperature, pressure and volume of the gas are closely related by the ideal gas equation. Temperature is measured in kelvin. A thermometer is used to measure temperature.

Temperature and heat are not the same. Heat is energy which moves from one object, thus cooling it, to another object, so heating it. If an object has a high temperature, it means the average speed of its molecules is fast. A thing might have a high temperature, but because it contains very few or light atoms it has very little heat.

In everyday use temperature are measured in degrees celcius or degrees fahrenheit. Temperature is how hot or cold a material is. The human body can feel the difference between something which is hot and something which is cold.

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