Most bodies have a complex structure, because they are composed of various substances. Therefore find them density using tables is almost impossible. To get an idea of \u200b\u200btheir structure, use a concept such as average density, which is calculated after measuring body weight and volume.
You will need
- - scales;
- - measuring cylinder;
- - table of densities of various substances.
Instruction manual
If the body is not composed of a homogeneous substance, find the mass with the help of the scales, and then measure the volume. If it is a liquid, measure with a measuring cylinder. If it is a solid body of the correct form (cube, prism, polyhedron, ball, cylinder, etc.), find its volume by geometric methods. If the body is irregular in shape, immerse it in water that is poured into a graduated cylinder, and determine its volume by lifting it. Divide the measured body weight by its volume, as a result, get the average density body? \u003d m / V. If the mass was measured in kilograms, express the volume in m ?, if in grams - in cm ?. Respectively density will it turn out in kg / m? or g / cm ?.
If it is not possible to weigh the body, find out density the materials it consists of, then measure the volume of each component of the body. Then find the masses of materials that make up the body, multiplying their densities by the volumes and total volume of the body, adding up the volumes of its component parts, including voids. Divide the total body weight by its volume, and get the average density body? \u003d (? 1 V1 +? 2 V2 + ...) / (V1 + V2 + ...).
If the body can be immersed in water, find its weight in water with a dynamometer. Determine the volume of the pushed out water, which will be equal to the volume of the body immersed in it. When calculating, keep in mind that density water is 1000 kg / m ?. To find the average density body, immersed in water, to its weight in Newtons, add the product of the number 1000 ( density water) to accelerate gravity 9.81 m / s? and body volume in m ?. Divide the resulting number by the product of the body volume and 9.81? \u003d (P +? In V 9.81) / (9.81 V).
When the body is floating in water, find the volume of the ejected fluid, the volume of the body. Then the average density body will be equal to the ratio of the product of the density of water to its volume pushed out by the body and the volume of the body itself? \u003d? in Vв / Vт.
Average strength - This is the average number of employees over a given period of time. A report on this indicator is submitted by all organizations to the tax inspectorate annually until January 20 for the previous year and upon creation (liquidation) of the enterprise by the 20th day of the next month.
You will need
- Time sheet
Instruction manual
This report is submitted in the form of KND-1110018 “Information on the average number of employees for the previous calendar year”. Average strength employees of the organization is very important when submitting the following forms of tax reporting: VAT, income tax, property tax, land tax, and also when obtaining the right to switch to a simplified taxation system.
First determine this indicator for each day. It takes into account all actually working and non-working, absent for any reason. Persons who have not worked full time are counted in proportion to the amount of time worked.
Then add the number of employees for the whole month and divide by the number of calendar days in that month.
Next, sum the average for each month and divide by the number 12 (the number of months in a year). The resulting figure will be - the average strength employees for the calendar year.
In the middle strength workers include even those who work under a contract of employment and seasonal workers. Workers for whom, for valid reasons, reduced working hours, are counted as whole units. Employees working under an employment contract but registered with another organization cannot be included in the average number of employees. The number of employees should be indicated in time sheets in the form of No. T-12 or T-13.
GENERAL PROPERTIES OF BUILDING MATERIALS » –
Determination of physical properties of materials
True density is the mass of a substance contained in a unit volume without internal pores and voids (in an absolutely dense state) (,). It is calculated by the formula:
where m - mass of material; V - the volume of absolutely dense material.
The average density is the mass per unit volume of material in its natural state along with pores and voids (,):
where m- mass of material, kg; V e- volume of material in its natural state,.
In contrast to the true average density of various building materials varies very widely due to the presence of pores and voids, the content of which can reach 90% of the total volume. For example, with a true quartz density of 2650, the average density of silica wool (glass wool, slag) can be 100 (appendices 1 and 2). Thus, the average density of materials is always less than their true density. Only for absolutely dense materials (glass, steel, bitumen and others), the average and true density values \u200b\u200bcoincide.
Most building materials have pores. The more they are in a unit volume of the material, the lower its density. For liquids and materials obtained from molten masses (glass, metal), the average density is practically equal in value to the true density.
The numerical value of the density depends on the chemical composition, crystalline structure and type of building material and product. Physicomechanical properties, for example, strength and thermal conductivity, largely depend on the density of a material. The value of the density of the material is used in determining its porosity, mass and size of building structures, calculations of transport and handling equipment. When determining the average density of the material, you can use samples of both regular and irregular geometric shapes. The method for determining the average density of the material depends on the shape of the sample.
Determination of the average density of samples correct
Geometric shape
The name of the material is determined. Samples are weighed and their geometric dimensions determined with a caliper or ruler with an accuracy of 0.1 cm:
a) for a sample having the shape of a cube, a parallelepiped, the volume V is calculated as the product of the base area by height;
b) for a sample of cylindrical shape, the volume V is calculated by the formula:
where V Is the sample volume, ().
Determine the average density of the material with an accuracy of 0.01. The results of the experiments are listed in table 1.
Table 1. The results of determining the average density of samples of the correct geometric shape
Determination of the average density of a dense sample of irregular geometric shape
A dry sample is weighed to the nearest 0.1 g and tied with a string. The measuring cylinder is filled with water, the initial volume of water is noted before immersion in the sample, and after the immersion of the sample, the volume of water displaced by it. The average density of the sample is determined by the formula:
The results are listed in table 2.
Table 2. The results of determining the average density of a dense sample of irregular geometric shape
The basis of this test is also the technical method for determining the volume of samples of material, but in a natural state, including the volume of pores and voids, which depends on their geometric shape. The average density is influenced by humidity, so standards set a specific moisture value at the time of testing for each material. It is recommended to determine the average density on samples of natural humidity or in a dry state (dried to constant weight at 105-110 0 С).
Main equipment
Vernier caliper or metal ruler, VLT-1KG technical scales, volume meter (Fig. 2.2), hydrostatic scales (Fig. 2.3), technical paraffin, thermostat.
Fig. 2.3. Hydrostatic scales:
1 - perforated (mesh) container; 2 - a vessel with a drain for water; 3 - rocker; 4 - a cup for weights; 5 - a glass with a fraction; 6 - weights
Test
There are two standard methods for determining the average density: on samples of regular and irregular geometric shapes. They differ in the way they measure volume.
The sample volume of any regular geometric shape (cube, parallelepiped, cylinder) is calculated from direct measurements with a caliper with an error of up to 0.1 mm for dense samples (size 50-100 mm) or a metal ruler with an error of up to 0.5 mm for porous samples ( size over 100 mm). The final size is found as the arithmetic average of the results of three measurements (for a cylinder, four measurements).
The sample volume of an irregular geometric shape (weighing more than 300-500 g) is determined using a volume meter or hydrostatic weighing.
The test dry sample is preliminarily coated with a thin layer of paraffin melted at 75-85 0 С using a brush or immersion, and weighed. You can pre-saturate the sample with water, remove excess tissue from the surface with a soft cloth and immediately determine the volume.
When tested with a volume meter (see Fig. 2.2), a sample tied with a strong thread is carefully immersed in water. After the dropping of drops from the tube into the glass stops, it is weighed and the mass of displaced water is calculated. The sample volume is calculated by the formula
but without waxing
Here is the mass of the dry sample;
The mass of the sample coated with paraffin;
The mass of displaced water;
The density of paraffin, \u003d 0.93 g / cm 3.
When using the hydrostatic weighing method, the sample volume is numerically equal to the buoyancy force. A sample of a material of a certain mass previously prepared by waxing or saturation in water is weighed in a vessel with water on a hydrostatic balance (Fig. 2.3). The sample volume is
but without waxing