# Advice 1: How to find the resistance force

At any movement between the surfaces of the bodies or in the environment in which it moves, there are always forces of resistance. They are called frictional forces. They can vary the friction surfaces, the support reactions of the body and its speed if the body moves in a viscous medium, for example water or air.
You will need
• - dynamometer;
• - table of coefficients of friction;
• calculator;
• - Libra.
Instruction
1
Find the resistance force, which acts on a uniformly rectilinearly moving body. For this dynamometer or another method to measure the force that must be applied to the body to move uniformly and rectilinearly. According to Newton's third law it will be numerically equal to the force of resistance to movement of the body.
2
Determine the resistance force of the body that moves along a horizontal surface. In this case, the friction force is directly proportional to the reaction force bearing, which in turn is equal to the force of gravity acting on the body. Therefore, the resistance force in this case or Fтр the friction force equals the mass m of a body measured weights in kilograms, the acceleration of gravity g≈9.8 m/S2 and the coefficient of proportionality μ, Fтр=μ∙m∙g. The number μ is called the coefficient of friction depends on the surfaces that make contact when moving. For example, for friction of steel on wood this coefficient is equal to 0.5.
3
Calculate the resistance force of a body moving on an inclined plane. In addition to the coefficient of friction µ, mass of the body m and the gravitational acceleration g, it depends on the angle of inclination of the plane to the horizon α. To find the resistance force in this case, you need to find the product of the friction coefficient, body mass, gravitational acceleration and the cosine of the angle at which the plane is inclined to the horizon Fтр=μ∙m∙g∙cos(α).
4
When the body moves in the air at low speeds the resistance force FC is directly proportional to the speed of the body v, Fc=α∙V. The factor α depends on the properties of the body and viscosity of the fluid and is calculated separately. When driving at high speeds, for example, when the body falling from a considerable height or the vehicle is moving, the resistance force is directly proportional to the square of the velocity Fc=β∙v2. Additionally, the coefficient β is calculated for high speeds.

# Advice 2: How to find force of air resistance

To determine the force of resistance of air create the conditions in which the body begins under the action of gravity to move uniformly. Calculate the value of gravity, it is equal to the force of air resistance. If the body moves in the air, picking up speed, the force of his resistance is by using Newton's laws, the force of air resistance can be found from the law of conservation of mechanical energy and special aerodynamic formulas.
You will need
• rangefinder, scale, speedometer or radar, ruler, stopwatch.
Instruction
1
The definition of air resistance a falling body uniformly Measure body weight using a scale. Dropping it from some height, make sure it is moving evenly. Multiply body weight in kg by the acceleration due to gravity, (9.81 m/S2), the result is the force of gravity acting on the body. And since it moves uniformly and rectilinearly, gravity will be equal to the force of air resistance.
2
The definition of air resistance the body will skorostrel gaining body mass by using weights. After the body began to move, using a speedometer or radar measure its instantaneous initial velocity. In the end, measure its instantaneous final velocity. Speed is measure in metres per second. If you measure it in kilometers per hour, divide the value by 3.6. In parallel to using the stopwatch to measure the time in which occurred the change. Subtracting the final speed from the start and dividing the result by the time, find the acceleration with which the body moves. Then find the force that causes a body to change velocity. If the body falls, then it is gravity, if the body moves horizontally, the thrust of the engine. From this power subtract the weight of a body on its acceleration (Fc=F+m•a). This will be the force of air resistance. It is important that the body did not touch the ground, for example, moving on an air cushion or fallen down.
3
Determination of air resistance to a body falling from vicotiasecret body weight and drop it from a height, which is known in advance. Upon contact with the surface of the earth clamp the velocity of the body using a speedometer or radar. Then find the product of the gravitational acceleration 9.81 m/S2 on the height from which the falling body, subtract from this value the speed squared. The result, multiply by body weight and divide by the height from which it fell (Fc=m•(9,81•H-v2)/H). This will be the force of air resistance.
In the General case the resistance force can be found if the density of the air multiplied by the square of the velocity with which the body moves, divide this value by 2 and multiplied by the cross-sectional area of the body perpendicular to the direction of the velocity. The result must be multiplied by the aerodynamic coefficient, which is determined experimentally. Therefore, the application of this formula difficult.

# Advice 3: What is measured by the resistance

If included in the electrical circuit of a single current source to different conductors and the ammeter can be observed that the readings of the ammeter when different conductors are different. This is due to the electric resistance of the portion, from which, like voltage, depends on current.

## Resistance as a physical quantity

The electrical resistance of a conductor is a physical quantity, denoted by the letter R. the unit of resistance adopted 1 Ohm – the resistance of the conductor in which the current strength is 1 ampere at a voltage of 1 volt at the ends. Briefly write down this formula:

1 Ohm=1V/1A.

The units of resistance can be and multiple. So 1 milliohm (mω) – is a 0.001 Ohm, 1 kilo Ohm (kω) 1000 Ohms, 1 Megohm (MW) Is 1,000,000 Ohms.

## What is the cause of electrical resistance in conductors

If orderly moving in a conductor the electrons have not experienced any obstacles in their path, they could move by inertia indefinitely. But in reality this does not happen, because the electrons interact with the ions arranged in the crystal lattice of the metal. Their movement from this slows down and in 1 second through a cross section of the conductor is smaller number of charged particles. Therefore, the charge carried by the electrons in 1 second decreases, i.e., decreases the current strength. Thus, every conductor as though resists moving in this current, opposing him.
The reason for the resistance – collision of moving electrons with the ions of the crystal lattice.

## What is expressed in Ohm's law for subcircuit

In any electrical circuit, a physicist has to deal with three physical quantities – current, voltage and resistance. These values do not exist separately in themselves, but are linked by a certain ratio. Experiments show that the strength of the current in the circuit is directly proportional to the voltage at the ends of this area and inversely proportional to the resistance of the conductor. This is Ohm's law, outdoor German scientist Georg Ohm in 1827:

I=U/R,

where I is the amperage on the circuit, U is the voltage applied to the ends of the plot, R is the phase resistance.

Ohm's law is one of the fundamental laws of physics. Knowing the resistance and current, you can calculate the voltage circuit (U=IR), and knowing the amperage and voltage, you can calculate the phase resistance (R=U/I).
The resistance depends on the length of the conductor, the cross sectional area and the nature of the material. The least resistance is typical of silver and copper and ebonite and porcelain almost do not conduct electricity.

It is important to understand that the resistance of the conductor, expressed from Ohm's law formula R=U/I, is a constant. It does not depend on current or voltage. If the voltage at this area will increase several times, at the same time increase the strength of the current, and their ratio will remain unchanged.

# Advice 4: How to measure the resistance of the resistor

The resistor is one of the main elements of any electrical circuit. Its main purpose is to create a certain resistance. Resistance can be measured with special devices or to identify a special marking on the body of the resistor.
You will need
• tester;
• calculator;
• table of markings.
Instruction
1
Take a tester who can work in ohmmeter mode. Connect it to the contacts of the resistor and perform measurement. Since the resistance of the resistors is very different, adjust the sensitivity of the instrument. If the tester can only measure current and resistance, take the current source and assemble electric circuit including a resistor. When connecting the chain, be sure to monitor the current which passes through it, so as not to cause a short circuit. After the change of current in amperes, switch the tester to measure voltage. Connect it in parallel with the resistor and take readings in volts. Then find the resistance of the resistorby dividing the voltage value U and the current I (R=U/I). If you are using a constant current source, connecting devices
2
If the resistor marked to find its resistance, without resorting to additional operations. Resistors are marked or digits or combination of numbers with letters, or a set of color bars.
3
If the resistor is indicated with three digits, the first two numbers identify tens and ones number, and the third digit is the degree 10 to which it should be raised to obtain the correct values. For example, if the resistor numbered 482, it means that its resistance is equal to 48∙102=4800 Ohms.
4
When the resistor marking SMD, the first two figures are taken as the coefficient, and the letter corresponds to the degree 10 to which it needs to multiply. All the values of the coefficients and the letters get in the table of marking of the SMD resistors EIA. The resistor may be the fourth letter of its accuracy. For example, if a resistor is marked 21ВF, its resistance will be equal to 162∙10=1620 Ohms ±1%.
5
If the drawn resistor color bands, use the table to determine the resistance of the resistor by the colored marking. The first three labels correspond to the numbers that make up a coefficient, and the fourth – power of 10 by which to multiply the resulting factor.
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