On the city the tourist walks, the car rushes, in air the plane flies. Some bodies move quicker than others. The car moves quicker than the pedestrian, and the plane flies quicker than the car. In physics the size characterizing speed of motion of bodies is speed.

## Instruction

1. At uniform motion of a body (the movement of a body with not changing speed) speed shows what way there passed the body per unit of time. Using a concept of speed it is possible to tell that in the given examples, bodies move with various speeds. At uniform the movement the value of speed remains to a constant eventually. If the bird flies by within 5 seconds the way equal to 25 meters, then her speed will be equal to 25m/5s =5 m/s (5 meters per second).

2. To determine speed at uniform motion, it is necessary to divide the way passed by a body into a period for which this way is passed. Speed = way / time. This rule will turn into a formula for calculation of speed if to write down the accepted designations. v – speed, s is a way, t is time. v = s/t. Body speed at uniform motion is the size equal to the relation of a way at the right time for which this way is passed by a body. Example 1. The mosquito in 30 seconds flew by a way of 5 m. Speed of a mosquito is equal to v = to s/t = 5 m / 30 with = 0.17 m/s.

3. In the international system of units the speed can be measured in meters for a second (m/s). It means that the speed of such uniform motion at which in 1 second the body passes the way equal to 1 meter is taken for unit of speed. Speed of a body can be measured in kilometers per hour (km/h); kilometers per second (km/s); centimeters a second (cm / c), etc. The numerical value of speed depends on the chosen unit of measure. Example 2. 54 km/h = 54000 m / 3600 with = 15 m/s. 36 km/h = 36000 m / 3600 with = 10 m/s.

4. Speed – vector size. Except numerical value it has the direction. The vector of speed is always directed towards the movement of a body. (cm drawing). If the body moves unevenly, then its movement characterizes a concept of average speed. For calculation of average speed it is necessary to divide all way passed by a body into all the time of the movement. Average does not explain speed how the body moved to various timepoints in different intervals of a way.