**Definition and calculation of speed**

Speed is a physical quantity that can be used to describe the rate of change of position of a body. The speed formula is used in kinematics and classical mechanics.

The SI unit of speed is 1 m/s (metres per second). Other common units of speed are, for example, 1 km/h (kilometre per hour), 1 cm/s (centimetre per second) or 1 kn (1 knot = 1 nautical mile per hour).

Using the formula for speed, you can determine:

- the speed in a uniform rectilinear motion

- the velocity in a uniformly accelerated straight-line motion

The main formula for speed in a uniform rectilinear motion is the ratio of distance to time, which we can describe with the formula: s = d/t, where:

- d = the distance

- t = time

To put it simply, to calculate the average speed with which a body moves, you need to divide the distance travelled (d) by the time (t) in which the body was moving.

**An example in which we use the formula for speed.**

Calculate the average speed at which a train is moving and which covered a distance of 35km in a time of 0.5h.

Data from the task:

d = 35km

t = 0.5h

Looking for

s = ?

Using the formula for velocity s = d/t, substitute

d= 35/0.5 = 70

s = 70km/h

Answer: According to the speed formula, the train driver covered the distance with an average speed of 70km/h.

**The formula for velocity in a uniformly accelerated straight-line motion**

In uniformly accelerated rectilinear motion, the speed increases by an equal amount in one second. Note that the acceleration in this motion has a constant value.

Thus, the formula for instantaneous velocity in uniformly accelerated rectilinear motion whose initial velocity is equal to 0 is defined as the relation V = a*t, where:

a = the acceleration,

t = time.

Often in tasks, we need to convert the units that are given in the task. In the formula for velocity, we use the generally known unit conversions such as:

1 km = 1,000m

1 cm = 0.01m

1 nautical mile = 1,852m

1 h = 60s

It is helpful in the speed formula to convert km/h into m/s and vice versa. To convert km/h to m/s, divide the speed value by 3.6. To convert m/s to km/h, multiply the speed value by 3.6.

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