Formula
Speed is the distance travelled by a body per unit of time: v = s / t.
ENT-level problem
The car traveled the first 30 minutes at 60 km/h and the next 20 minutes at 90 km/h. Find the average speed of the car for the whole trip.
1. Find the distance of each leg:
$$s_1 = v_1 \cdot t_1 = 60 \cdot 0.5 = 30 \text{ km}$$$$s_2 = v_2 \cdot t_2 = 90 \cdot \frac{1}{3} = 30 \text{ km}$$2. Total distance and total time:
$$s = s_1 + s_2 = 30 + 30 = 60 \text{ km}$$$$t = t_1 + t_2 = 30 + 20 = 50 \text{ min} = \frac{5}{6} \text{ h}$$3. Average speed:
$$v = \frac{s}{t} = \frac{60}{5/6} = 72 \text{ km/h}$$Speed shows how fast a body moves: it is the distance s covered in time t. In the SI system it is measured in metres per second (m/s).
To convert km/h to m/s, divide by 3.6. Average speed is the total distance divided by the total time; it is constant for uniform motion and is found from a segment of the path otherwise.
Divide the value by 3.6: 72 km/h = 72 / 3.6 = 20 m/s.
Average speed is the total distance divided by the total travel time: v = s / t. It is constant for uniform motion.
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