6
Velocity of uniform rectilinear motion
Textbook: pp. 28–31
GoalKnow υ = s/t; convert velocity units and solve problems.
New words
velocity · tezlikmetre per second · m/skilometre per hour · km/soatpath formula s = υt · yo‘l formulasi s = υt
Explanation
The velocity of uniform rectilinear motion is a vector equal to the ratio of the displacement to the time in which it occurred: υ = s/t. Its magnitude shows the distance covered in unit time. The SI unit of velocity is the metre per second (m/s); km/h is also used in practice. The conversion is 1 m/s = 3.6 km/h, since 1 m/s = (1/1000 km)/(1/3600 h) = 3.6 km/h. The formula also gives the path s = υt and the time t = s/υ. When solving, first bring all quantities to the same units.
Worked examples
A bus covering 120 km in 2 hours has υ = 120 : 2 = 60 km/h = 60 : 3.6 ≈ 16.7 m/s.
A cyclist riding at 5 m/s for 2 minutes covers s = υt = 5 · 120 = 600 m.
Class activity
“Measuring velocity”: on a straight 20 m strip, under the teacher’s eye, one pupil walks at a normal pace while another times it; the class computes υ = s/t. Safety: do not run; check the surroundings.
Practice
1
What is velocity and what is its formula?
The ratio of displacement to time: υ = s/t.
2
A body covers 40 m in 8 s. What is its velocity in m/s?
5
3
A train at 90 km/h travels for 3 hours. How many km does it cover?
270
4
Why is 1 m/s = 3.6 km/h?
1 m = 0.001 km and 1 s = 1/3600 h, so 1 m/s = 0.001 · 3600 = 3.6 km/h.