Lessons 9 · 1 lessons · P.Q. Habibullayev, A. Boydedayev, A.D. Bahromov, S.O. Burxonov. Physics Grade 7, revised and enlarged 4th edition. “O‘zbekiston milliy ensiklopediyasi” State Scientific Publishing House, Tashkent, 2017
9
Acceleration in uniformly variable motion
Textbook: pp. 37–39
GoalKnow acceleration, a = (υ − υ₀)/t and its unit; tell speeding-up from slowing-down motion.
New words
uniformly variable motion · tekis o‘zgaruvchan harakatacceleration · tezlanishmetre per second squared · m/s²initial velocity υ₀ · boshlang‘ich tezlik υ₀
Explanation
Motion in which the velocity changes by the same amount in any equal time intervals is called uniformly variable motion. The quantity describing how fast velocity changes is acceleration: the change of velocity divided by the time in which it occurred, a = (υ − υ₀)/t, where υ₀ is the initial and υ the final velocity. The SI unit of acceleration is m/s². For example a = 2 m/s² means the velocity changes by 2 m/s every second. Acceleration is a vector: if it points the same way as the velocity the body speeds up; if opposite, it slows down (the formula then gives a negative value).
Worked examples
A car raises its velocity from 0 to 20 m/s in 5 s: a = (20 − 0) : 5 = 4 m/s².
A cyclist reduces velocity from 10 m/s to 6 m/s in 4 s: a = (6 − 10) : 4 = −1 m/s²; the minus sign means slowing down.
Class activity
“Speeding up?”: the teacher names situations (a train starting, a bicycle braking); pupils say which way the acceleration points relative to the velocity. Safety: do it in imagination only, no running in the room.
Practice
1
What is acceleration and what is its formula?
The change of velocity divided by time: a = (υ − υ₀)/t.
2
A body raises its velocity from 2 m/s to 14 m/s in 4 s. Acceleration (m/s²)?
3
3
What does a = 3 m/s² mean?
The velocity changes by 3 m/s every second.
4
Why is the acceleration of a braking car negative?
Because the final velocity is smaller than the initial, and the acceleration points against the velocity.