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Basic concepts of kinematics

Lessons 3 · 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
3

Basic concepts of kinematics

Textbook: pp. 14–17
GoalTell apart material point, trajectory, path and displacement.
New words
material point · moddiy nuqtatrajectory · trayektoriyapath (distance) · yo‘ldisplacement · ko‘chish
Explanation

Kinematics describes the motion of bodies without studying what causes it. If a body’s size is very small compared with the distance it covers, we treat it as a material point. The line a body traces during motion is its trajectory; it may be straight or curved. The length measured along the trajectory is the path (distance) s — a scalar quantity, with the metre as its unit. The directed segment from the body’s initial position to its final position is the displacement — a vector quantity. On a curved route the path is longer than the magnitude of displacement; for motion along a straight line in one direction the two values are equal.

Worked examples
A boy walked 400 m east from home to a shop and then back along the same road. The path is s = 400 + 400 = 800 m, but the displacement is zero (the start and end positions are the same).
A cyclist rode 3 km due east and then 4 km due north. The path is s = 3 + 4 = 7 km, while the magnitude of displacement, found by measuring the segment from start to finish on a scale drawing (1 cm = 1 km), is 5 km.
Class activity

“Path and displacement”: a pupil walks 4 steps forward and 2 steps back from a starting point; the class works out the path and the displacement. Safety: clear the floor first.

Practice
1
What is a trajectory?
2
A body moved 6 m right and then 6 m left. What are the path and the magnitude of displacement?
3
Is path a scalar or a vector? And displacement?
4
Why is the path longer than the displacement magnitude on a curved route?