Lessons 1–2 · 2 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
1
Motion of bodies
Textbook: pp. 8–10
GoalUnderstand mechanical motion and the relativity of motion; know what a reference body is.
New words
mechanical motion · mexanik harakatrelativity of motion · harakatning nisbiyligireference body · sanoq jismiobserver · kuzatuvchi
Explanation
A change of a body’s position in space relative to other bodies with time is called mechanical motion. We always observe motion relative to some body: a car relative to trees, a boat relative to the shore. So both rest and motion are relative. For example, a book in a cyclist’s rear basket is at rest relative to the bicycle but moving relative to a post at the roadside. The body relative to which motion is considered is called the reference body. Without other bodies we cannot speak about the motion of a single body.
Worked examples
A passenger sitting in a train is at rest relative to the carriage but moving relative to a person standing on the platform.
A child standing still on an escalator is at rest relative to the escalator but rising relative to the floor below.
Class activity
“Find the reference body”: the teacher names a situation (for example “pupils sit in the classroom”) and pupils say relative to which body they are at rest and relative to which they move. Safety: observe traffic only from the pavement.
Practice
1
What is mechanical motion?
A change of a body’s position in space relative to other bodies with time.
2
What is a reference body?
The body relative to which the motion is considered.
3
A passenger in a flying aeroplane: relative to which body is the passenger at rest, and relative to which moving?
At rest relative to the aeroplane seat; moving relative to houses on the ground.
4
Why do we say that “being at rest is relative”?
One body can be at rest relative to one reference body and moving relative to another; for example, a house is at rest relative to the Earth but moves relative to the Sun.