Lessons 19–20 · 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
19
Newton’s first law — the law of inertia
Textbook: pp. 72–75
GoalState Newton’s first law (the law of inertia) and explain inertia.
New words
inertia · inersiyaNewton’s first law · Nyutonning birinchi qonuniinertial reference frame · inersial sanoq sistemasibalanced forces · muvozanatlashgan kuchlar
Explanation
Newton’s first law: there exist reference frames in which a body keeps its state of rest or uniform rectilinear motion if no other bodies act on it, or if the actions balance each other. The property of a body to keep its velocity is called inertia, which is why the first law is also called the law of inertia. A frame in which this law holds is inertial; a frame attached to the Earth is taken as approximately inertial in most problems. Because of inertia, passengers lean forward when a bus brakes and are pushed back when it starts suddenly. To change a body’s velocity, the action of another body is required.
Worked examples
If you pull a paper strip quickly from under a glass, the glass stays in place: by inertia it keeps its state of rest.
When a dusty carpet is beaten, the carpet stops but the dust keeps moving by inertia and flies out.
Class activity
“Inertia trick”: with a plastic cup, a paper strip and a coin: pulling the strip quickly drops the coin into the cup. With an adult; use plastic only. Safety: no breakable containers.
Practice
1
State Newton’s first law.
A body keeps rest or uniform rectilinear motion if no other bodies act or their actions balance.
2
What is inertia?
The property of a body to keep its velocity.
3
Why does a passenger lean forward when a bus stops?
The bus slows, but the passenger’s body keeps its previous velocity by inertia and moves forward.
4
How does a body move if the resultant force is zero?
It stays at rest or moves uniformly in a straight line.