☰ Contents · Physics

Newton’s second law

Lessons 21 · 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
21

Newton’s second law

Textbook: pp. 78–81
GoalKnow Newton’s second law F = ma and solve problems.
New words
Newton’s second law · Nyutonning ikkinchi qonuniF = ma · F = maresultant · teng ta’sir etuvchiacceleration a · tezlanish a
Explanation

Newton’s second law: the acceleration of a body is directly proportional to the resultant force acting on it, inversely proportional to its mass, and directed along the force: a = F/m, or F = ma. Force is in newtons, mass in kilograms, acceleration in m/s²: 1 N = 1 kg · m/s². Doubling the force doubles the acceleration; doubling the mass halves it. When several forces act, F means their resultant. The law includes the case F = 0 giving zero acceleration, i.e. the law of inertia.

Worked examples
m = 4 kg, F = 20 N: a = 20 : 4 = 5 m/s².
A 1500 kg car accelerating at 2 m/s² needs a resultant F = 1500 · 2 = 3000 N.
Class activity

“Force and acceleration”: push a light and a heavy cart with the same push on a flat floor (with an adult) and explain the result using F = ma. Safety: not on roads or stairs.

Practice
1
Write Newton’s second law as a formula.
2
m = 5 kg, F = 30 N. Acceleration (m/s²)?
3
a = 3 m/s², m = 8 kg. Resultant force (N)?
4
Why does a lorry accelerate more slowly than a car under the same force?