Lessons 27–28 · 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
28
Weight of a body
Textbook: pp. 102–104
GoalKnow weight (P) and how it differs from the force of gravity; at rest P = mg.
New words
weight of a body · jism og‘irligisupport · tayanchsuspension · osmasupport reaction force · tayanch reaksiya kuchi
Explanation
The force with which a body presses on a support or pulls a suspension because of the Earth’s attraction is called the weight of the body, denoted P. The force of gravity acts on the body, whereas weight acts on the support or suspension — they act on different bodies. When the support or suspension is at rest or moves uniformly, P = mg. By Newton’s third law the support reaction N equals the weight in magnitude and is opposite in direction. Weight is measured in newtons, mass in kilograms; saying “60 kg weight” is wrong — correctly: “mass 60 kg, weight about 600 N”. With an accelerating support the weight changes.
Worked examples
A 40 kg child at rest presses on the floor with P = 40 · 10 = 400 N.
If a hanging load reads 5 N on a dynamometer, its mass is 5 : 10 = 0.5 kg.
Class activity
“Bathroom scale”: with an adult, read your mass on a scale and compute your weight with P = mg. Safety: do not jump on the scale; avoid wet floors.
Practice
1
What is the weight of a body?
The force with which a body, because of the Earth’s attraction, presses on a support or pulls a suspension.
2
A 25 kg load rests on a table. Weight (g = 10 m/s²), N?
250
3
On which bodies do weight and gravity act?
Gravity acts on the body; weight acts on the support (suspension).
4
Why is “mass 60 kg, weight 600 N” correct but “weight 60 kg” wrong?
Mass is measured in kg, weight (a force) in newtons.