☰ Contents · Natural science

Gravity and friction

Lessons 53–54 · 2 lessons · Y.V. Malikova, S.V. Kosyanenko. Natural Science Grade 4, Part 1. Novda Edutainment, Tashkent, 2023; S.V. Kosyanenko, Y.V. Malikova, O.E. Tigay. Natural Science Grade 4, Part 2. Novda Edutainment, Tashkent, 2023
53

The force of gravity

Textbook: Part 2, pp. 86–89
GoalKnow gravity (the Earth’s pull), why bodies fall, the difference between mass and weight, and that gravity is weaker on the Moon.
New words
gravity · og‘irlik kuchimass · massaattraction · tortishishkilogram · kilogramm
Explanation

The Earth pulls all bodies towards its centre. This force is called gravity. That is why a stone let go of falls to the ground, rain falls down, and we do not fly off the Earth. The mass of a body shows how much matter it holds and is measured in kilograms; it is the same everywhere. Weight is how hard a body presses on its support and depends on the pull of gravity. On the Moon gravity is about six times weaker than on Earth, so an astronaut bounces lightly there, yet the mass does not change. A parachute falls slowly because of air resistance. Things that fall from a height are dangerous: do not throw objects from above.

Worked examples
A cat’s mass is 4 kg — also 4 kg on the Moon, but its weight is about six times smaller.
Class activity

Take a sheet of paper and an identical sheet crumpled into a ball. Drop both from the same low height (while sitting). Which falls faster and why?

Practice
1
What is gravity?
2
What is mass and how is it measured?
3
Why does an astronaut bounce lightly on the Moon?