الأسبوع 24 · المحور الرابع «التغير والثبات»، الوحدة الرابعة «الأنماط في السماء»: ابدأ، نظرة عامة على مشروع الوحدة، المفهوم 4.1 «تأثير الجاذبية» (أ) · 3 دروس
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What is gravity?
Discovery Education Science P5: Term 2, Unit 4, pp. 112–114
GoalDescribe gravity as a pull between objects with mass, and say how mass and distance change it.
New words
gravity · a pull between any two objects that have massmass · how much matter there is in an objectorbit · the curved path of one object around another
Explanation
Gravity is a force of attraction. Every object that has mass pulls on every other object that has mass. It works even when the objects do not touch. You, a pencil and a rock all pull on each other, but the pulls are far too small to notice. Earth has a huge mass, so its pull is strong and we feel it all the time. It keeps us on the ground, holds water in the sea and makes a dropped ball fall. The more mass an object has, the stronger its pull. Earth has more mass than the Moon, so Earth pulls on a ball or on you more strongly than the Moon would. Distance matters too: the farther apart two objects are, the weaker the pull between them. The Moon stays in an orbit around Earth because of Earth’s pull. Gravity is not a push, and it does not come from Earth spinning.
If …
the pull of gravity …
an object has more mass
gets stronger
an object has less mass
gets weaker
the objects are farther apart
gets weaker
the objects are closer together
gets stronger
Worked examples
Why does a ball you throw up come back down? Earth pulls it toward Earth. The ball also pulls on Earth, but Earth has so much mass that we see only the ball move.
Earth pulls on the Moon, and the Moon pulls back on Earth with an equal force. But the Moon has much less mass, so it is the Moon that is pulled round. This pull keeps the Moon moving round Earth in an orbit instead of flying away.
Two classmates sit close together, but they do not move toward each other. Their masses are small, so the pull between them is tiny. Gravity is there, but too small to see.
Class activity
Gravity true or false. Read these statements to a friend and decide: (1) Gravity pulls things toward the middle of Earth. (2) Gravity only works when things touch. (3) A bigger mass pulls harder. (4) Objects far apart pull more strongly than objects close together. Then drop a book, a ball and a sheet of paper from knee height onto a mat and write what happened. Make sure your feet are clear.
Practice
1
What is gravity?
A pull between any two objects that have mass.
2
True or false: gravity only works when two objects touch.
False. It works across a distance, like Earth pulling on the Moon.
3
The Moon and Earth are close together in pair A. In pair B they are much farther apart. Which pair has the stronger pull between them? Say the letter.
A, because the two objects are closer together.
4
Why do we feel Earth’s gravity but not the pull of a pencil?
Earth has a huge mass and the pencil has a tiny mass, so the pencil’s pull is far too small.
