الأسبوع 25 · المحور الرابع «التغير والثبات»، الوحدة الرابعة «الأنماط في السماء»، المفهوم 4.1 «تأثير الجاذبية» (ب): السقوط والكواكب · 3 دروس
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Gravity holds the planets
Discovery Education Science P5: Term 2, Unit 4, pp. 125–129
GoalExplain that the Sun’s gravity keeps the planets in orbit, and write a claim, evidence and reasoning about gravity.
New words
revolve · to travel in a path around another objectplanet · a large round world that orbits a starellipse · a shape like a slightly squashed circle, an oval
Explanation
Our planet is moving. As you read this, Earth is travelling around the Sun. The Sun has far more mass than all the planets together, so its gravity is very strong. This pull acts like an invisible rope that keeps each planet moving in a curved path around the Sun instead of flying off in a straight line. The path of a planet is called its orbit, and it is an ellipse that is almost a circle. Going round another object in an orbit is called revolving. Earth takes about a year to go once round the Sun, and the planets nearer the Sun take less time than those farther away. The Moon revolves round Earth for the same reason, and Earth’s pull keeps it there. Gravity is the force that holds the solar system together.
Planet
Place from the Sun
Time for one orbit (about)
Mercury
1st, nearest
88 days
Earth
3rd
365 days
Neptune
8th, farthest
165 years
Worked examples
Model
Real world
ball
planet
hand
the Sun
string pull
gravity
Think of a ball on a string that you swing in a circle, away from faces (a grown-up watches). The string keeps pulling the ball inward, so it goes round instead of flying away. The Sun’s gravity does the job of the string for a planet. If the string breaks, the ball flies off straight.
365 days→52 weeks→+ 1 day
A year on Earth is about 365 days. That is 52 weeks and 1 day, because 52 × 7 = 364. Earth takes this long to go once round the Sun.
Part
Example
claim
gravity affects motion
evidence
falling balls, orbits
reasoning
it pulls objects together
Claim, evidence and reasoning: Claim: gravity affects the motion of objects in many ways. Evidence: dropped balls fall to the ground, and planets stay in orbit round the Sun. Reasoning: gravity pulls objects toward each other, so things fall toward Earth and planets curve round the Sun.
Class activity
Orbit model (a grown-up helps). Put a marble in a wide, smooth bowl on a table. Give it a gentle push sideways so it rolls round the inside of the bowl. Watch it curve round the middle instead of rolling away. The bowl is a model: the curved bowl pulls the marble inward like the Sun’s gravity pulls a planet. Write one way the model is like a real orbit and one way it is different (for example, the marble slows down and stops because of friction, but planets do not).
Practice
1
What keeps the planets moving round the Sun instead of flying away?
The Sun’s gravity pulls on them.
2
What is an orbit?
The curved path of one object around another, such as Earth around the Sun.
3
In the ball-on-a-string model, what stands for gravity? Say the letter.
C, the string.
4
A year is about 365 days. How many full weeks is that?
