الأسبوع 9 · الدرس الثالث «قوى الجاذبية»: قوى التلامس وقوى المجال ومجال جاذبية الأرض والمد والجزر والحركة المدارية والكتلة والوزن · 3 دروس
2
Gravity everywhere: Earth, tides and orbits
Science Prep 1 Term 1, Unit 2: pp. 47–54
GoalExplain that all objects attract each other, and use gravity to explain tides and orbits.
New words
newton (N) · the unit of forcetides · the rise and fall of the sea level, caused mainly by the Moon’s pullorbital motion · the curved path of an object going round another because of gravity
Explanation
Isaac Newton found that all objects attract each other. The pull between two objects is equal and opposite on each of them. It grows with their masses and gets weaker as they move apart: if the distance doubles, the pull becomes four times smaller (2 × 2). Between small objects the pull is very weak, so we do not notice it. Force is measured in newtons (N). The Moon pulls on the Earth and its oceans, causing tides. Most places have about two high tides a day, roughly 12 hours apart, and the biggest tides come at new moon and full moon. Tides can also be used to make renewable electricity. Gravity keeps the Moon going round the Earth, the Earth going round the Sun and satellites going round the Earth: this is orbital motion. A black hole forms when a very massive star collapses. Its gravity is so strong that even light cannot escape.
Worked examples
A 5 kg object and a 20 kg object pull each other. The two pulls are equal and opposite: the 20 kg object does not pull harder. The lighter object just moves more easily.
The distance between two objects doubles. The pull between them becomes 2 × 2 = 4 times smaller. If it triples it becomes 3 × 3 = 9 times smaller.
high tide→low tide→high tide→low tide
On a coast there are usually two high tides and two low tides a day. The water rises and falls because of the Moon’s pull on the ocean. One cycle takes a little more than 12 hours.
Class activity
Gravity map. Draw the Earth as a circle with arrows pointing to its centre from four different places around it. Then draw the Moon at a distance with an arrow showing the pull on the ocean. Write one sentence on tides and one on orbits. With an adult, search for a picture of how tides can make electricity and describe it.
Practice
1
The distance between two objects becomes three times bigger. How does the pull change?
It becomes 3 × 3 = 9 times smaller.
2
Which causes the tides?
The Moon’s pull on the Earth and its oceans.
3
What keeps the Moon going round the Earth?
Gravity.
4
What is a black hole?
What forms when a very massive star collapses; its gravity is so strong that even light cannot escape.
