Page 11 - Spotlight A+ Physics Form 4.5

Form
4
Chapter 3 Gravitation Physics

Thread loop with circumference 32 cm 4. The ellipse is drawn by moving the pencil
slowly around F and F .
1 2
5. The thumb tacks and thread are removed.
16 cm
6. A small circle is drawn at F to represent
Minor axis 1
the Sun and another small circle on the
circumference of the ellipse to represent the
Thread Earth.
Thumb Discussion:
tack 1. The shape of the ellipse shows that when
F F
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1 2 the Earth moves in its orbit along the
Major 12 cm circumference of the ellipse, the distance
axis between the Sun and the Earth changes.
Figure 3.15 2. The Earth is nearest to the Sun when it is on
the major axis to the left of F . The Earth is
1
furthest away from the Sun when it is on the
major axis to the right of F .
2
Video: Drawing an ellipse
https://bit.ly/2UhHkhH

CHAP
3
A Kepler's First Law B Kepler's Second Law
• Kepler’s First Law states that the orbit of • Kepler’s Second Law states that a line that
each planet is an ellipse, with the Sun at one connects a planet and the Sun sweeps out
focus. equal areas in equal time intervals as the
• The planets in the Solar System have elliptical planet moves in its orbit.
shaped orbits and the Sun is always at one • If a planet takes the same time to move from
focus of the ellipse as shown in Figure 3.16. A to B and C to D in Figure 3.17, the area of
region AFB is equal to the area of region CFD.
• The distance AB is larger than the distance
Minor axis
CD as the planet moves with a higher linear
speed from A to B compared to from C to D.
Ellipse
Planet
B
C
F
Major axis
Sun Sun
D
A
Equal area in equal time
Planet
Figure 3.17 The motion of a planet in its orbit
Figure 3.16 Try question 2 in Formative Zone 3.2
• The orbits of most planets in the Solar System C Kepler's Third Law
have the major axes and minor axes of almost
the same length. • Kepler’s Third Law states that the square of
• Therefore, elliptical shape of the orbits the period of a planet is directly proportional
2
3
of planets in the Solar System are actually to the cube of the radius of its orbit, T ∝ r .
almost round. • Therefore, planets that are further away from
• The radius of the orbit is the average distance the Sun will take a longer time to complete
between the planet and the Sun. one orbit around the Sun.
Try question 1 in Formative Zone 3.2

3.2.1 3.2.2 75