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Earth and Space - The Solar System and Planetary Motion

Grade 5IB

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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The Solar System consists of the Sun, eight planets, their moons, and other objects like asteroids and comets. The Sun contains more than 99.8%99.8\% of the total mass of the system.

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Planets move in two distinct ways: Rotation (spinning on an axis) and Revolution (orbiting the Sun). Earth's rotation takes approximately 24 hours24 \text{ hours}, while its revolution takes about 365.25 days365.25 \text{ days}.

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Gravity is the invisible force that pulls objects toward each other. The Sun's massive gravity keeps planets in their orbits. The force of gravity FF depends on the masses m1,m2m_1, m_2 and the distance rr between them.

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An Astronomical Unit (1 AU1 \text{ AU}) is the average distance from the center of the Earth to the center of the Sun, which is approximately 1.5×108 km1.5 \times 10^{8} \text{ km}.

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The inner planets (Mercury, Venus, Earth, Mars) are terrestrial and rocky, while the outer planets (Jupiter, Saturn, Uranus, Neptune) are gas giants or ice giants with much larger masses mm.

📐Formulae

1 AU≈1.5×108 km1 \text{ AU} \approx 1.5 \times 10^{8} \text{ km}

W=m×gW = m \times g

F=Gm1m2r2F = G \frac{m_1 m_2}{r^2}

Speed=DistanceTime\text{Speed} = \frac{\text{Distance}}{\text{Time}}

💡Examples

Problem 1:

If a student has a mass of 40 kg40 \text{ kg} and the gravity on Earth is g≈9.8 m/s2g \approx 9.8 \text{ m/s}^2, calculate their weight in Newtons (NN).

Solution:

W=40 kg×9.8 m/s2=392 NW = 40 \text{ kg} \times 9.8 \text{ m/s}^2 = 392 \text{ N}

Explanation:

Weight is the force of gravity acting on an object's mass. Even if the student travels to the Moon, their mass remains 40 kg40 \text{ kg}, but their weight would decrease because the Moon's gg is smaller.

Problem 2:

Light from the Sun travels at a speed of c≈3×105 km/sc \approx 3 \times 10^{5} \text{ km/s}. How long does it take light to travel 1 AU1 \text{ AU} (1.5×108 km1.5 \times 10^{8} \text{ km})?

Solution:

t=1.5×108 km3×105 km/s=500 secondst = \frac{1.5 \times 10^{8} \text{ km}}{3 \times 10^{5} \text{ km/s}} = 500 \text{ seconds}

Explanation:

By dividing the average distance of Earth from the Sun by the speed of light, we find that it takes approximately 8 minutes8 \text{ minutes} and 20 seconds20 \text{ seconds} for sunlight to reach us.

Problem 3:

Why does the Earth stay in orbit around the Sun instead of flying off into space?

Solution:

The balance between Earth's forward velocity and the Sun's gravitational pull (F=Gm1m2r2F = G \frac{m_1 m_2}{r^2}) creates a stable orbit.

Explanation:

Gravity acts as a centripetal force, constantly pulling the Earth toward the Sun, while the Earth's inertia tries to keep it moving in a straight line, resulting in an elliptical path.