Kepler's Laws and Newtonian Gravity
Kepler found three laws that describe how the planets move; Newton explained why with gravity. Learn the three laws, the inverse-square law, and how T squared is proportional to r cubed - with the calculations to match.
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How the planets move 🪐
Two people cracked planetary motion. **Johannes Kepler** found three laws that *describe* how the planets move. **Isaac Newton** then *explained* why - with gravity. This module covers Kepler's three laws, Newton's inverse-square law of gravitation, and the key relationship **T squared is proportional to r cubed** - with the calculations to go with them.
The words you need 🔑
Four terms describe an orbit:
Match each law to what it says 🔗
- Kepler's first law
- Kepler's second law
- Kepler's third law
- Newton's law of gravitation
- Orbits are ellipses with the Sun at one focus
- A line to the Sun sweeps equal areas in equal times
- Period squared is proportional to radius cubed
- Every mass attracts every other mass
The shape of an orbit 🥚
According to Kepler's first law, what shape is a planet's orbit?
- An ellipse
- A perfect circle
- A square
- A spiral
The inverse-square law 📉
Newton's law of gravitation is **F = G m1 m2 / r squared**. The force depends on the two masses and, crucially, on 1 divided by the distance **squared**. That "squared" matters: if you **double** the distance, the force falls to a **quarter** (because 2 squared = 4). Gravity weakens quickly as things move apart.
True of gravity 🌍
Select the TWO statements that are true of gravity and orbits.
- Gravity provides the centripetal force that keeps a planet in orbit
- If the distance doubles, the gravitational force falls to a quarter
- Gravity only acts on very large objects
- Gravity pushes planets away from the Sun
Describe versus explain ⚖️
Kepler and Newton did different jobs. Keep them apart.
Kepler's third law: find the period 🔢
An interactive activity.
Fast and slow ☀️
Kepler's second law (equal areas in equal times) means a planet moves FASTEST when it is...
- Closest to the Sun (at perihelion)
- Farthest from the Sun (at aphelion)
- It always moves at a constant speed
- Over the Sun's poles
The inverse-square law in numbers 🧲
An interactive activity.
Put it together 🧱
A planet's orbit is an _____ with the Sun at one _____. It moves fastest at _____, when closest to the Sun. Kepler's third law states that T squared is proportional to r _____. Newton explained all this with _____, whose force obeys an inverse-square law.
Order by orbital period 🪜
An interactive activity.
Plot Kepler's third law 📈
An interactive activity.
The grade-9 link 🌟
The top answers **connect Newton to Kepler**: gravity is the *cause*, Kepler's laws are the *consequence*. The Sun's gravity provides the centripetal force, and that same inverse-square force is why orbits are ellipses and why T squared is proportional to r cubed. Be ready to **calculate**: use T squared = r cubed to find a period, and remember that doubling a distance quarters the gravitational force. Describe with Kepler, explain with Newton, and back it with numbers - top band.