Newton's Law of Gravitation
Earth and Space

Gravity exists in space

Teaching Guidance for 11-14 Supporting Physics Teaching

A universal force

Wrong Track: There is no gravity out in space, so things just float around.

Wrong Track: Astronauts in spacecraft are weightless, you see them floating around. This is because there's no gravity.

Right Lines: Gravity is a universal force acting throughout space. For example it's the gravitational pull of the Sun that keeps Pluto in its orbit. It is impossible to find even the remotest part of the universe where gravitational forces are not acting. Astronauts orbiting the Earth in craft such as the International Space Station do experience weightlessness. They do float around. But bear in mind that the International Space Station orbit is only about 400 km above the surface of the Earth. This is relatively close to Earth and the Earth's gravitational pull certainly works there. In fact it is the Earth's pull that keeps both the astronauts and the Space Station in orbit and gives rise to the condition of weightlessness. As both the astronauts and the Station orbit the Earth, they are falling towards the Earth at the same rate and the astronauts appear to be weightless (the floor of the space station does not support the astronauts).

Gravity is everywhere

Thinking about the learning

A major challenge is getting across the idea that gravity is a force that acts everywhere in the universe and is not simply restricted to the surface of the Earth. A more limited view of gravity takes pupils down various wrong tracks.

Newton's Law of Gravitation
is expressed by the relation F=G(m_1)(m_2)/r^2
can be used to derive Kepler's First Law

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