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    The gravity well is a physics analogy. It aids with the visualisation of abstract physical concepts.

    This interactive displays some of the simulations that can be demonstrated with a gravity well. Click on the labels for background information, demonstration videos and links to related content.

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    Acknowledgement: Background image, julvil,123RF Ltd.

    Transcript

    Newton’s first law of motion

    Newton’s first law states that an object will travel in a straight line at a constant speed unless a force is applied. Gravity is a force that can cause objects to travel in curved lines, and the stronger the gravity, the more strongly objects are pulled off the straight line trajectory. The gravity pulling towards the Sun keeps the planets in our Solar System in their orbits.

    Transcript

    Gravity is a force that can cause objects to travel in curved lines, and the stronger the gravity, the more strongly objects are pulled off of the straight line trajectory.

    Demonstrate how a massive object exerts a force that affects the motion of nearby objects by rolling a ping pong ball across a gravity well.

    Place a heavy ball on the gravity well and roll the ping pong ball again.

    Experiment by using a heavier ball to represent a more massive object.

    Related resources:

    Direction of planetary orbits

    Planets in our Solar System all go in the same direction around the Sun. This is a likely outcome for a system that starts out with matter in orbit around a star going in random directions. Collisions between objects going in different directions slows them down, and they fall into the Sun. Objects going in the same direction are much less likely to collide and therefore survive as planets.

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    Transcript

    To demonstrate the direction of planetary orbits, place a heavy ball on your gravity well.

    Roll two groups of marbles in opposite directions around the central mass. The marbles will collide, and the collisions cause the marbles to lose speed and fall into the central mass. Usually, the result is that the remaining marbles will go in the same direction around the central mass.

    Orbital speeds of planets

    The further a planet is from the Sun, the slower it moves and the longer it takes to complete an orbit. Mercury – the closest planet to the Sun – takes 88 Earth days to complete an orbit. The furthest planet – Neptune – takes 164 Earth years to complete an orbit.

    Transcript

    To demonstrate the orbital speed of planets using a gravity well, place a very heavy ball in the centre of the well to represent the Sun. Roll one marble midway on the sheet to represent the orbit of a planet such as Mars. Roll a second marble onto the sheet to represent a planet with a larger orbit such as Neptune. The period (length of time for one revolution) is much longer for the marble that is further from the Sun.

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    Circular and elliptical orbits

    Planets have orbits that are close to circular. However, comets have elliptical orbits.

    To demonstrate the different orbits on the gravity well, begin by placing a heavy ball on the sheet to represent a central mass like the Sun. Simulate the circular orbit of a planet by rolling a marble so that it forms a circular orbit. To simulate the elliptical orbit of a comet, roll a marble so it narrowly misses the central mass. This will cause it to have an elongated elliptical orbit.

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    IMAGE: NASA Robert Simmon

    Moon orbits around planets

    Most planets in our Solar System have moons – Mercury and Venus are the exceptions. The moons and the planets stay together as they travel around the Sun.

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    To demonstrate the orbit of a moon around a planet as they both orbit the Sun, roll a small and a large marble around a large central mass.

    The large marble will distort the sheet close to it enough to hold the small marble close as they both make their trip around the large central mass.

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    The pull of gravity on all objects

    Gravity is a force that attracts all objects towards each other – every object with mass pulls on every other object with mass. When a person jumps off a chair, the person is attracted to the Earth and the Earth is attracted to the person. The Earth moves a tiny distance towards the person as the person moves towards the Earth. However, the forces are quite small, and it takes a great deal of mass to exert an easily detectable force.

    Transcript

    Gravity is a force that attracts all objects towards each other.

    Demonstrate the pull of gravity on two objects by placing a heavy ball onto the gravity well. Roll another heavy ball onto the gravity well.

    Observe the movement of the two balls as they get closer to each other.

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    Gravity assist for spacecraft

    Gravity assist occurs when a spacecraft enters the orbit of other planets and moons and uses their gravity to propel or kick itself forward.

    When NASA was planning the mission to the Moon, they sent rockets to the Moon on free return trajectories, which is a trip that begins at the Earth, orbits around the Moon and comes back to Earth.

    Gravity assist features in other missions such the ESA’s Rosetta mission and NASA’s Parker Solar Probe.

    Transcript

    Demonstrate gravity assist on the gravity well by placing two metal balls on the sheet with a distance between them. Place small magnets under the balls to hold them in place. The balls represent the Earth and the Moon.

    Use a marble to represent a rocket.

    When given the correct trajectory, the rocket can be sent from the Earth to travel around the Moon and back to the Earth in a classic figure-of-eight shape.

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    Formation of planets and stars

    Planets and stars are formed when dust and small pieces of matter clump together and attract nearby matter – eventually becoming a massive object such as a planet. If enough mass is present, the matter is crushed together and ignites as a star.

    Transcript

    To demonstrate the concept of planet and star formation, place a few marbles on your gravity well. Roll a marble or two onto the sheet. The marbles eventually group together to make a clump that attracts the other objects.

    Related resource:

    Black holes

    A black hole has so much mass tightly packed into a small space that, close up, its gravity is so strong that nothing nearby can escape it.

    To simulate a black hole on the gravity well, place a small metal ball on the sheet. Attach a strong magnet to the ball under the sheet and attach a heavy ball to the magnet. The sheet stretches downwards and simulates the infinite stretch of spacetime caused by an actual black hole.

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    Professional learning and development

    The Hub has resources to help educators plan and teach about the Physical World concept of gravity and unpack students’ alternative conceptions.

    Rights: The University of Waikato Te Whare Wānanga o Waikato Published 19 June 2019 Size: 1 MB Referencing Hub media