GED Science Practice Test: Motions in Space
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Read the following passage and answer the question below:
There are three main motions we can talk about in space: straight-line motion, rotation, and revolution (or orbit).
Straight Line Motion: Typically, objects in space move in a straight line due to inertia. Inertia is the tendency of an object to continue moving in the direction it is currently moving. Objects experiencing inertia tend to move in straight lines. A car traveling on an icy road may try to turn at a curve, but because the car has inertia, it continues moving in a straight line off the road. The inertia of the car can be thought of to “beat out” the frictional forces on the road. In space, galaxies continue moving outward in a straight line, despite the fact that there are small gravitational forces between them. The inertia beats out those small gravitational forces.
Rotation: Rotation is the spin of an object on its own axis. Earth spins on its axis once every 24 hours. Many objects in space rotate, though their rates may be different. For example, Jupiter rotates on its axis once every 9.9 hours. Our Milky Way galaxy is also rotating, though at a much slower rate, once every 200 million years.
Revolution: Revolution is the circling of one object around another. Another word for the revolution of one object around another is orbit. Revolution is the balance of the inertia of an object (the tendency of it to continue moving in a straight line) and gravity, which pulls the orbiting object inward. Remember we talked about inertia and gravity as battling each other. If inertia wins, we see straight line motion. If inertia and gravity are balanced, we see revolution, or orbit. The following picture shows the balance of these forces in creating the orbit of the moon around the earth:
In reality, in terms of the orbit of the moon around the earth, the earth pulls on the moon, but the moon also pulls on the earth. For this reason, orbits are rarely circular, but rather more elliptical in shape, as pictured below in the revolution of the earth around the sun. The distance of the orbiting object away from the center of the orbit also impacts the shape of the orbit. Kepler described the relationship between the speed, distance, and mass of planets.
We have talked briefly about some objects in space, such as the sun and the earth. Next, we will explore some other space, or celestial objects.
What allows an object to orbit, or revolve, around a second object?