MadSci Network: Astronomy
Query:

Re: How does gravity in space effect motion?

Date: Mon Jun 2 17:29:26 2003
Posted By: Richard Goode, Faculty, Physical Sciences, Porterville College
Area of science: Astronomy
ID: 1054142332.As
Message:

Hello, Andy,
This is a good question and will require some explaining so be patient. The two ideas are not contradictory, just slightly different views of the same idea.

Your first statement about an object continuing in motion until an outside force acted on it is true. This is Newton's first law of motion. All objects that have mass have inertia. Inertia is an objects resistance to its state of motion. If I am in the shuttle for example and I push a ball away from me, the ball will continue until another force acts upon it.

Newton's second law describes how an object will react when a force is applied. An applied force will cause an object to accelerate with an acceleration that is proportional to the force. A small force will cause a small acceleration and a big force applied to the same object will cause a large acceleration.

Now, let's look at the ball again. Inside the shuttle, the ball moves until it hits something. Nothing will change its motion unless there is a force, it hits the wall or another person touches it. Yet that is not the only motion the ball has. The ball and I are in the shuttle which is orbiting the Earth. A force, gravity, is pulling the shuttle down towards the Earth but the shuttle is moving fast enough that it falls around the Earth. The shuttle's inertia wants to keep going in a straight line, but gravity applies the force that curves the motion around the Earth. Newton used this idea of an object falling in orbit to formulate his law of gravity.

So you can see that the two apparently contradictory ideas in your question are not really contradictory, just applications of Newton's first and second laws. Because of this you could say that everything in the solar system is falling into the Sun, but the will rarely hit the Sun because their inertia has them moving in an orbit that takes them around the Sun. If I give an object enough velocity, it can escape the gravity of the Sun and leave our solar system. This is what has happened to Voyager 1, Voyager 2, Pioneer 10, and Pioneer 11. These spacecraft will keep going because they are traveling fast enough to escape the Sun, but they still have gravity acting on them. Gravity exists throughout space, even in the space between galaxies. Gravity will be very small there so an object will have to travel a very long way before you would notice a change in its motion.

I hope that this answers your question. Feel free to ask for clarification if you need too.

Richard Goode

[Another aspect of this question to consider is the direction of forces and motion. The Sun's gravity pulls the planets toward the Sun. However, their direction of motion is perpendicular to this direction. That's what helps them continue moving around the Sun. If an object starts at rest with respect to the Sun, then it initially has no motion with respect to the Sun. The only force acting on it will be gravity. It will begin to move toward the Sun under the influence of the Sun's gravity. Barring any interaction with the gravity of one of the planets, it will simply fall directly into the Sun, accelerating always under the force of the Sun's gravity. Moderator]


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