Day 12 2-15-11 Topic: Universal Gravitational Objective: I can describe the relationship between gravity, distance, & mass. Science Starter: Aaron Rogers throws a football straight up and it reaches its pinnacle in 2.1 seconds how high did it go? L: G: R: S:

“g” on other Planets

“g” on other Planets NOTES The value of acceleration due to gravity is not the same everywhere in the universe. On other planets its value is dictated by the mass of the planet (or star or satellite) and the distance of the object from the center of the planet. For objects that do not get too far from the planet, one can use the value at the surface of the planet. The following table lists the values of g on the surface of a few heavenly bodies.

“g” on other Planets *Mass varies

Universal Gravitation When Newton first proposed the law of universal gravitation he was quoted as saying, “Though I have calculated its effect, exactly how gravity operates is still a mystery.” He realized that it did not make sense that gravity should instantly transmit forces between planets regardless of their distance. The idea of a field had not yet been thought of during Newton’s time. Therefore, his law of gravitation described the strength and direction of the force of gravity, but not how the force got from one body to the next.

Like the force from a magnet, the force of gravity comes from a field Like the force from a magnet, the force of gravity comes from a field. The gravitational field is created by mass. All mass creates a gravitational field, even a single atom. However, gravity is a relatively weak force and it takes a planet-sized mass to create a field strong enough to create a significant force.. Mass creates a gravitational field first. Then, the gravitational field exerts forces on other masses.

Distance and gravity The distance between objects, measured from center to center, is also important when calculating gravitational force. The closer objects are to each other, the greater the force between them. The farther apart, the weaker the force. The decrease in gravitational force is related to the square of the distance. Doubling the distance divides the force by four (22). If you are twice as far from an object, you feel one-fourth the gravitational force. Tripling the distance divides the force by nine (32). If you are three times as far away, the force is one-ninth as strong.

Force and Gravity The force of gravity between two objects is proportional to the mass of each object. If one object doubles in mass, then the gravitational force doubles. If both objects double in mass, then the force doubles twice, becoming four times as strong.

“g” on other Planets *Mass varies *Distance Varies r r r r