Planetary Motion 4/28/14

19-1b pgs. 515-518

IN: What causes night and day?

Planetary MotionPlanetary Motion

Most objects in space move Most objects in space move according to strict physical laws.according to strict physical laws.

The way in which the Earth moves, The way in which the Earth moves, for example, causes seasons and for example, causes seasons and even day and night.even day and night.

RotationRotation – the spinning – the spinning motion of a body on its axis.motion of a body on its axis.•On Earth rotation causes night On Earth rotation causes night and day.and day.

RotationRotation As the Earth rotates, only one-half of the As the Earth rotates, only one-half of the

Earth faces the sun at any given time.Earth faces the sun at any given time.

Day Night CycleDay Night Cycle

Night and DayNight and Day

http://www.cs.sbcc.net/~physics/flash/LengthofDay.swf

Revolution – the elliptical motion of a body as it orbits another body in space.

The period of revolution for the Earth to revolve arou the sun is 365.25 days.

Kepler’s Laws of MotionKepler’s Laws of Motion11stst Law Law –Planets orbit the sun in a –Planets orbit the sun in a

closed curve called an ellipse.closed curve called an ellipse.

2nd Law2nd Law –Planets seem to move faster when they are –Planets seem to move faster when they are close to the sun and slower when they are farther close to the sun and slower when they are farther away.away.

3rd Law3rd Law –Planets close to the sun orbit faster –Planets close to the sun orbit faster than planets far away from the sun.than planets far away from the sun.

Planets

GravityGravity – the attractive force – the attractive force of one object on another of one object on another object.object.

Newton’s Law of Universal Gravity

The strength of the gravitational force The strength of the gravitational force between two objects is dependant between two objects is dependant on two factors:on two factors:

1.1. mass of the objectsmass of the objects • the more massive the object the the more massive the object the

greater the gravitational forcegreater the gravitational force

2.2. the distance between the two the distance between the two objectsobjects

• the closer the objects are together, the closer the objects are together, the greater the gravitational force the greater the gravitational force between thembetween them

The two biggest gravitational forces that we notice are:

1. The sun (1.98892 x 1030 kg) is the most massive object in our solar system and exerts a huge gravitational force. This is why all the planets stay in orbit around the sun.

2. The Earth (5.9742 x 1024 kg) is so massive that its gravitational force keeps us from falling off the planet.

When astronauts go into space, they “float” because they are far enough away from the center of the earth that the gravitational force isn’t strong enough to hold them down.

OUT: Will a planet or comet be moving faster in its orbit when it is farther from or closer to the sun? Explain.