© Copyright all rights reserved Bell Ringer What is gravity?

© Copyright all rights reserved Bell Ringer What is gravity? Gravity is a non-contact attractive force. It is what keeps us on Earth.

© Copyright all rights reserved Attraction? So gravity is a force of attraction. That means that objects feel a “pull” towards each other. This “pull” is focused on the center of the mass. This means everything gets pulled in not “down”. So far stuff you know, right?

© Copyright all rights reserved Center of mass We said the “pull” of gravity is towards the center of mass. What do we remember about mass? What has mass?

© Copyright all rights reserved Center of mass We said the “pull” of gravity is towards the center of mass. What do we remember about mass? What has mass? Right! All matter has mass! Why is this important?

© Copyright all rights reserved Why? Well, if everything has mass then everything exerts(has) a gravitational force or “pull” Earth, the Sun, your desk, you, your pencil Everything is exerting a gravitational “pull” on everything else! Sir Issac Newton observed this natural phenomenon and after much recorded experimental observations over time we have.....

© Copyright all rights reserved The Law of Universal Gravitation! This is a scientific law that states that any two objects exert a gravitational force of attraction (“pull”) on each other. This force can be described as a straight line between the two objects. The strength of the force is affected by the mass and distance of the two objects.

© Copyright all rights reserved But... If my pencil is exerting a gravitational force on me, why isn't it stuck to my forehead? Good question! That is because......

© Copyright all rights reserved But... If my pencil is exerting a gravitational force on me, why isn't it stuck to my forehead? Good question! That is because the masses are too small.

© Copyright all rights reserved Factors affecting Strength of Gravitational force Mass: larger mass means larger force Distance: larger distance means smaller force Let's explore!

© Copyright all rights reserved Mass A larger object exerts a stronger/larger gravitational force. (that is why the arrows are larger) A smaller object exerts a weaker/smaller gravitational force. (that is why the arrows are smaller)

© Copyright all rights reserved Distance A shorter distance between the objects means there will be stronger/larger gravitational force. (that is why the arrows are larger) A longer distance between the objects means there will be weaker/smaller gravitational force. (that is why the arrows are smaller)

© Copyright all rights reserved Gravity and the Formation of Solar Systems A star starts off as a nebula which is basically a cloud of gas and dust. Some of the matter in the nebula sticks together increasing its mass. More mass means more particles are attracted and stick together. So what do you think happens next?

© Copyright all rights reserved Gravity and the Formation of Solar Systems That's right—it gets bigger (more massive) which means more gravity and more particles are attracted and so on..... So the “ball” of gas and dust eventually gets so big its center gets very compacted. It gets so compacted it starts to get hot then it combusts. (we will cover this more when we talk about star life cycle) Now it is a star....

© Copyright all rights reserved Gravity and the Formation of Solar Systems Inside the star there is a lot of activity, this makes the star spin and spin (rotate) Some of the mass gets flung off the star. Over time this mass clumps together into the planets (kind of like rain drops on the window) Dense rocky planets are closer to the star because the heavier elements couldn't go as far. Less dense gas planets are further out because lighter elements traveled farther.

© Copyright all rights reserved Gravity and the Formation of Solar Systems These objects are spherical because gravity pulls matter to the center of the object

© Copyright all rights reserved Gravity and the Motion of Solar Systems They revolve around the star for the same reason. The star is constantly pulling the planet towards it ( like a big string) and only the planets forward velocity keeps it from falling into the star. What would happen if the planets sped up? What if they slowed down? Let's Explore!

© Copyright all rights reserved Gravity and the Motion of Solar Systems If the planet slowed down enough it would be pulled into the star! If it sped up enough it would fly out of orbit!

© Copyright all rights reserved Dear Uni Verse Dear Uni Verse is an intergalactic advice column for concerned celestial bodies. Uni Verse is ill and has asked us to respond to some of the letters sent in. Now that you are experts in gravity I think we can do it. Lets try some together first.

© Copyright all rights reserved Dear Uni Verse Dear Uni Verse: I am a large star. One of the planets in my solar system has an amazing moon! It is the planet that is the furthest out and while it is a large planet it is still much smaller than me. I want that moon! I know that mass is a factor in gravitational force so why doesn't the moon orbit me? Is there any thing I can do or will I just have to admire the moon from afar? Distressed by Distance.

© Copyright all rights reserved Dear Uni Verse What is the problem in the letter? What should they do? How can they solve their problem? Why should they do that? What facts can we give to them to reinforce our answer?

© Copyright all rights reserved Dear Uni Verse What is the problem in the letter? The star is jealous and wants the moon that orbits the planet. What should they do? How can they solve their problem? There is nothing the star can do. Why should they do that? What facts can we give to them to reinforce our answer? Mass isn't the only factor in gravity, distance is too and the moon is much closer to the planet.

© Copyright all rights reserved Dear Uni Verse Now let's take our answers and write a response to Distressed by Distance. Dear Distressed by Distance, I understand you dilemma but unfortunately there is nothing you can do. While it is true that mass is a factor of gravitational force, it is not the only factor. Distance affects gravitational force as well, and it sounds like the moon is much closer to the planet than it is to you. I am sorry but you will just have to admire that amazing moon from afar. Sincerely, Uni Verse

© Copyright all rights reserved Dear Uni Verse Dear Uni Verse, I am a newly formed star. I am not sure where I came from. I hope you can help me. I rotate and I know I am revolving around something in the center of my galaxy. What I don't understand is why these other things are revolving and rotating around ME! Should I be concerned? Will they keep revolving around me or will that change? Where did they all come from? I know I have a lot of questions but this is all so new and I am overwhelmed. Help me understand! Confused Celestial

© Copyright all rights reserved Dear Uni Verse What is the problem in the letter? What should they do? How can they solve their problem? Why should they do that? What facts can we give to them to reinforce our answer?

© Copyright all rights reserved Dear Uni Verse What is the problem in the letter? The star is new and it doesn't understand it's solar system. What should they do? How can they solve their problem? They shouldn't be worried, it is all normal. Why should they do that? What facts can we give to them to reinforce our answer? The objects revolving around the star are probably planets and they are revolving around the star because of the pull of gravity. It is kind of like a string holding the planets in orbit. They will orbit around the star for a very long time. The planets probably came from left over material that spun off as the star formed and rotated. Your Turn, Take our answers and write a response to Confused Celestial.

© Copyright all rights reserved Dear Uni Verse Let's share some of our responses!

© Copyright all rights reserved Group Work Work with your groups to answer the letters to Dear Uni Verse. Make sure you are responding in letter form. Give the letter writer a solution and back up your answer with facts about gravity. Ask yourself: What is the problem in the letter? What should they do? How can they solve their problem? Why should they do that? What facts can we give to them to reinforce our answer?