Law of universal gravitation

• Misconceptions about falling objects videohttp://www.youtube.com/watch?v=_mCC-68LyZM (Veritasium)

• Video from the universe beginning http://www.youtube.com/watch?v=DmUiCweDic4 (TED)

Lesson 1 Focus questions

• What do we mean when we say gravity is always an “attractive” force?

• What is G?• Why is the law of universal gravitation called

an “inverse square law”• Is gravity a field or contact force?Handout: Planetary Data Sheet (NASA)

Notes- Law of Universal Gravitation

• Any two masses are gravitationally attracted to each other

• Newton’s Law of Universal gravitation: Fg = G m1 m2 G= 6.673x10-11N•m2

r2 kg2

r is the distance from the center of one object to

the center of the other

rm1

m2

Notes- Law of universal grav.

• As the distance r between the objects increase what happens to the force? ________

• If mass increases and the distance between the objects stays the same, how does this affect the force? ____________

• Sooooo.. the closer the objects the ________the more massive the objects the ______

Notes- Law of universal grav.

• As the distance r between the objects increase what happens to the force? decreases

• If mass increases and the distance between the objects stays the same, how does this affect the force? increases

• Sooooo.. the closer the objects the greater the forcethe more massive the objects the the greater the

force

Notes Example……

• What is the gravitational force of attraction between Pluto and the sun?

Note: masspluto = 1.309 x 1022 kg

mass sun = 1.98x1030 kg

r pluto-sun= 5.914x 109 m

Applying Newton’s Law of Universal gravitation- example

• Two students are 5.0m apart. One has a mass of 35kg, the other has a mass of 70 kg. What is the force of gravity from one student on the other?

Now you try

• Calculate gravity on the surface of the Earth.

Circular Motion- The Big Idea

Big idea: to maintain circular motion requires a force directed inward. This is caused by gravity (for planets)

Putting it all togher!!!

• From the last unit we learned to maintain circular motion you need an inward directed force . IN THE CASE OF A PLANET ORBITING THE SUN THAT INWARD DIRECTED FORCE IS THE FORCE OF GRAVITY!!!!!!!

orbital motion: Fc = Fg

So mv2 = Gm1m2

r r2

Worksheet

• In small groups work on the worksheet showing your steps to proper problem solving, boxing answers and including appropriate units!