Chapter 9 Gravity The Earth Sucks

1.THE UNIVERSAL LAW OF GRAVITY 4From Law 3 of Kepler, Newton deduced the inverse square law of attraction. 4Newton compared the fall of the moon to the falling of an apple. 4Moon's fall from straight line is (0.05 in/s).

Newton's Law of Universal Gravitation r m1m1 m2m2

2.The Universal Gravitational Constant, G

3.Gravity and Distance: The Inverse-Square Law 4Gravity is the most dominant force in nature. 4Yet it is the weakest.

1 unit 2 units 3 units 4 units Let’s look at the inverse-square nature for the force of gravity.

4.WEIGHT AND WEIGHTLESSNESS 4W = mg (the gravitational force acting on m) 4Weighing on an elevator and Weighing in outer space or in orbit 4We define the weight of something as the force it exerts against the supporting floor or the weighing scale. (Remember the elevator video.)

5.OCEAN TIDES 4demo - Earth-moon system 4Force of gravity decreases with distance from moon. Moon Earth

4 Differences in gravity on different parts of the Earth produce tidal bulges. 4 mpeg/320px/heic0705i.html High Tides North Pole

4 Should expect two high tides, two low tides per day. –Moon moves in its orbit so that it returns to the same place in your sky every 24 h 50 m. 4 Sun has one-half the influence of the moon –The gravitational force is greater but tides depend on the gravitation difference on opposite sides of the Earth.

Sun Moon SFA

Sun Earth Spring Tides SFA New Moon Full Moon First Quarter

Sun Earth First Quarter Last Quarter Neap Tides SFA

NASA Animation MPEG SFA

4 Complications –Friction with ocean bottom –Irregular shape of continents 4 Bay of Fundi –Tides can come in very fast and exceed 15 m. SFA

Tides in the Earth and Atmosphere 4 Tides in ionosphere produce electric currents and alter the magnetic field. –Affects penetration of cosmic rays –Can have effects on living things 4 Tides in the crust mean that volcanoes and earthquakes are more common at new or full moon. SFA

Tides on the Moon 4 Just as the earth has “land tides” the moon has moon tides. 4 Because the moon always keeps the same face toward the earth, the moon tides are always in the same place. SFA Go to link

6.GRAVITATIONAL FIELDS 4 Alternate view of gravitational influence –Properties of space surrounding object are changed so that another mass entering the region experiences a force. Earth SFA

4 Objects accelerate in the direction of the arrows. 4 Field is stronger where lines are closer together. Earth SFA

The Gravitational Field Inside a Planet 4 Inside the Planet a = g a = 0 a = ½g SFA

Gravitational Field 4 Uniform Sphere –Linear inside - like Earth outside 4 Hollow Sphere –Zero inside - like Earth outside SFA

7.EINSTEIN'S THEORY OF GRAVITATION Warped Space SFA

Bending of Starlight Sun Light from star bent by the gravity of the Sun Apparent position of star in this direction Eye SFA

8.BLACK HOLES 4Let’s observe a star that is shrinking but whose mass is remaining the same. 4What happens to the force acting on an indestructible mass at the surface of the star? 4What effect is there on the escape velocity? SFA

R2R2 F R2R2 F R2R2 F R2R2 F R R R R which means the force is 64 times larger Remember that the force between the two masses is given by SFA

4If a massive star shrinks enough so that the escape velocity is equal to or greater than the speed of light, then it has become a black hole. 4Particles entering it would suffer disintegration. 4How are black holes detected? SFA

Near a Black Hole SFA

Another Look SFA

9.UNIVERSAL GRAVITATION 4Earth is a "sphere" because of gravitation. 4The universal law of gravitation was a great accomplishment. 4Its accuracy is phenomenal. 4Perturbations in planet's orbits Examples: Neptune was discovered because of Uranus's wobble. Pluto was discovered in a similar way. 4Oscillating universe. Wow!

Chapter 9 Review Questions

If somehow the moon were moved to a distance twice as far away from the earth as it is now, the force that the earth has for the moon would (a) be twice what it is now (b) be one half of what it is now (c) be four times greater than it is now (d) be one fourth as much as it is now

What must be the phase of the moon if neap tides are occurring? (a) first quarter (b) full (c) lunar eclipse (d) new

Newton said that the path of a planet curves because a force acts on it. Why does a planet's path curve according to Einstein? (a) because a force acts on it (b) because it is spinning (c) because it is in a geometric warp of space and time End of Course End of Course End of Chapter End of Chapter Test 3 Questions Test 3 Questions

4 Discuss final 4 Parting thoughts next slide