1. 4.4 & 4.5 Gravity, Tides and Orbits

2. Newton discovered that gravity is universal. Everything pulls on everything else in a way that involves only mass and distance. Newton’s Law of Universal Gravitation

3. The force of gravity between objects depends on the distance between their centers of mass.

4. Newton’s law of universal gravitation states that every object attracts every other object with a force that for any two objects is directly proportional to the mass of each object. Newton deduced that the force decreases as the square of the distance between the centers of mass of the objects increases. Newton’s Law of Universal Gravitation

6. Butter spray travels outward from the nozzle in straight lines. Like gravity, the “strength” of the spray obeys an inverse-square law. Gravity and Distance: The Inverse-Square Law

7. Gravitational force is plotted versus distance from Earth’s center. Gravity and Distance: The Inverse-Square Law

8. Newton showed that the ocean tides are caused by differences in the gravitational pull of the moon on opposite sides of Earth. Ocean Tides

9. The moon’s attraction is stronger on Earth’s oceans closer to the moon, and weaker on the oceans farther from the moon. This is simply because the gravitational force is weaker with increased distance. This difference in pulls across Earth slightly elongates it. Ocean Tides

10. The oceans bulge out about 1 meter on average, on opposite sides of Earth. The two tidal bulges remain relatively fixed with respect to the moon while Earth spins daily beneath them, producing two sets of ocean tides per day—two high tides and two low tides. Ocean Tides

11. Tidal Range

12. Spring tide –the largest tidal range, occurs when the tidal effects of the Sun and Moon reinforce each other –Happens at either new moon or full moon Tides & Phases

13. Neap tide –the smallest tidal range, when the tidal effects of the Sun and Moon partially cancelled each other –Happens at either first quarter or third quarter Tides & Phases

14. Tidal Friction Tidal friction gradually slows Earth rotation (and makes Moon get farther from Earth). Moon once orbited faster ; tidal friction caused it to “lock” in synchronous rotation.

15. How do gravity and energy together allow us to understand orbits? What actually causes the speed to change? More gravitational energy; Less kinetic energy Less gravitational energy; More kinetic energy Total orbital energy stays constant

16. In an elliptical orbit, at all points on the orbit—except at the perihelion and aphelion — some of the gravitational force is parallel to the direction of the planet’s motion. This part of the force changes the speed and, thus, the KE. Energy Conservation and Satellite Motion

17.  Is there anything that can cause an object gain or lose orbital energy? –Friction or atmospheric drag –A gravitational encounter. Changing an Orbit

18. If an object gains enough orbital energy, it may escape (change from a bound to unbound orbit) Escape velocity from Earth ≈ 11 km/s from sea level (about 40,000 km/hr) Escape Velocity