Making Sense of the Universe Understanding Gravity and Motion

Astronomical Motion Planets move along curved (elliptical) orbits Speed & direction are constantly changing Is a force present? Yes!

Gravity is that force!

Gravity gives Universe its structure Universal attractive force that causes all objects to pull on all other objects everywhere Holds Moon in orbit around Earth Holds Earth in orbit around Sun Sun in orbit around Milky Way Milky Way in path within Local Group of galaxies

Sir Isaac Newton Invented reflecting telescope Invented calculus Connected gravity & planetary forces Philosophiae naturalis principia mathematica

Universal Laws of Motion “If I have seen farther than others, it is because I have stood on the shoulders of giants.” Sir Isaac Newton (1642 – 1727) Physicist

How do we describe motion? Speed: Rate at which object moves speed of 10 meters/second = 10 m/s speed of 22 miles/hour = 22 mph Velocity: Speed and direction 10 m/s, due east 22 mph down Hesperian Blvd. Basic vocabulary of motion. Emphasize that turning, slowing, and speeding up are all examples of acceleration.

How do we describe motion? Velocity: Speed and direction 10 m/s, due east Acceleration: Any change in velocity; either in direction, magnitude, or both Basic vocabulary of motion. Emphasize that turning, slowing, and speeding up are all examples of acceleration.

The Acceleration of Gravity As objects fall, they move faster & faster. They accelerate.

The Acceleration of Gravity Acceleration from gravity is constant near Earth’s surface Falling speed increases by 10 meters/second each second

The Acceleration of Gravity g = ~10 meters/sec/sec g = ~22 miles/hour/sec

The Acceleration of Gravity Falling from rest… after 1 second, moving 10 m/s (about 22 mph) after 2 seconds, moving 20 m/s (about 44 mph)

The Acceleration of Gravity Acceleration from gravity is constant FOR ALL OBJECTS near Earth’s surface Why don’t heavier things fall faster than lighter ones???

Acceleration of Gravity (g) Galileo demonstrated g same for all objects, regardless of mass! Heavier objects (with more mass) must be pulled more to accelerate at same rate. And Gravity indeed pulls more on heavier objects!

How is mass different from weight? Mass = amount of matter in an object (protons, neutrons, electrons) Weight = force from gravity that acts upon an object from another mass

Question: On the Moon….? your weight is the same, your mass is less. your weight is less, your mass is the same. your weight is more, your mass is the same. your weight is more, your mass is less.

Question: On the Moon…. your weight is the same, your mass is less. your weight is less, your mass is the same. your weight is more, your mass is the same. your weight is more, your mass is less.

Newton’s Laws of Motion 1 A body at rest or in motion at a constant speed along a straight line remains in that state of rest or motion unless acted upon by an outside force.

Newton’s 1st Law Planets orbit stars stay in motion, but are continually being pulled in their orbit by the star. Rockets heading to the moon or Mars, once launched, can coast along a straight line.

Newton’s 2nd Law of Motion

Newton’s 2nd Law The change in a body’s velocity due to an applied force is in the same direction as the force and proportional to it, but is inversely proportional to the body’s mass. F = m a Launch a rocket – as fuel is used up, mass decreases, and rocket accelerates even faster! “Staging” rockets is even smarter!

Thought Question Is there a net force for each of the following? A car coming to a stop. A bus speeding up. An elevator moving up at constant speed. A bicycle going around a curve. A moon orbiting Jupiter. Use to check if students can figure out where a net force is acting. For bonus questions, ask if they can identify the forces.

Thought Question Is there a net force for each of the following? A car coming to a stop. Yes A bus speeding up. Yes An elevator moving up at constant speed. No A bicycle going around a curve. Yes A moon orbiting Jupiter. Yes Use to check if students can figure out where a net force is acting. For bonus questions, ask if they can identify the forces.

Newton’s 3rd Law of Motion

Newton’s 3rd Law For every applied force, a force of equal size but opposite direction arises. As rocket exhaust pushes backwards, the rocket itself moves forwards As Earth pulls on the Moon, the Moon pulls on Earth

Universal Law of Gravitation Between every two objects there is an attractive force, the magnitude of which is directly proportional to the mass of each object and inversely proportional to the square of the distance between the centers of the objects.

Universal Law of Gravitation Between every two objects there is an attractive force, the magnitude of which is directly proportional to the mass of each object and inversely proportional to the square of the distance between the centers of the objects.

Law of Gravity Between every two objects there is an attractive force, the magnitude of which is directly proportional to the mass of each object and inversely proportional to the square of the distance between the centers of the objects. Gravity is ONLY attractive! There is no “anti-gravity”

Gravity Between every two objects there is an attractive force, the magnitude of which is directly proportional to the mass of each object and inversely proportional to the square of the distance between the centers of the objects. What kind of Mass? It doesn’t matter ! What shape, size, temperature, state? It doesn’t matter!!

Why are astronauts weightless in space? There is no gravity in space. The force of gravity is much less. The moon is pulling astronauts in the other direction. The Earth’s magnetic field holds them up. They are massless.

Why are astronauts weightless in space? There is no gravity in space. The force of gravity is much less. The moon is pulling astronauts in the other direction. The Earth’s magnetic field holds them up. They are massless.

Why are astronauts weightless in space? There is no gravity in space. The force of gravity is much less. The moon is pulling astronauts in the other direction. The Earth’s magnetic field holds them up. They are massless. Gravity IS pulling them towards Earth. They ARE falling!

Why are astronauts weightless in space? There is gravity in space! Gravity is always attractive! They *have* to be coming towards Earth! Weightlessness is due to a constant state of free-fall.

How can they ORBIT?

How can they ORBIT? As spacecraft fall, they also move sideways fast At 350+ miles above Earth: Falling towards Earth continuously, but… Moving SIDEWAYS at 17,000 miles per hour Orbits are continuous falling “around” Earth!

Illustrating Gravity with Tides Why are there two high tides each day? Why are tides on Earth caused primarily by the Moon rather than by the Sun? Why is Earth’s rotation gradually slowing down? Why does the Moon always show the same face to Earth?

Tides Gravitational force decreases with (distance)2,

Tides Since gravitational force decreases with (distance)2, the Moon’s attractive pull on Earth is strongest on the side closer to the Moon & weakest on the opposite side.

Tides

Tides Earth gets stretched along the Earth-Moon line. Oceans rise relative to land at these points.

Tides - Observations Every place on Earth passes through these points, called high tides, twice per day as the Earth rotates. SF Bay Tide Tables

Tides - Observations High tides occur every 12 hours… plus! Average difference between high tides = 12 hours and 25 minutes Why not exactly 12 hours? The Moon moves!

Tides - Observations Tides are tied to the phases of the moon

Tides - Observations High tides occur every 12 hours 25minutes the Moon moves! The Sun’s tidal effect on Earth is not as strong About ½ as large as the Moon But when BOTH stretch in the same direction, even larger tides!

Tides - Observations Tides are tied to the phases of the moon

“Spring” Tides When Sun & Moon pull in the same direction (new & full phases) high tide is HIGHER than usual

“KING” Tides When Sun & Moon pull in the same direction (new & full phases) AND… Earth is closest to the Sun (January!) high tide is HIGHER than usual

Spring Tides in Miami

Spring Tides in California

Spring Tides in California

“Neap” Tides When Sun & Moon pull at right angles (first & last quarter phases) high tide is LOWER than usual

Tidal Friction Reaction between Moon’s pull & Earth’s rotation. Earth’s rotation slows down (1 sec every 50,000 yrs.) Moon moves farther away from Earth.

Where’s the PROOF? Stromatolites! Earth’s rotation slows down

Synchronous Rotation When rotation period of a moon, planet, or star equals its orbital period about another object. Tidal friction on the Moon (caused by Earth) has slowed its rotation down to 1 month. The Moon now rotates synchronously. We always see the same side of the Moon.

Orbital Paths Extending Kepler’s Law #1, Newton found that ellipses were not the only orbital paths. possible orbital paths ellipse (bound) parabola (unbound) hyperbola (unbound)

Newton’s Version of Kepler’s Third Law Using calculus, Newton was able to derive Kepler’s Third Law from his own Law of Gravity. In its most general form: P2 = 42 a3 / G (m1 + m2) If you can measure the orbital period of two objects (P) and the distance between them (a), then you can calculate the sum of the masses of both objects (m1 + m2).

Changing Orbits orbital energy = kinetic energy + gravitational potential energy conservation of energy implies orbits can’t change spontaneously An object can’t crash into a planet unless its orbit takes it there.

Changing Orbits An orbit can only change if it gains/loses energy from another object, such as a gravitational encounter If an object gains enough energy so that its new orbit is “unbound” it has reached escape velocity.

Changing Orbits We can use the gravitational pull of planets to “slingshot” spacecraft to other parts of the solar system! “Gravitational Assist” cuts travel time to outer solar system by years!