Isaac Newton - Late 17th Century Discovered Three Laws of Motion Discovered Universal Laws of Gravitation Modified Kepler’s Laws Invented calculus Invented new telescope Studied Light’s spectrum Etc. Newton

Newton’s Three Laws of Motion Objects go in straight lines at constant velocity unless you push on them When a force acts on an object, it changes speed (accelerates) according to When one object pushes on another, the other pushes back with exactly the same force F = ma

Newton’s First Law

Newton’s Third Law Q. 7: Understanding Newton’s Laws

m M r Newton’s Law of Universal Gravitation All objects have mass which tells us how much stuff they have in them Any two objects will be attracted to each other depending on their separation Force is proportional to product of their masses and inversely proportional to the square of their separation. m M r Q. 8: Newton’s Law of Gravity and Forces

Kepler’s Laws – Newton’s Version Planets move on ellipses with one focus at the Sun , parabolas, or hyperbolas

Kepler’s Laws – Newton’s Version 2. A line joining the Sun and the planet sweeps out equal area in equal time Unchanged by Newton 3. The square of the period (in years) equals the cube of the semi-major axis (in AU). times the mass of the combined objects (in solar masses) (M + m) P2 = a3 Far away move slower Massive moves faster

Solar Masses – a New Unit For describing stars, kilograms are too small (M + m)P2 = a3 This symbol means Sun Planets are much lighter than the Sun Q. 9: Using Newton’s Version of Kepler’s Third Law

Gravity and Orbits – Not Free Fall What happens when something holds you up. Gravity pulls you down Floor pushes you up Forces balance - you don’t go anywhere. You know which way is up because that is the way the floor is pushing you

Gravity and Orbits – Free Fall Cut the cable The elevator starts falling The floor no longer holds you up You fall too You float around – no gravity Because you and elevator are falling at the same rate, feels like weightlessness Free fall can be up, down, or sideways Help!

Gravity and Orbits - Circular Orbits “The knack [to flying] lies in learning how to throw yourself at the ground and miss” - The Hitchhiker’s Guide to the Galaxy trilogy, in Life, the Universe, and Everything

Q. 10: Why Is There “No Gravity” in Space? Gravity and Orbits Q. 10: Why Is There “No Gravity” in Space?

Non-Circular Orbits If velocity is more or less than this, you get an ellipse If velocity is big enough, it leaves on a parabola – escape velocity If velocity is bigger, it leaves on a hyperbola