Copernicus put the Sun at the center. This was WAY simpler than Ptolemy’s model. In science, one basic rule is that the simplest explanation for an observation is most likely the correct one. We call this Ocham’s razor. In this case, Ocham’s razor makes it pretty obvious to us that planets making circles around a huge star makes a lot more sense than everything in the universe making strange loopdy-loops around the Earth. At the time though, people were not comfortable with Copernicus’s new theory. Ptolemy’s model had reigned for millenia, and had become part o the mythology of the time, and the dogma of the church.
Galileo provided proof that the Earth was not the center of a system of glass spheres, using the telescope he pointed upward, which we learned about last week. Remember that Galileo did not invent the telescope, but he took the idea of putting two lenses together in a tube, and he pointed it upward for the first time. He saw amazing things. He saw the phases of Venus.
Galileo provided proof that the Earth was not the center of a system of glass spheres, using the telescope he pointed upward, which we learned about last week. Remember that Galileo did not invent the telescope, but he took the idea of putting two lenses together in a tube, and he pointed it upward for the first time. He saw amazing things. He saw the phases of Venus.
Galileo provided proof that the Earth was not the center of a system of glass spheres, using the telescope he pointed upward, which we learned about last week. Remember that Galileo did not invent the telescope, but he took the idea of putting two lenses together in a tube, and he pointed it upward for the first time. He saw amazing things. He saw the phases of Venus.
Kepler’s laws described very well wht was going on in the solar system, but they didn’t really say WHY.
Planets move in elliptical orbits with the Sun at one focus of the ellipse.
The second law tells us that the how fast a planet is moving around the Sun changes so that a line joining the Sun and the planet will sweep out equal areas in equal time intervals. So the triangles on the left and right here are the same area, so that means it will take a planet the same amount of time to move from here to here, as here to here.In other words, the closer a planet is to the Sun, the faster it moves.Keep in mind that although it is true that planets orbit the Sun in an ellipse, not a perfect circle, the ellipse is not as pronounced as in these pictures. In fact, they look almost like circles. But they are really ellipses. Still, it doesn’t bring us close enough to the Sun for us to notice any major differences between when we are here on the left, and when we are here on the right.
Kepler’s third law says that the length of time that it takes a planet to circle the sun, P is related to how far away that planet is from the Sun. This law implies that a planet with a larger average distance from the Sun, which is the semimajor axis distance, a, will take longer to circle the Sun. The amount of time a planet takes to orbit the Sun is related to its orbit’s size…here they call the time it takes for a planet to go around the sun the “period”…what do we commonly call the time it takes for a planet to go around the Sun? (A year.)Third law can be used to determine the semimajor axis, a, if the period, P (or year), is known, a measurement that is not difficult to make.Still, Kepler’s laws were only describing what he observed, they did not provide a deeper reason for why the planets did what they did.
Newton had the why. Newton invented the reflector, the telescope with mirrors instead of lenses that we saw last week. He was also kind of a brilliant loony, he kept to himself, remained a virgin his entire life, and invented calculus without telling anyone. He did tell people eventually, but he kept it locked up in his trunk for years first. And he came up with these three essential laws of motion, all of which can be used to explain at a basic level what Kepler observed. These were Newton’s 3 laws.
So moving this way at 2 miles per hour is NOT THE SAME VELOCITY as if I moving this way at 2 miles per hour. Velocity means both speed, and direction.We can define a force very generally using this law. It is something, anything that causes a change in the velocity of an object. So that means that it causes a change in either the speed, or the direction, or both. So if this ball is rolling straight, and I push it so it goes FASTER, that is a change in speed, and a force has been applied. By me in this case. If I push it to the side so it is still going the same speed, but in a new direction, I have also applied a force. If it is rolling and I stop it, I have applied a force, because the velocity has changed from something, to 0.An objects velocity will NEVER CHANGE unless there is a force acting on it. That is what this law is saying. If there is no force, an object will just go forever at whatever speed and direction it is already going in. So that means that if I roll this ball, it will keep moving forever unless there is a force acting on it. Let’s try. (The ball does not roll forever). Okay, WHAT HAPPENED?The answer is, FRICTION happened. Friction is a FORCE. It is the force caused by when one kind of matter rubs against another kind of matter. On Earth, we can try to make friction as low as possible, but we cannot get rid of it all together. If the floor was made of ice, for example, I could roll this ball for much longer before it stops. There is much less friction force being exerted on the ball when it is on ice. But still , there is a force rubbing between the ice and the ball, and eventually it will stop. In space, however, something will float forever at the exact same speed, unless a force is applied. I could kick this in space, and it would never stop unless it floated close by a planet or a star, or someone else applied a force.
So somebody apply a force to this apple. How much force did you apply? That is the question addressed by Newton’s second law. It says that the force on an object is determined by the mass of the object, how much STUFF is in it, and the ACCELERATION of that object. What is acceleration? It is the change in the velocity of an object. This law is actually very simple. It is serving to tell us what a force is in a more specific way than what we said earlier. F, force, is equal to a mass, that is accelerating! A force is something that causes a mass to accelerate, and it is determined by how much that mass accelerates. That is all this law is telling us. So in the case of this apple, the mass of the apple let us say is about 1/10 kg. So if I push it so that in the first second it changes from 0 meters per second velocity, to 10 meters per second velocity, its acceleration is 10 meters per second per second, or 10 m/s2. So in that case, the force I put on it was 1m kg/s2, which we call a Newton. I put one Newton of force on this apple.
Every action has an equal and opposite reactionLet’s watch a clip from Wall-E, that I think exemplifies this fairly well.http://youtu.be/pVRgfDSAGOASo when I push this apple, it is actually pushing back on me with the EXACT same force that I am pushing on it. EXACTLY. What does this mean? Why am I not moving if it is pushing on me with the same force?F = (mass of apple)(acceleration of apple) = (mass of Zoe)(acceleration of Zoe)=mA=Ma Big mass, so small acceleration. It’s so small, in fact, I barely notice it, and it doesn’t even make my feet move. Now if I push this wall, it is much more massive than the apple. If I push hard enough, the force will be enough to make my feet move.
The most famous story about Newton is that of the falling apple. Legend has it that the apple fell on his head, but no one knows if this is really true. What he did do, was wonder out loud, what is the difference between the Moon, and an apple falling from a tree? What if they were both governed by the same force? What if they were both doing the same thing? Turns out he was right, and he revolutionized the way we think about the Universe, and we are going to talk more about that, later!
