Astronomy Exam 2 (Ch. 4)
Momentum
Mass x Velocity
What do we need to know if we want to measure an object's mass with "Newton's version of Kepler's third law"?
-"Newton's version of Kepler's third law" allows us to calculate the mass of a distant object, if we measure the orbital period and distance of another object orbiting around it ex) We can measure the mass of the sun from Earth''s orbital period (1 year) and its average distance (1 AU)
Law of Conservation of Momentum
-As long as there are no external forces, the total momentum of interacting objects cannot change; that is, their total momentum is conserved. -An object can gain or lose momentum only if another object's momentum changes by a precisely opposite amount. Ex) In space, objects always move at same speed because there are no external forces
Law of Conservation of Angular Momentum
-As long as there is no external torque, the total angular momentum of a set of interacting objects cannot change -An objects angular momentum can change ONLY by transferring some angular momentum to or from another object. Ex) Earth constantly moves in the same orbit at the same speed because there is no external torque to change it.
Law of Conservation of Energy
-Energy can't appear out of nowhere or disappear to into nothingness. -Objects can gain or lose energy only by exchanging energy with other objects Ex) All actions on Earth involve exchanges of energy or the conservation of energy from one form to another.
Universal Law of Gravitation
-Every mass attracts every other mass through the force called Gravity -The strength of the gravitational force attracting any two objects is directly proportional to the product of their masses. ex) Doubling the mass of one object doubles the force of gravity between the two objects. -The strength of gravity between 2 objects decreases by the "square" of the distance between their centers ex) doubling the distance between two objects weakens the force of gravity by a factor of 2^2, or 4.
Explain why orbits cannot change spontaneously, and how a "gravitational encounter" can cause a change. How can an object achieve "escape velocity"?
-Orbits cannot change spontaneously, UNLESS another object causes the planet to gain or lose energy. -Left undisturbed, planets will forever keep the same orbits in their galaxies. -Gravitational Encounter: 2 objects exchange energy if they pass near enough that each can feel the effects of the other's gravity. ex) When a comet passes very close to a planet, the comet's orbit will change dramatically due to the gravity exerted from the planet. Escape Velocity: The minimum amount of energy required to escape Earth's gravity for a spacecraft that starts on the surface.
Acceleration of Gravity
-The acceleration of a falling object (abbreviated g) -As an object falls, its speed is increased 10m/s ex) When a rock is dropped off the top of a building, it goes 0m/s when its dropped, 10m/s after 1 second, 20 m/s after 2 seconds, etc. THEREFOR, we say that the Acceleration of Gravity is 10 meters per second per second, or 10 m/s^2
Bound vs. Unbound orbit
-Unbound: Paths that bring an object close to another one just once (parabolas or hyperbolas) -Bound: An object orbits around the same object over and over again (make ellipses)
Newton's 3 Laws of Motion, with examples:
1) An object moves at constant velocity if there is no net force acting upon it -ex) A car parked on a flat street won't suddenly start moving. When the car is driven, and then the foot is taken off the pedal, it will go at the same speed until air and friction slows it down. 2) Force = Mass x Acceleration Force = Rate of Change in momentum -ex) You can throw a baseball further than a shot put because your arm gets more acceleration with a baseball. 3) For any force, there is always an equal and opposite reaction force. -ex) Force is what keeps us on the ground of earth, but earth itself is also pushing us at an equal force towards space. Since they're equal, we stay on the ground.
Velocity
Both speed and direction
What is free-fall, and why does it make you feel weightless? Briefly explain why astronauts are weightless in the space station.
Free-fall: falling without any resistance to slow you down. The floor drops away at the same rate that you fall, allowing you to float freely above it, causing you to be weightless. Astronauts are constantly weightless because they're in a constant rate of free-fall. Everything orbiting around Earth is constantly "falling around" (orbiting) earth -EX) the running and jumping example from the book
Speed
How far something will go in a certain amount of time
What do we mean by "mass-energy"? Define e=mc squared.
Mass-Energy: Mass itself is a form of energy -e=mc^2 tells us that a small amount of mass contains a huge amount of energy. -It also tells us that energy can be converted into mass ex) The Big Bang Theory turned energy into mass, creating everything in the universe including us.
What do we mean when we say that momentum can be changed only by a net force?
Momentum is not necessarily changed by force, but it is changed when its net force (overall force) is not zero.
Temperature vs. Thermal Energy
Temperature: measures the AVERAGE kinetic energy of the particles. Thermal Energy: measures the TOTAL kinetic energy of all the randomly moving particles in a substance -SIMILAR because thermal energy depends on temperature -DIFFERNT because TEMPERATURE measures the avg. speed of movement, and THERMAL measures the mass of a substance
Acceleration
The change in velocity in any way, whether in speed, direction, or both.
Force
The only way to change an object's momentum is to apply force to it
