Chapter 4 Notes: Jessica Horn

Ch 4 Making Sense of the Universe
4.1 Describing Motion: Examples from Daily Life

• How do we describe motion? Speed: Rate at which object moves. Velocity: Speed and direction. Acceleration: Any change in velocity; units of speed/time. Acceleration of Gravity. All falling objects accelerate at the same rate (not counting friction of air resistance). On Earth, g approximately = 10 m/s^2. Speed increases 10 m/s with each second falling. Acceleration of Gravity (g). Galileo showed that g is the same for all falling objects, regardless of their mass. Momentum and Force. Momentum = mass x velocity. A net force changes momentum, which generally means an acceleration (change in velocity). The rotational momentum of a spinning or orbiting object is known as angular momentum. Question: Is a net force acting on 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
• How is mass different than weight? Mass: the amount of matter in an object. Weight: the force that acts on an object. *You are weightless in free-fall. On the moon, your weight is less; your mass is the same. Why are astronauts weightless in space?
• There is gravity in space 
• Weightlessness is due to a constant state of free-fall
• How to describe motion.
• Speed = distance/time
• Speed and direction = velocity
• change in velocity = acceleration 
• momentum = mass x velocity
• force causes change in momentum, producing acceleration
• How is mass different from weight?
• mass = quantity of matter
• weight = force acting on mass
• objects are weightless in free-fall

4.2 Newton's Laws of Motion

• How did Newton change our view of the universe? He realized the same physical laws that operate on Earth also operate in the heavens: => one universe. He discovered laws of motion and gravity. Much more...experiments with light; first reflecting telescope, calculus...
• What are Newton's 3 laws of motion? 1st Law: An object moves at constant velocity unless a net force acts to change its speed or direction. 2nd Law: Force = mass x acceleration. 3rd Law: For every force, there is always an equal and opposite reaction force.
• Is the force that Earth exerts on you larger, smaller, or the same as the force you exert on it? Earth and you exert equal and opposite forces on each other.
• A compact car and large truck have a head on collision: 
• The force of car on truck is = and opposite to the force f truck on the car. True; 3rd law
• The momentum transferred from truck to car is = and opposite to momentum transferred from car to truck. True; 2nd law
• Change of velocity of car is same as the change of velocity of truck. False 
• Recap: Newton discovered laws of motion and gravitation. He realized same laws of physics were identical in the universe and on Earth.
• Newton's 3 Laws of Motion
1. An object moves at constant velocity if no net force is acting
2. force = mass x acceleration
3. for every force there is an equal and opposite reaction force

4.3 Conservation Laws in Astronomy

• Conservation of Momentum: The total momentum of interacting objects cannot change unless an external force is acting on them. Interacting objects exchange momentum through equal and opposite forces.
• What keeps a planet rotating and orbiting the Sun? Conservation of Angular Momentum. Angular momentum = mass x velocity x radius. The angular momentum of an object cannot change unless an eternal force (torque) is acting on it. Earth experiences no twisting force as it orbits the Sun, so it will... 
• Where do objects get their energy? Energy makes matter move. Energy is conserved, but it can..transfer from one object to another, change in form.
• Basic Types of Energy
• kinetic (motion)
• radiative(light)
• stored or potential 
• *energy can change type but cannot be destroyed 
• Thermal Energy: The collective kinetic energy of many particles (i.e. in a rock, in air, in water). Thermal energy is related to temp. but is NOT the same. Temp- average kinetic energy of the many particles in a substance. Thermal energy is a measure of the total kinetic energy of all the particles in a substance. It therefore depends on both temperature AND density.
• Gravitational potential energy: On Earth, it depends on...
• an object's mass (m)
• the strength of gravity (g)
• the distance an object could potentially fall
• Gravitational Potential Energy
• In space, an object or gas cloud has more gravitational energy when it is spread out than when it contracts. A contracting cloud converts gravitational potential energy to thermal energy.
• Mass - Energy: mass itself is a form of potential energy. E=mc^2
• A small amount of mass can release a great deal of energy
• Concentrated energy can spontaneously turn into particles (i.e. in particle accelerators)
• Conservation of Energy: energy can be neither created nor destroyed. It can change form or be exchanged between objects. The total energy content of the universe was determined in the Big Bang.

4.4 The Force of Gravity

• What determines the strength of gravity? The Universal Law of Gravitation:
1. Every mass attracts every other mass
2. Attraction is directly proportional to the product of their masses
3. Attraction is inversely proportional to the square distance between their centers 
• How does Newton's law of gravity extend Kepler's Laws? Kepler's first two laws apply to all orbiting objects, not just planets. Ellipses are not the only orbital paths. Orbits can be bound (ellipses), unbound (parabola, hyperbola). Newton generalized Kepler's 3 Law: Newton's version of Kepler's 3 Law: "IF a small object orbits a larger one and you measure the orbiting object's orbital period AND average distance, THEN you can calculate its mass of the larger object." 
• How do gravity and energy together allow to understand orbits? Total orbital energy (gravitational + kinetic) stays constant if there is no external force. Orbits cannot change spontaneously. Changing an Orbit => So what can make an object gain or lose orbital energy? Friction or atmospheric drag, a gravitational encounter.
• Escape Velocity: If an object gains enough orbital energy, it may escape (change from a bound to unbound orbit). Escape velocity from Earth is approximately = 11 km/s from sea level (about 40,000 km/hr).
• How does gravity cause tides? The Moon's gravity pulls harder on near side of Earth then on far side. The difference in the Moon's gravitational pull stretches Earth.
• Tides and Phases: Size of tides depend on the phase of the Moon
• Tidal Friction:  Gradually slows Earth's rotation (and makes the Moon get farther from Earth). Moon once orbited faster (or slower); tidal friction caused it to "lock" in synchronous rotation. The Moon's gravity stretches Earth and its oceans.