4.1 Describing Motion: Examples from Everyday Life
How do we describe motion?
- Speed: Rate at which objects move. Speed = distance / time (units of meters / seconds)
- Velocity: Speed and direction
- Acceleration: Any change in velocity; units of speed / time (m / s^2)
- Acceleration of Gravity
- All falling objects accelerate at the same rate (not counting friction in air resistance).
- Acceleration of Gravity (g)
- Galileo showed that g is the same for all falling objects, regardless of their mass.
- Feather and hammer drop
- 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 fore acting on each of the following?
- A car coming to a stop: Yes
- A bus speeding up: Yes
- An elevator moving up at a constant speed: No
- A bicycle going around a curb: Yes
- A moon orbiting Jupiter: Yes
- Mass: The amount of matter in an object
- Weight: The force that acts on an object
- Question: On the Moon...
- A. Your weight is the same; your mass is less.
B. Your weight is less; your mass is the same.
C. Your weight is more; your mass is the same.
D. Your weight is more; your mass is less.
- A. Your weight is the same; your mass is less.
- There is gravity in space.
- Weightlessness is due to a constant state of free falling.
4.2 Newton's Law of Motion
How did Newton change our view of the universe?
- He realized the same physical laws that operate in the heavens - one universe.
- He discovered laws of motion and gravity.
- Experiments with light (optics), first reflecting telescope, calculus
- First law of motion: An object moves at constant velocity unless a net force acts to change its speed or direction.
- Second law of motion: Force = mass X acceleration
- Third law of motion: For ever force, there is always an equal and opposite reaction force.
- Question: Is the force the Earth exerts on your larger, smaller, or the same as the force you exert on it?
- A. Earth exerts a larger force on you.
- B. You exert a larger force on Earth.
- C. Earth and you exert equal and opposite forces on each other.
- Question: A compact car and a large truck have a head-on collision. Are the following questions true or false?
- The force of the car on the truck is equal and opposite to the force of the truck on the car. : True
- The momentum transferred from the truck to the car is equal and opposite to the momentum transferred from the car to the truck.: True
- The change of velocity of the car is the same as the change of velocity of the truck.: False
4.3 Conservation of Laws in Astronomy
What keeps a planet rotating and orbiting the Sun?
- 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.
- Conservation of Angular Momentum
- Angular momentum = mass X velocity X radius
- The angular momentum of an object cannot change unless an external twisting force (torque) is acting on it.
- Earth experiences no twisting force
- Energy makes matter move and is conserved, but it can transfer from one object to another
- Basic types of energy:
- Kinetic (motion)
- Radiative (light)
- Stored / potential
- Energy can change type but cannot be destroyed
- Thermal Energy: The collective kinetic energy of many particles
- Thermal energy is related to temperature but is not the same. Temperature is the 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, gravitational it depends on:
- An object's mass (M)
- The strength of gravity (g)
- The distance an object could potentially fall
- 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.
- On Earth, gravitational it depends on:
- Mass Energy
- A small amount of mass can release a great deal of energy.
- Concentrated energy can spontaneously turn into particles (particle accelerator).
- Conservation of Energy
- Energy can be neither created nor destroyed.
- It can change form or be exchanged between objects.
What determines the strength of gravity?
- The Universal Law of Gravitation
- Every mass attracts every other mass.
- Attraction is directly proportional to the product of their masses.
- Attraction is inversely proportional to the square of the distance between their centers.
- Kepler's first two laws apply to all orbiting objects, not just planets.
- Ellipses are not only orbital paths.
- Bound (ellipses)
- Parabola
- Hyperbola
- Third law: If a small object orbits a larger one and you measure the object's orbital period and average the orbital distance, then you can take the mass of the larger object.
- Total orbital energy (gravitational kinetic) stays constant if there is no external force.
- Orbits can't change spontaneously.
- Changing an orbit can make an object gain or lose orbital energy:
- Friction or atmospheric drag
- 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 ≈ 11 km/s from sea level
- The Moon's gravity pulls harder on the near side of Earth than the far side.
- The difference in the Moon's gravitational pull stretches Earth.
- Tides and Phases
- Size of tides depends on the phase of the Moon
- Tidal Friction
- Tidal friction gradually slows Earth's rotation (makes the Moon get further from Earth).
- Moon once orbited faster; tidal friction caused it to lock in synchronous rotation.
Posts
No comments:
Post a Comment