Chapter 4 Notes Jessica Brandon

Chapter 4 making sense of the universe understanding motion, energy, and gravity.

4.1 describing motion: examples from everyday life

*****How do we describe motion?******

-precise definitions to describe motion:

-Speed: rate at which objects moves

-Velocity: speed and direction

-Acceleration : any change in velocity units of speed/time.

Acceleration of gravity

-All falling objects accelerate at the same rest ( not counting friction of air resistance)

-On earth speed increases 10 m/s with each second of falling.

- Galileo showed that g is the same for all falling objects, regardless of their mass.

Momentum and force

-Momentum= massX 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 the net acting on each of the following?

Answer: a car coming to a stop (yes) a elevate moving at a consent speed ( no)

****How is mass different from weight?****

-Mass: the amount of matter in an object

-Weight: the force that acts on an objects

Question: on the moon..

Answer: your weight is less your mass is the same.

Why are astronauts weightless on earth?

-There is gravity in space

-Weightlessness is due to a constant state of free-fall

4.2 newtons laws of motions

*****How did newton change our view of the universe?*****

-Sir Isaac newton he realized the same physical laws that operate on earth also operate I the heavens

-He discovered laws of motion and gravity

-Much more: experiments with light first reflecting telescope, calculus.

*****What are newtons three laws of motions?*****

-Newton's first law of motion: an object moves at constant velocity unless a net force acts to change its speed or direction.

- Newton's second law of motion: force=mass X acceleration.

-Newtons third law of motion: for every force there is always an equal and opposite reaction force.

Question: is the force that earth exerts on you large, smaller, or the same as the force you exert on it?

Answer: earth and you exert equal and opposite force on each other.

4.3 conservation 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

-The angular momentum of an object cannot change unless an external twisting force (torque)  is acting on it

-Earth experiences no twisting force as it orbits the sun,

****Where do objects get their energy?*****

-Energy makes matter move

-Energy is conserved but it can... Transfer from one object o another. Change in form.

-Types of energy: kinetic(motion) radiative(light) stored of potential

-Energy can change type but cannot be destroyed

-Thermal energy: the collective kinetic energy of many particles for example in a rock, in air, in water.

-Thermal energy is related to tempura true but is not the same as temperature is the average kinetic energy for the particles in a substance must also lead to a higher total energy.

-Thermal energy is a measure of the total kinetic energy of all particles in a substance. It therefore depends on both temperature and density.

Gravitational potential energy

-On earth it depends on... An objects mass, the strength of gravity, 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.

-Mass energy: mass itself is a form of potential energy e=mc square

-A small amount of mass can release a great deal of energy.

-Concentrated turn into spontaneous turn into particles for example in particles accelerators.

Conservation of energy

Energy can be neither created nor destroyed

It can change form or be exchanges between objects.

****4.4 the force of gravity****

What determines the strength of gravity

The universal law of gravitation.

1. Every ,ass attracts every other mass

2. Attraction is directly proportional to the product of their mass.

3. Attraction is inversely proportional to the square of the distance between their centers.

How does newtons law of gravity extend keplers laws?

-Keplers  first two laws apply to all orbiting objects not just planets

-Ellipses are not the only orbiting path. Orbits can be... Bound (ellipses) unbound: parabola or hyerbola.

-Newtons generalized keplers third law.. Newtons version of keplers third law: if a small object orbits a large one and you measure the orbiting objects orbital period and distance then you can calculate the mass of the large object

Example: calculate the mass of the sun from earth orbital period (1 year).

How do gravity and energy together allow us to understand orbits?

-Total orbital energy (gravitational + kinetic) stays constant if there is no external force

-Orbits cannot change spontaneously

-less gravitational  energy more kinetic energy

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 11 km/s from sea level (about 40,00 km/hr)

How does gravity cause tides?

-the moons gravity pulls harder on near side of earth than on far side

-the difference on the moon gravitational pull stretches earth

-size of tides depend on the phase of the moon

Tidal friction

-Tidal friction gradually slows earths rotation (and makes the moon get farther from the earth)

-moon once orbited faster ( or slower) tidal friction caused it to lock in synchronous rotation.