Class 9/19/12

CHAPTER 4 MAKING SENSE OF THE UNIVERSE: UNDERSTANDING MOTION, ENERGY, AND GRAVITY

·         Speed: Rate at which object move.

·         Velocity: Speed and direction

·         Acceleration: Any change in velocity; units of speed/time m/s²

ACCELERATION OF GRAVITY

·         All falling objects accelerate at the same rate (not counting friction of air resistance).

·         On Earth, g almost equal to 10 m/s²: speed increases 10 m/swith each second offalling.

·         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.

·         Mass: the amount of matter in an object

·         Weight: the force that acts on an object.

WHY ARE ASTRONAUTS WEIGHTLESS IN SPACE?

·         There is gravity in space.

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

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…

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 external twisting force (torque) is acting on it.

·         Earth experiences no twisting force as it orbits the
Sun, so its rotation and orbit will continue indefinitely.

Angular momentum conservation also explains why objects rotate faster as they shrink in radius.

WHERE DO OBJECTS GET THEIR ENERGY?

·         Energy makes matter move.

·         Energy is conserved,but it can…

ü  Transfer from one object to another.

ü  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 (for example, in a rock, in air, in water)

Thermal energy is related to temperature but it is NOT  the same thing.

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, it depends on…

ü  An object’s mass (m).

ü  The strength of gravity (g).

ü  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 = mc²

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

·         Concentrated energy can spontaneously turn into particles (for example, 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 and remains the same today

4.4 THE FORCE OF GRAVITY

 HOW DOES GRAVITY CAUSE TIDES?

·         The Moon’s gravity pulls harder on near side of Earth than on far side.

·         The difference in the Moon’s gravitational pull……

Size of tides depends on the phase of the Moon