Chapter 4

4.1 Describing Motion: Examples from Everyday Life

Speed= Rate at which an object moves

- speed= distance/time (units of m/s)

Velocity= Speed and direction

- 10 m/s due east

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 of air resistance)
• On Earth, g is approximately 10 m/s^2; speed increases 10 m/s with each second of falling 
• Galileo showed that g is the same for all falling objects, regardless of 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 and Weight

• 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.

4.2 Newton's Laws of Motion

• Newton realized the same physical laws that operate on Earth also operate in the heavens. 
• He discovered laws of motion and gravity
• He also did more: Experiments with light, first reflecting telescope, calculus.

Newton's Three Laws of Motion

1. An object moves at constant velocity unless a net force acts to change its speed and direction.
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. 

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.

Energy

• Energy makes matter move
• Energy is conserved, but it can transfer from one object to another and change in form.

Basic Types of Energy

• Kinetic (motion)
• Radiative (light)
• Stored or potential
• Energy can change but cannot be destroyed

Thermal Energy

• Thermal energy is related to temperature but it is not the same, temperature is the average kinetic energy of the many particles in the 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, the strength of gravity, and 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^2)
• A small amount of mass can release a great deal of energy
• Concentrated energy can spontaneously turn into particles (for example, in particle accelerators)

4.4 The Force of Gravity

Tides

• The Moon's gravity pulls harder on the near side of the Earth than on the far side.
• The difference in the /moon's gravitational pull stretches Earth.
• Size of the tides depends on the phase of the Moon.