Practical Physics for College Students - Tim L

Most motion examples involve movement in the same or opposite directions, termed linear or translational motion.
Translational motion involves movement from one point to another, disregarding spins or rolls.
One-dimensional motion involves movement in one way or its exact opposite.
Objects can move in two dimensions (2-D) or three dimensions (3-D) simultaneously.
2-D motion includes movement right and up, or north and east.
3-D motion includes movement east, north, and up into the sky.

Compass points are used for precise direction.
North, south, east, and west are primary points; angles are used for more precise measurements.
North is 0°, east is 90°, south is 180°, and west is 270°.
Northeast is halfway between north and east, at 45°.
A position slightly east of north, like 10° east of north, is not 10° northeast but rather the same as 80° north of east.

The word momentum has common uses, but in physics, it has a specific definition.
The physics definition of momentum is a mathematical formula.

Momentum equals mass times velocity.
$Momentum = mass \times velocity$
Objects with more momentum are harder to stop.
Momentum has direction and is a vector.
Inertia and momentum are different; inertia remains constant whether an object is still or moving, but momentum is zero when an object is still.
Impulse is related to momentum, involving force applied over time.
Impulse is defined as force times time.
$Impulse = force \times time$
Impulse changes an object's momentum.
Applying brakes to a moving car slows it down; harder or longer braking increases the effect.
Momentum is conserved unless an outside force acts upon the object.
A change in momentum is associated with force and time, or impulse.
Impulse causes a change in momentum, and its magnitude equals the change in momentum.

Work has a specific definition in physics, requiring observable motion.
If you push on a wall and it doesn't move, no work is done on the wall. (Though your muscles may be doing work internally.)

Work is defined as the force on an object times the distance the object moves in the direction of that force.
$Work = Force \times Distance$
Only forces parallel to the displacement do work.
If the force is not in the same direction as the motion, use the component of the force in the direction of the motion.
Sliding a table across the floor is work; carrying it upstairs is more work because it requires greater force.
The force required to lift something straight up at a steady speed is equal to the force of gravity but in the opposite direction.
The force used for objects moving horizontally at a steady speed is equal but opposite to the force of friction.
If there is an acceleration, the force is calculated using Newton's Second Law.
A net force of zero produces no acceleration or a steady speed.
If something is moving with a steady speed, the force moving the object is exactly equal to a force in the opposite direction.
To drag a box across the floor, your pull must equal the resistance of friction.
To carry a box upstairs, you must lift the weight of the box by exerting an equal force upward.

Physics laws describe how, when, or where things happen, rather than why.
They describe predictable patterns mathematically.
Theories attempt to address the "why" but often rely on laws.
The part of physics that deals with the causes of motion is called dynamics.
Isaac Newton developed the laws of motion in his twenties during a plague in England (around 1665).
He formulated concepts explaining gravity, leading to books published about 20 years later.
His book, "Philosophiæ Naturalis Principia Mathematica" (Mathematical Principles of Natural Philosophy), describes most motion seen today.

An object at rest tends to remain at rest, and an object in motion tends to remain in motion in a straight line unless acted upon by an outside force.

Physics includes the study of anything that moves.
Chapter 2 covers the initial ideas of observing and describing motion.
Most students have some correct and some incorrect concepts of motion like speed and acceleration.
The study of motion is kinematics, and the study of the causes of motion is dynamics.

Start with the concept of sitting still.
Even when sitting still, one is moving with the Earth's spin at about 1,000 miles per hour.
Treating a room as sitting still is called a frame of reference.
The starting position is the initial or original position, and the later position is the final position.
Position can be described by an address, room number, or coordinates.
Positional descriptions often give two pieces of information.
Position is often given as x, y coordinates or longitude and latitude, which need an origin to make sense.
The x-y plane uses (0,0) as the origin, and a position on the globe is measured from the equator and the prime meridian.

The change in position can be called the distance moved.
Distance should include a unit, and the direction is often important.
Sometimes the distance is the actual motion, but sometimes the straight-line distance is needed.