Physics Principles: Motion, Forces, and Momentum

Posted on Mar 30, 2025 in Physics

Describing Motion

  • Motion: Change of position relative to other objects.
  • Reference point: A place or object used for comparison to determine if something is in motion.
  • International System of Units (SI): The system used to describe motion measurements.
  • Distance: The length of the path between two points.

Speed and Velocity

  • Speed: The distance an object moves per unit of time.
  • Calculating speed: To calculate speed, divide the distance by the time (Speed = Distance / Time).
  • Calculating average speed: To find the average speed, divide the total distance traveled by the total time.
  • Instantaneous speed: The speed at which an object is moving at a given instant in time.
  • Velocity: Speed in a given direction.
  • Graphing Motion: You can show the motion of an object on a line graph by plotting distance vs. time. By convention, time is shown on the horizontal axis (x-axis), and distance is shown on the vertical axis (y-axis). A point on the line represents the distance an object has traveled at a particular time.
  • Slope: The steepness of a line on a graph.
  • What slope tells you: The slope indicates how fast one variable changes in relation to the other variable in the graph; in other words, the slope represents the rate of change. Since speed is the rate that distance changes in relation to time, the slope of a distance vs. time graph represents speed. A steeper slope indicates a greater speed. A constant slope represents motion at a constant speed.

Acceleration

  • Acceleration: The rate at which velocity changes. This can involve changes in speed, direction, or both.
  • Calculating acceleration: Calculate acceleration by subtracting the initial speed from the final speed, then dividing the change in speed by the time (Acceleration = (Final Speed – Initial Speed) / Time).
  • Graphing Acceleration: You can use both a speed vs. time graph and a distance vs. time graph to analyze acceleration.

Forces

  • Force: A push or a pull.
  • Newton (N): The strength of a force is measured in the SI unit called the newton.
  • Net force: The overall force acting on an object when all individual forces are combined. The net force determines if and how an object will accelerate.

Friction and Gravity

  • Friction: The force that two surfaces exert on each other when they rub against each other.
  • Sliding Friction: Occurs when two solid surfaces slide over each other. Sliding friction makes moving objects slow down and stop (e.g., helps a penguin stop before hitting a wall).
  • Static Friction: Acts between objects that are not moving. Static friction is the force that resists initial movement (e.g., the force you must overcome to push a couch across the room).
  • Fluid Friction: Occurs when a solid object moves through a fluid (like air or water). Fluid friction is often easier to overcome than sliding friction (e.g., why sidewalks get slippery when wet, reducing friction).
  • Rolling Friction: Occurs when an object rolls across a surface. Rolling friction is much easier to overcome than sliding friction (e.g., allows a skateboard to roll easily).
  • Gravity: A force that pulls objects towards each other.
  • Gravity acts everywhere on Earth, pulling objects towards the center of the planet.
  • Mass: The amount of matter in an object.
  • Weight: A measure of the force of gravity on an object (Weight = Mass × Acceleration due to gravity).
  • Inertia (Newton’s First Law of Motion): The resistance of any physical object to any change in its state of motion (including changes to its speed or direction).
  • Inertia depends on mass: objects with more mass have more inertia, and objects with less mass have less inertia.
  • Newton’s Second Law of Motion: An object’s acceleration depends on its mass and the net force acting on it (Acceleration = Net Force / Mass).
  • Determining acceleration: You determine acceleration by dividing the net force acting on an object by its mass.
  • Newton’s Third Law of Motion: States that if one object exerts a force on another object, then the second object exerts a force of equal strength in the opposite direction on the first object (often stated as “for every action, there is an equal and opposite reaction”).

Momentum

  • Momentum: A characteristic of a moving object related to its mass and velocity.
  • Calculating momentum: To find momentum, multiply the object’s mass by its velocity (Momentum = Mass × Velocity).
  • Law of Conservation of Momentum: States that in the absence of outside forces (like friction), the total momentum of objects that interact does not change.

Free Fall & Circular Motion

  • Free Fall: The motion of an object where gravity is the only force acting on it.
  • Centripetal Force: A force that causes an object to move in a circular path. It is directed towards the center of the circle.