PHYS 1600 Notes - Forces and Concepts

Forces Vocabulary

  • Force: A push or pull on an object resulting from its interaction with another object.
  • Mass: A measure of the amount of matter in an object, usually in kilograms (kg).
  • Inertia: The resistance of an object to any change in its state of motion; an object at rest stays at rest, and an object in motion stays in motion unless acted upon by a net external force.
  • Static Force: Force acting on an object at rest, preventing motion.
  • Dynamic Force: Force acting on an object in motion, affecting its velocity.
  • Resultant Force: The vector sum of all forces acting on an object.
  • Equilibrium: A state where all forces acting on an object are balanced, resulting in no net force and no motion change.
  • Net Force: The overall force acting on an object after all forces are combined.
  • Contact Force: Forces that occur when two objects are in contact with each other (e.g., friction, tension).
  • Long-range Force: Forces acting at a distance, such as gravitational force.
  • Weight: The force exerted by gravity on an object's mass.
  • Tension: The pulling force transmitted along a string, rope, or wire.
  • Normal Force: The force perpendicular to the surface that supports the weight of an object resting on it.
  • Incline: A slope or angle that can affect the forces acting on an object.
  • Free-body Diagram: A graphical representation used to visualize the forces acting on an object, showing all vectors and their points of application.

Equations

  • F⃗x = m⃗ax

  • Variables:

    • F⃗x = net force in the x-direction
    • m = mass
    • ax = acceleration in the x-direction

  • Use: To calculate acceleration or net force when the mass and net force in the x-direction are known.

  • F⃗y = m⃗ay

  • Variables:

    • F⃗y = net force in the y-direction
    • m = mass
    • ay = acceleration in the y-direction

  • Use: To calculate acceleration or net force in the y-direction.

  • FG = mg

  • Variables:

    • FG = weight (gravitational force)
    • m = mass
    • g = acceleration due to gravity (approximately 9.81 m/s² on Earth)

  • Use: To find the weight of an object based on its mass.

Questions

  1. Constant Velocity with a Single Force:
  • An object can move with a constant velocity when the net force acting on it is zero, meaning all forces balance out.
  1. Zero Acceleration with External Forces:
  • Yes, an object's acceleration can be zero even when external forces are present if those forces balance out (net force is zero).
  1. Net Force on a Rock at the Top:
  • The net force acting on the rock at the top of its trajectory is equal to its weight (downward) since it has zero velocity but is subject to gravitational force.
  1. Moving Object with Zero Net Force:
  • A good example is an object moving at constant velocity in a frictionless environment with no net external forces acting on it.
  1. Ignoring Internal Forces:
  • Internal forces can be ignored when calculating the net force because they operate within the system and do not influence its overall motion (e.g., forces between parts of a single system like a car).
  1. Normal Force on a Cup:
  • Yes, the normal force from the desk on the cup should be considered because it counters the weight of the cup.
  • Yes, the ground's normal force on the desk also matters as it influences the overall system stability.
  • Internal normal forces from the spoon sitting in the cup can be ignored in this context as they do not affect the cup's equilibrium.
  1. Bug on Windshield:
  • The reason for the different outcomes is due to the large difference in mass; the bug, having much less mass, experiences far greater acceleration according to Newton's second law (F=ma).
  1. Normal Force Equal to Weight:
  • Normal force equals weight when an object rests on a flat surface without additional vertical forces.
  • They are not equal on slopes or when additional forces act vertically (like pushing down on the object).
  1. Net Force in Equilibrium:
  • The net force acting on an object in equilibrium is zero; all opposing forces balance out each other effectively.

Introductory Problems

  • Free-body Diagram for a Ball Thrown Upward:

  • Forces to show: Weight (downward) and any upward forces if present.

  • Free-body Diagram for a Chair:

  • Forces to show: Weight (downward), normal force from the ground (upward), and any other forces acting on it.

  • Free-body Diagram for a Refrigerator:

  • Forces to show: Applied force (if pushing), frictional force (if applicable), weight (downward), and normal force (upward).