Newton's Laws

Newton's First Law

  • A body at rest or moving at a constant speed in a straight line will remain at rest or keep moving in a straight line at constant speed (constant velocity)  unless it is acted upon by a force
    • In a straight line at constant speed means constant velocity
  • Translational equilibrium: the net force acting on the object must be zero when an object is traveling at a constant velocity or in stationary.

Note: An object's resultant force might not be zero moving at constant speed, but an object's resultant force is zero if it's moving at a constant velocity.

Newton's Second Law

  • The Newton's second law states that a net force of an object can be calculated by the formula of F=ma, where F is the net force, m is mass of the object, and a is the acceleration.
  • Credit: https://www.thegreatcoursesdaily.com/understanding-newtons-laws-of-motion/

Apparent Weight

  • Apparent is a common misconception of Newton's Law and a classical physical problems known as the elevator problems. One important concept to understand is that a scale in an elevator with a person standing on it measured the normal force of the person, explaining changes in the scale's reading when an elevator accelerates or decelerates.

Some Classical Force Models

Newton's Third Law

  • If an object A exerts a force on object B, then object B must exert a force of equal magnitude and opposite direction back on object A. (Action = Reaction)

https://www.wired.com/2013/10/a-closer-look-at-newtons-third-law/

Questions to Think About 

  1. How does rocket launching relate to Newton's third Law

  2. How does rocket launching relate to Newton's first and second Laws

  3. Sketch the velocity vs time graph for the rocket in the first two videos. (drag force cannot be ignored

Answers

  • 1st Law - A rocket will remain on the launch pad until an unbalanced force is exerted, propelling the rocket upward.
  • 2nd Law - The amount of force depends upon how much air is pumped inside the rocket. You can increase the force further by adding a small amount of water, which increases the mass expelled by the air pressure in the rocket .
  • 3rd Law - Finally, the action force of the air (and water) as it rushes out of the nozzle creates an equal and opposite reaction force propelling the rocket upward.

Rocket Launching Countdown

Rocket Launching