Newton’s Laws

12PHYS - AS91171

Finn LeSueur

2020

Newton’s First Law

Every object persists in its state of rest or uniform motion in a straight line unless it is compelled to change that state by forces impressed on it.

TLDR: A non-zero net force will induce a change in velocity (an acceleration).

Source

Examples

  • Car Crash: A person not wearing a seatbelt during a crash will continue their motion forwards as the car suddenly stops.
  • Ball: A ball rolling on a soccer field will come to a stop because of the friction force acting upon it, creating a negative acceleration (deacceleration).
Source

Newton’s Second Law

Force is equal to the change in momentum per change in time. For a constant mass, force equals mass times acceleration. \(F=m \times a\)

TLDR: The acceleration created by a force depends directly upon the mass of the object.

Source

Examples

  • A student pushing a trolley can cause a great deal of acceleration
  • A student pushing a car will cause very little (or no acceleration)
Source

Newton’s Third Law

For every action, there is an equal and opposite reaction.

TLDR: Objects exert an opposite and equal force as a kind of support.

Source

Examples

  • Rockets: Gas is ignited causing it to heat and expand. It is expelled out a narrow nozzel at a high velocity. An equal and opposite force is exerted on the spacecraft causing it to move forwards.
  • Bouncing Balls: A ball thrown on the ground exerts a force on contact, the ground exerts an equal and opposite force which causes the ball to bounce.
Source

Pātai

With the person next to you, write a paragraph describing how Newton’s 1st and 2nd Laws relate to projectile motion.

Whakatika

  • Newton’s 1st Law
    • A non-zero net force will induce a change in velocity (an acceleration)
    • In the x-direction there are no forces acting (friction is negligible), and in the y-direction there is only the weight force of the object. This means the resultant force is in the downwards direction.
    • An acceleration is therefore induced in the downwards direction (constant).
  • Newton’s 2nd Law
    • The acceleration created by a force depends directly upon the mass of the object.
    • In Earth’s gravitational field at sea level, \(g=9.81ms^{-2}\) at all times, and therefore the object is accelerated uniformly down.

Whakawai/Practise

  • Try Q1-3 of Activity 10A: Using Newton’s Laws in your textbook.
  • Draw a diagram each time!