IV Momentum, Weight, Force, Gravity (Newton's Laws of Motion)

Newton's Laws of Motion

Based on the following assumptions:

  • mass=density×volumemass = density \times volume
  • momentum=mass×velocitymomentum = mass \times velocity
  • force=an action on an object that changes its motionforce = \text{an action on an object that changes its motion}
  • the motion of an object is uniform, if its velocity is constant\text{the motion of an object is uniform, if its velocity is constant}

First Law of Motion

Unless acted on by some force an object stays at rest or in uniform motion.

force=anything capable of changing the motion of an objectforce = \text{anything capable of changing the motion of an object}

Equal forces cancel each other out (e.g. a tug-of-war stalemate, an object hanging from a support by a string).

Second Law of Motion

The force on an object is proportional to the rate of change of momentum of the object.

F=kmaF = kma

k=1k = 1

  • an object decelerates whena force acts on it in the opposite direction of its movement
  • force of gravity is produced by an objects weight pushing it down

Unit Force

NN (newtons)

Mass and Weight

W=mgW = mg where

m=mass in kilogramsm = \text{mass in kilograms}
g=acceleration of a freely falling objectg = \text{acceleration of a freely falling object}

Since g=9.8m/s2g = 9.8m/s^2 near earth's surface, the weight of a mass 1kg1kg on earth is 9.8N9.8N.

Since weight is a force, its unit is newtons.

Gravitational Field Strength

The force of gravity per unit mass on an object at that position.

g=9.8N/kgg = 9.8N/kg or
g=9.8m/s2g = 9.8m/s^2

Third Law of Motion

When two bodies interact, they exert equal and opposite forces on each other.



A physical quantity with direction and size.

  • Force
  • Velocity
  • Acceleration
  • Momentum

Vectors in diagrams are depicted as arrows pointing in directions.


A physical quantity without direction.

  • Distance
  • Speed
  • Mass

Center of Gravity / Center of Mass

A concept that keeps objects in stable equilibrium when they stand straight upright.

For free standing objects, the lower the center of gravity the more stable the object stands.