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General Physics I:

Sunil Karna

Subsection 4.1.2 Newton’s Laws of Motion

  1. Newton’s first law provides the definition of force which states that every object in a state of rest remains at rest or in a state of uniform motion tends to remain in that motion along the same direction, unless an external force is applied onto it. Force is not required to continue the motion of anything which is already in motion.
    The first law is also called a law of inertia. Inertia is an ability of an object to resist changes in its motion. In other words, we can say the laziness of an object to change its mechanical state is called an inertia. Mass is related to inertia, the bigger the mass the more difficult it is to change its state of rest or motion.
  2. Newton’s second law provides the measurement of force which states that the rate of change of momentum is directly proportional to the applied force and its direction is along that change.
    \begin{align*} \vec{F}\amp =\frac{\,d\vec{p}}{\,dt}\\ \amp = m\frac{\,d\vec{v}}{\,dt} \end{align*}
    \begin{equation} \therefore \quad \vec{F} = m\vec{a}\tag{4.1.1} \end{equation}
  3. Newton’s third law provides the property of force which states that for every action there is an equal and opposite reaction. Meaning, force always comes in pair. If an object \(A\) applies the action force on object \(B\) then the object \(B\) applies the reaction force on the object \(A\text{.}\) These pair of forces always act on two different objects. For example, if action force is acting on object B then reaction force must be acting on the object A.
    \begin{equation*} \vec{F}_{AB} = -\vec{F}_{BA} \end{equation*}
    If you stand on a weighing scale, your weight push the scale down with a force equal to your weight, then the scale applies the reaction force on you equal to your weight as a normal force. Remember that even if weight and normal force are equal they are not always equal and they are completely different forces. You get a normal force only when you are in contact with the other surface but you have the weight even if you are above the surface (while jumping) of anything. Suppose you are pushing the book against a wall, so that it does not fall. In that case, the book pushes against the wall and the wall pushes back. This interaction is the normal force and acts on both the book and the wall. None of these forces were equal to the gravitational force or weight of the book.
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