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phys: add third law of motion

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eggy
2020-10-21 17:55:15 -04:00
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docs/sph3u7.md
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@@ -394,9 +394,22 @@ If an object as a **zero net force** in all dimensions, the forces acting on it
### Newton's laws of motion
Newton's first law of motion (also known as the **law of inertia**) states that every object will remain at rest or at constant velocity unless acted upon by an unbalanced force.
Newton's first law, also known as the **law of inertia**) states that every object will remain at rest or at constant velocity unless acted upon by an unbalanced force.
Newton's second law of motion states that the **net** force on an object is equal to its mass multiplied by its acceleration, such that $\Sigma \vec{F} = m\vec{a}$.
Newton's second law states that the **net** force on an object is equal to its mass multiplied by its acceleration, such that $\Sigma \vec{F} = m\vec{a}$.
Newton's third law states that every force applied on an object as an equal and opposite reaction force on another object. These pairs of forces are known as **action/reaction pairs**. Forces are *only* part of the same pair when they:
- have the same magnitude but opposite directions
- are of the same type (e.g., both are weight or normal force)
- start and stop at the same time
- act on **two different objects**, thus requiring **two** force diagrams to show both of them
!!! example
If a block is lying on the ground, the force from the weight of the Earth on the block is the same as the force of the block on the Earth.
!!! warning
The weight of an object and the normal force resisting that weight is *not* an action/reaction pair as they appear on the *same* force diagram and act on the *same* object.
## 2.3 - Work, energy, and power