## Newton's Laws

• Dynamics is the study of how bodies interact with one another; it answers the question, "Why does the motion of a ball change?"
• Isaac Newton expressed a set of three rules which allow one to predict the result of interactions between objects. These are known as Newton's Three Laws of Motion.
• The quantities which these rules govern are
• acceleration (m/s^2)
• mass (kg)
• force (kg-m/s^2)
• Inertia is a property of objects: the degree to which they resist changes in their motion. It turns out that the inertia of an object is directly proportional to its mass: the more massive it is, the harder one has to push to move it.
• Weight is the force exerted downwards by a body in a gravitational field. It is not the same as mass.
• Newton's First Law:
```     A body at rest stays at rest, and a body in motion continues
in motion with constant velocity, unless some external force
acts upon it.
```
• Newton's Second Law
```     The acceleration of an object is directly proportional to the
force applied to it, and inversely proportional to its mass.
```
In other words,
```              F = ma
```
• Newton's Third Law
```
If a body exerts a force on a second body, then the second
body exerts an equal and opposite force on the first.
```
But note that the result of these opposing forces depends on the mass of each body.

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