Chapter 5
The Laws of Motion
5.0 Introduction to Newton's Laws
There
are three classical laws of motion, all developed by Sir Isaac Newton, and
are known as Newton's laws of motion. A key concept of these laws is
force. It is important to note that by 1670, when Newton was only 30
years old, he had already developed the Laws of Mechanics, the Laws of
Gravity, and Calculus in mathematics.
Classical Laws of Motion
This only applies to ordinary objects. Atomic level particles move at a speed near the speed of light and require the theories of Quantum physics that were developed in the 1900s, and which are beyond the scope of this course.
Equilibrium state of a system
The first condition of equilibrium is that
the vector sum of the forces acting on an object is zero. That is an object
is said to be under equilibrium condition or state, if the total sum of
external forces acting on it is zero i.e., (
). (Note that: sum of forces 
(net force)
º
(resultant force) - all these terms will be used
interchangeable during lectures hours or in exams). Equilibrium means
that acceleration is 0, but NOT that no forces are present.
Force
is the agent that changes the equilibrium state of a given body. While
Inertia provides resistance to a change of state either in motion or at
rest.
There are two types of equilibrium conditions, these are:
· Static Equilibrium - a body at rest will stay at rest if no external force is exerted
on the body.
· Dynamic Equilibrium - Dynamics deal with why objects move the way they do.
For example, a moving body will continue moving if no external force is applied.
5.1 The Concept of Force
Force is described as an agent that changes the equilibrium state of a given body. Alternatively, it can be described as an external interaction that causes an acceleration of a body. This acceleration is described as the change in direction or magnitude. The forces can have the same direction as the object. The forces can have just the opposite direction of the object. The forces can also occur at right angles (perpendicular) to the objects. Or, finally, forces, which do not have identical, opposite, or right angle direction to an object, fall into another category. Note that force is often called net force; this is simply all of the forces acting on an object together. In the examples, however, don't think that if net force is zero that no force is present at all.
5.1.1 General motion of an object experiencing external force.
Four different scenarios can occur when we deal with forces acting on moving objects, see Fig. 5.1:
- Forces of the first kind will increase an object's speed.
Recall the equation, 
, in this case, a, is positive.
- Forces of the second kind will decrease an object's speed.
Recall the equation, , in this
case, a, is negative.
- Forces of the third kind will simply deflect an object, i.e., change it's direction.
- Forces of the fourth kind will result in a combination of speed change and deflection.
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Fig. 5.1 The four cases of forces acting on a moving object.
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