•Aristotle (384-322 BC) believed that all object had a “natural
place” and that the tendency of an object was to reside in its
•All objects were classified into categories of earth, water, air,
•“Natural motion” occurred when an object sought to return to
its “natural place” after being moved from it by some type of
•To keep an object moving would require a force.
Causes of Motion
These views remained widely
supported until the 1500s
when Galileo Galilei (1564-1642)
Isaac Newton (1642–1727)
proposed that the tendency of
an object was to maintain its
current state of motion.
Sir Isaac Newton (1642 – 1727)
• Built on Galileo, Kepler, and others
• Worked out the “three laws of
motion” governing the movement of
all objects at all times an in all
• He published them in his book
Philosophiae Naturalis Principia
Mathematica (mathematic principles
of natural philosophy) in 1687.
Sir Isaac Newton
Law – an object at rest or in motion stays at rest or
in motion unless acted upon by an outside force
* Known as the “Principle of Inertia”
Law – describes how an object accelerates or
changes direction when a force is applied to it
* F = ma
Law – for action there is an equal and opposite
What is a Force?
• Force can be defined as a push or a pull.
– (Technically, force is something that can accelerate
• Force is measured by N (Newton).
• A force that causes an object with a mass of 1 kg
to accelerate at 1 m/s is equivalent to 1 Newton.
1 Newton = kg * m/s2
What are the types of Forces?
Type of Force Force Symbol
Contact Friction Ffric or Ff
Contact Normal FN
Contact Applied FAppl
Contact Spring FS
Contact Tension FT
Contact Thrust Fthrust
Long-Range Weight Fgrav
Contact Forces vs. Long-Range Forces
– Contact Force = acts on an object only by
touching it. (ie. Books resting on a desk)
– Long-Range Force = forces that are exerted
without contact or forces resulting from action-
at-a-distance, (ie. force of gravity).
– Types of Forces = see Table 6-2 in your books!
Forces have Agents
– Each force has a specific, identifiable, immediate
cause called the agent.
– How to solve for agents:
– 1. Create a pictorial model of situation.
– 2. Circle the system and identify every place where the
system touches the environment.
– 3. It is at these places that contact forces are exerted.
– 4. Then identify any long-range forces on the system, ie.
Force of gravity (Fgrav).
Example of Forces having Agents
♦A physics book resting on a desk.
Law of Motion
• An object at rest tends to stay at rest and an
object in motion tends to stay in motion with the
same speed and in the same direction unless acted
upon by an unbalanced force. (outside force)
The velocity of an object remains constant
unless acted on by an unbalanced force.
““Law of Inertia”Law of Inertia”
Law of Motion
• Inertia - the resistance an object has to
a change in its state of motion. [Newton’s
• Equilibrium - if the net force on an object
is zero, then it is said to be in equilibrium.
– An object is in equilibrium when: at rest or
moving at constant velocity.
Some Examples from Real Life
Two teams are playing tug of war. They are both
exerting equal force on the rope in opposite
directions. This balanced force results in no change
A soccer ball is sitting at rest. It takes
an unbalanced force of a kick to change
• After you have added
and subtracted all the
forces you are left with
the net force acting on
• There are several
common forces acting on
objects that you need to
Friction Ffric or Ff
Applied FAppl or Fa
Weight Fgrav or Fg
Free Body Diagrams
• To keep track of how all these forces are
affecting a single object, it is a good idea to draw
a free body diagram.
• A free body diagram is just a simple sketch of the
object showing all the forces that are acting on it.
– Draw a quick sketch of the object.
– Draw an arrow showing every force acting on the object.
– To calculate the net force, add any vectors acting on the
same axis (x and y), making sure to pay attention to the