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FORCES
I N T R O D U C T I O N T O T H E S T U DY O F
DY N A M I C S
DYNAMICS
• Dynamics is a branch of physics that studies forces and how they effect the motion of
objects.
• So what is a force?
FORCE
• Force  Any push or pull on an object
• There are two main categories of forces that we will talk about
• There are also four fundamental forces that are the basis for all of physics
• It is the study of these fundamental forces that has led to much of our understanding of
physics today.
CATEGORIES (TYPES) OF FORCES
• The two main categories of forces are…
– Contact forces
• These forces occur when objects are in physical contact with each other
– Examples
• Pushing on a wall, or friction between your hands when you rub them together
– Long range forces
• These forces occur between objects that are not in contact with each other
– Examples
• Magnetic forces, gravity
FUNDAMENTAL FORCES
• Electromagnetic force
– The attractive and repulsive long range forces between electric charges and magnetic poles
• Gravitational force
– The long range attractive force all matter has on all other matter
• Strong Nuclear force
– The force that holds the nucleus of atoms together
• Weak nuclear force
– The force that allows for and causes nuclear decay
FORCES IN THIS CLASS
• In our discussion of forces, we will focus on the “big picture” and concentrate mainly on the
two large categories of forces…contact and long range.
• But what causes a force?
• The agent
– The agent of a force is what CAUSES the force
FORCES
• Let’s talk some more about forces…
• As we said before a force is any push or pull acting on an object
• The symbol for force is F
• The SI unit of measure of forces is the newton (N)…
– As you probably guessed, it is named after Isaac Newton.
• Force is a vector quantity…
– Which means direction matters
NEWTON AND FORCES
• As you may have been taught before, Isaac Newton developed the three laws of motion.
– (Newton’s Laws)
• As you will now learn, these laws of motion are dependent upon forces and how they affect
matter in motion.
• We will discuss Newton’s Laws in detail in later lessons, today we will just touch on the basic
statements
NEWTON’S 1 ST LAW
• Newton’s first law of motion – this law is also called the law of inertia.
• Inertia is the tendency of an object to resist changes in motion.
• The law of inertia states that “an object in motion will remain in uniform motion and an
object at rest will remain at rest unless acted upon by an outside force”
NEWTON’S 2 ND LAW
• Newton’s Second Law of motion – This law is also called the law of acceleration.
• The 2nd law was actually developed first by Newton, but since it is slightly more complicated,
it is discussed second.
• The second law states that “When an object has a net force, Fnet, acting upon it, it will
experience an acceleration proportional to its mass.”
NEWTON’S 2 ND LAW
•
A net force occurs when all of the forces acting on an object DONOT cancel each other
out.
•
This is the only one of the three Newton’s laws that has a formula associated with it:
F = ma
NEWTON’S
RD
3
LAW
•
Newton’s third law of motion – this law is also called the law of reaction forces.
•
The third law is the most well known, yet it is the least understood.
•
It states that “for every action force there exists a reaction force that is equal in magnitude
but opposite in direction.”
MASS AND WEIGHT
• Mass –
– The amount of matter in an object. Mass is NOT a force!
– Measured in kilograms (kg)
– Mass DOES NOT CHANGE when you change locations (ie. Earth to moon)
• Weight –
– Defined as the FORCE of GRAVITY acting on an object
– Measured in newtons (N)
– Weight is dependent upon the force of gravity and therefore WILL CHANGE if you go to a place
where gravity is different (ie. The moon)
FREE-BODY DIAGRAMS
• A free-body diagram is a way of showing all of the forces acting on an object with their relative
magnitudes and directions.
• Free-body diagrams will ALWAYS be extremely helpful in solving force problems. ALWAYS
draw a free-body diagram before attempting to solve a force problem.
• Let’s see how to do this.