FUNdamental Forces &
FBD’s
What is a force?
• A push or a pull on an object.
•Let’s demonstrate
•Pick a partner (size appropriate)
•Stand 50cm apart from each other (feet
shoulder width apart).
•Put your hands out to your sides with your
palms facing your partner.
•Now the object is to keep your balance and
knock your partner off balance by avoiding their
attempts or by you hitting their hands ONLY.
Strong Nuclear Force
• It has the shortest range, meaning that particles
must be extremely close before its effects are
felt.
• Its main job is to hold together the subatomic
particles of the nucleus .
• Strongest of the 4 fundamental forces.
Electromagnetic Force
• There are two kinds of electrical charge - positive and negative.
• The electrical force can be either attractive or repulsive - like
charges repel, opposite charges attract.
• Although the Electromagnetic force is of order 1039 times stronger
than Gravity, it is not as apparent because of its dual
(attractive/repulsive) nature
• 2nd strongest of the 4 fundamental forces
Weak Nuclear Force
• It causes radioactive beta decay and other
subatomic reactions.
• The particles that carry the weak force are called
weakons (or intermediate vector bosons).
• 3rd Strongest of the 4 fundamental forces.
Gravitational Force
• It is a purely attractive, long-range force.
• We will learn Newton’s Law of Universal
Gravitation tomorrow (stay tuned)
• Weakest of the 4 fundamental forces
Electroweak Force
• A force that unifies the electromagnetic and weak
nuclear forces under conditions of extreme temperature
prevalent much earlier in the history of the universe.
• Physicists concluded that the weak and electromagnetic
forces have essentially equal strengths. This is because
the strength of the interaction depends strongly on both
the mass of the force carrier and the distance of the
interaction.
How is a force measured?
• A Newton (N) is the unit of force
• 1 N = 1 kg * m
s2
• By looking at the units, the equation for force is F=ma (your
new best friend)
• Forces are usually measured using a spring scale
Free Body Diagram’s (FBD’s)
Here is the step by step how to draw one:
1. Draw a north arrow and a scale
2. Sketch of the object (don’t be too detailed)
3. Locate the approximate centre of the object
and put a dot there.
4. Draw on the force arrows to represent each
force on the object (just like vector diagrams).
5. Do NOT include the forces that the object
exerts on other objects.
Common Forces
• Normal Force (FN) – acts on an object
perpendicular to the surface it is resting on
• Friction (Ff) – ALWAYS opposes motion
• Gravity (FG) – related to mass and is
straight down
• Applied (FApp) – given by a external body
Example FBD’s
Adding Non-Collinear Forces
• It is the same as adding vector quantities that we
did in chapter 1, but now we can use:
• Tail to Tip method again.
• Trig  - Sine Law
• Cosine Law  c2 = a2 + b2 - 2ab·cos(C)
Homework
• p. 119 # 1,2
• Have Fun 