Chapter 3: Forces
Types of Forces
Review

What is a force?
 A force is an interaction between two objects.
All forces come in pairs.


All forces are measured in units of Newtons (N)
How do forces between 2 objects interact?
 According to Newton’s 3rd Law, every action force
has an equal in magnitude and opposite in
direction reaction force.
 Newton's Third Law Song
Force Vectors

Forces are represented by arrows called
vectors which indicate both magnitude and
direction.


The length of the arrow indicates the magnitude
of the force
The direction the arrow is pointing indicates the
direction of the force
Balanced & Unbalanced Forces

Objects on Earth usually experience more
than one force at a given time

The combination of all the forces acting on an
object is called the net force

When the net force on an object equals 0, we say the
forces are balanced

Example: Because forces have direction, a 5N force upward
plus a 5 N force downward, both exerted on an object,
would add up to 0


Therefore the net force on an object would be 0 and the
forces would be balanced
When the net force on an object does not equal 0, we
say the forces are unbalanced
Bowling Ball Physics

What do I need to do to make a bowling ball
start moving?

Stop moving?

Continue moving at a constant speed?
Forces and Motion

Motion does not require a cause (force)

Motion can be separated into two categories:
1. Constant velocity (no acceleration)
2. Changing velocity (acceleration)

When an object has a constant velocity, its
forces are balanced

When an object has a changing velocity (speed
up, slow down, change direction) it’s forces are
unbalanced
Newton’s First Law


Every object moves at a constant speed in a
straight line, unless an unbalanced force is
applied to it
This is also referred to as inertia
Tablecloth Trick
Types of Forces

There are two main types of forces:
 1. Long Range Forces
 The 2 objects do not need to be in contact with
each other for the force to be applied


Gravitational, magnetic, electrostatic
2. Contact Forces
 The 2 objects must be in contact with each
other in order for the force to be applied

Normal, tension, friction, applied, buoyancy, lift
Long Range Forces:
Gravitational Force

Gravitational Force (Fg)
 The attractive force applied by the Earth
on an object
 Objects
are “pulled” towards the center
of the Earth
 Depends on the objects’ masses and the
distance from the center of the Earth
 The


more mass and the closer the objects
are, the stronger the gravitational force
Any object with mass has a gravitational
force! Most objects we encounter do not
have enough mass for us to notice
Family Guy Clip
Gravitational Force Examples
www.universetoday.com
Other examples of gravitational force:
 The sun’s Fg keeps all of the planets in
the solar system in orbit


The planets do not crash into the sun, because
they are travelling so fast
Tides on Earth are caused by the Fg of
the sun and the moon on Earth’s water
Contact Forces

Normal Force (FN)
 A force applied by a surface on an object
 The
force is always applied perpendicular
(90˚ angle) to the surface

Tension Force (FT)
 A force applied by a rope, string, cable,
etc. on an object
 The
force is always applied parallel to the
rope
Contact Forces:
Friction Force

Friction Force (Ff)

A force applied by a surface with
texture on an object


The force is always applied parallel to the surface in a
way that resists slipping.
This can slow an object down or speed it up

i.e. sliding into home plate vs. wearing cleats to run the
bases
Contact Forces:
Applied Forces

Applied force (Fapp)

Any other type of force applied directly by you or
another person on an object
Force Diagrams

Used to illustrate the forces on an object
Step 1: Draw a dot. This dot represents your entire object.
Step 2: Draw a vector to represent the force of gravity. Label it Fg.
Step 3: If the object is on a surface, draw a vector to represent the
normal force. Label it FN.
Step 4: If the object is being pulled/suspended by a rope, string, etc.,
draw a vector to represent the tension force. Label it FT.
Step 5: If the object is being pushed or pulled directly by a person,
draw a vector to represent the applied force. Label it Fapp.
Step 6: If a parallel force is being applied to the object while on a
surface with a texture, draw a vector to represent the friction
force. Label it Ff. Assume all moving objects have a friction
force unless stated otherwise.