Do Now
• What causes an object to start or stop its
motion, or to change its direction?
• Think of examples from everyday life. (What
would a driver do to avoid a crash? How
does a person start riding a bike or
skateboard?)
• How can you describe these examples using
what we’ve already learned about motion?
Aim: What are forces and
how are they represented?
SWBAT: describe forces, draw free-body
diagrams, and find the value of net forces
on a system
Recall
 Acceleration causes changes in motion
(accelerations change an object’s
velocity)
 Vectors have both a magnitude (#) and
direction (+, -); scalars only have
magnitude (#)
Forces and Motion
 A force is a push or pull exerted on an
object
 Causes an object to speed up, slow
down, or change direction
 A force causes an acceleration!
 Represented by F (vector: magnitude
and direction) and F (just magnitude)
 Unit: Newton [1 N = kg*m/s2]
Terminology
 Object is called the “system”
 Anything that exerts a force on the
object is called the “external world”
 The specific and identifiable cause of a
force is called the “agent”
 A force can only exist if there is both an
agent and a system
Contact vs. Field Forces
 Two kinds of forces
 Contact forces exist when an object
from the external world touches the
system (ex: hand on a book, brake on
a tire)
 Field forces exist when forces are
exerted without contact (ex: gravity,
magnets)
Free Body Diagrams
 Visual representation of forces
 Sketch identifies the system and all contact
and field forces
 Free-body diagram shows forces as vectors
acting on the object
 Since forces are vectors, we can simplify the
free-body diagram further by sketching the
vector sum of all the forces on the object (the net
force)
 If the net force is zero, there is no
acceleration.
 If an object is moving at a constant
velocity, there are either (1) no forces
acting on the object, or (2) the sum of the
forces acting on the object is zero.
system
gravity
(field force)
Fgravity
Fair
Air caught in
parachute
(contact)
system
gravity
(field)
Fgravity
Examples
A sky diver falls downward
through the air at a constant
velocity.
A cable pulls a crate at a
constant speed across a
horizontal
surface.
The
surface provides a force that
resists the crate’s motion.
Two horizontal forces, 225 N and 165 N, are
exerted on a canoe. If these forces are applied
in the same direction, find the net horizontal
force on the canoe.
If the same two forces (225 N and
exerted on the canoe in opposite
what is the net horizontal force of
Be sure to indicate the direction
force.
165 N) are
directions,
the canoe?
of the net
Three confused sleigh dogs are trying to pull a sled
across the Alaskan snow. Alutia pulls east with a force
of 35 N, Seward also pulls east but with a force of 42 N,
and big Kodiak pulls west with a force of 53 N. What is
the net force on the sled?