Transcript Force and Motion Section 6.1

Force and Motion
Section 6.1
Physics
Let’s Get Started
• Your assignment is to define ALL
vocabulary words on page 144.
• This is due at the end of class today.
Objectives
• Define a force and differentiate between
contact forces and long-range forces.
• Recognize the significance of Newton’s
second law of motion and use it to solve
motion problems.
• Explain the meaning of Newton’s first law
and describe an object in equilibrium.
Types of Forces
• Two Types:
– Contact Forces
– Long-Range Forces
Contact Forces
• A contact force acts on an object only by
touching it.
• Give an example of a contact force.
Long-Range Forces
• A long-range force is exerted without
contact.
– Magnets
– Gravity
• The force of gravity is an attractive force that exists
between any two objects and is dependent on
distance and mass of the objects.
Forces and Their Agents
• Each force has a specific, identifiable,
immediate cause called the agent.
• When solving problems, you must identify
and draw the agents.
F
• Example: Hand holding a ball.
Hand on Ball
Ball
FEarth’s mass on Ball
Practice Problems
• P 119
• #1
Newton’s Second Law of Motion
• Force = Mass x Acceleration
• F = m(a)
F
m
a
How are All Three Related?
• How are force, mass, and acceleration
related?
• Explain the equation: F = m(a)
What if there is more than one
force?
• Often times, more than one force acts on
an object.
• The forces need to be combined to form
the vector sum of the different forces.
• A free-body diagram is drawn to show
the different forces that act on an object.
Free-Body Diagrams
• Objects are represented by dots.
• Forces are drawn on the dot in the
direction and magnitude of the force.
• Because forces are vectors, the total force
on an object is the vector sum of all forces
exerted on the object.
• You are looking for the net force on the
object.
Strategy
• First identify all forces acting on the object.
• Draw the free-body diagram showing the
direction and relative magnitude of each
force acting on the system.
• Use Newton’s second law to calculate
the acceleration.
• Use kinematics to find the velocity and
position of the object.
Units of Force
• If Force = Mass(Acceleration), what are
the units of force?
• The SI Unit of force is the Newton.
– 1 N = 1 kg (m/s²)
Practice Problems
• Pg 122
• 2-6
Newton’s First Law of Motion
• An object at rest will stay at rest or an
object in motion will stay in motion if the
net force acting on that object is zero.
• Net Force: The sum of all the forces on an
object.
• Newton’s first law is often called the law of
inertia.
• Inertia: the tendency of an object to resist
change. NOT a FORCE!
Newton’s First Law of Motion Cont.
• If the net force on the object is zero, what
will be the object’s motion?
• It will not move or continue with
unchanging velocity.
• When the net force on an object is zero,
the object is said to be in equilibrium.
Practice Problems
• Pg 124
• 7-11
Common Misconceptions about
Forces
• A force is needed to keep an object
moving.
– If there is no net force, the object will keep
moving.
• When a ball has been thrown, the force of
the hand that threw it remains on it.
– NO! It is only a contact force.
Common Misconceptions about
Forces Cont.
• Inertia is a force
– No, it is just a tendency of an object to resist a
change.
• Air does not exert a force.
– Air exerts a huge force, there is just no net
force. All the forces are balanced.