Transcript Four Basic Forces

Kinematics – the study of how things move
Dynamics – the study of why things move
Forces (the push or pull on an object) cause things to move
Newton proposed three Laws
of Motion that allow us to
understand how forces cause
things to move
Four Basic Forces
Applied force (Fa) – a force that is done by an external
cause (agent). It can be any direction.
Gravitational force (Fg or W) -- the force caused by
gravity (weight). It always acts
downward.
Frictional force (Ff) -- a force that opposes motion and
slows down objects. It is always parallel
to the surface.
Normal force (Fn) – the force exerted by a surface on
which an object is resting. It is always
perpendicular to the surface.
Net force (ΣF) – the sum of all forces that act on an object.
A free body diagram (FBD) shows all of the forces that
are present on an object both in the horizontal and
vertical direction.
Example: book
being pushed on
table
book
FBD
Net Force
Equations
Fn
Fa
Ff
book
Fg
Σ Fx = Fa - Ff
Σ Fy = Fn - Fg
Mass
•A measurement of an object’s quantity of matter
•A measurement of an object’s inertia
Inertia – the tendency
of objects to maintain
their state of rest or to
maintain constant
velocity
Example: car slams
on brakes and items
on seat fall to floor
Objects with a larger mass have a greater
inertia. Therefore, they are harder to accelerate
(speed up or slow down)
Inertia can give the impression that forces are
being applied.
Force is not being applied to rider. Rider is
moving at constant velocity because of inertia.
Newton’s First Law – an object at rest tends to
stay at rest and an object in uniform motion tends
to stay in uniform motion (constant velocity)
unless acted upon by a net external force
Newton’s First Law is also
known as the Law of Inertia.
Newton observed some things about
accelerating objects:
•The bigger the force, the greater the acceleration
•The larger the mass, the smaller the acceleration
Newton’s Second Law – the acceleration of an
object is directly proportional to the net force
acting on the object and inversely proportional to
its mass
Σ F = ma
F = force (Newtons)
m = mass (kg)
a = acceleration (m/s2)
What net force is required to bring a 1500-kg
car to rest from a speed of 28 m/s within a
distance of 55 m?
A 70-kg person traveling at 100 km/hr strikes a
parked car. At the instant of impact, the seat belt
restrains the person with a force of 21,000 N bring
them to rest in the car. How far does the person
travel before coming to rest?
When a force is applied to an object, it is always exerted
by another object.
Examples: a hammer hits a nail
a child pulls a sled
an apple is pulled to the Earth
Newton believed that the “force-providers” also are
“force-receptors.”
Examples: the nail pushes back on the hammer
the sled pulls back on the child
the Earth is pulled to the apple
Newton’s Third Law – Whenever one object exerts a
force on an second object, the second exerts an equal
force in the opposite direction on the first
If every force has an equal and opposite force, why dos
objects ever move?
The forces are NOT exerted on the same object.
Example: If a
hammer exerts a
50-N force on a
nail, the nail
exerts a 50-N
force on the
hammer in the
other direction.
Evidence:
Hammer causes
the nail to
accelerate (+ force)
while the nail
causes the
hammer to
decelerate (- force)
Weight and Normal Force
• A measure of the
gravitational force
on an object
• Always directed
downward (toward
the center of the
Earth
Fg = mg
A person’s mass does not change, but his weight
does depending on the magnitude of gravitation
force.
An average man has a weight of 686 N on the
Earth.
1. What is the man’s mass?
1. What would his weight be if he was standing
on the moon (ag = 1.6 m/s2)
A person pulls upward on string attached to a
box with a force of 150 N. The box has a mass
of 12 kg. Does the box move upward and if
so, with what acceleration does it move?
• A contact force
that is
perpendicular to
the surface
• The force that
pushes up on the
object resting on
the surface
Since the statue is at rest FN is equal and opposite
to FG. FN has another equal and opposite force
(F’N is reaction force on table)
Friction
Friction is the resistance that an object experiences when
moving.
Caused by a rough surface.
Object on a rough surface
actually has to move up and
down because the two rough
surfaces catch on each other.
Because energy is used to
move the moving object up and
down, less energy is used to
move the moving object
forward.
The force of friction is
influenced by two
factors – the surface on
which an object is
moving and the weight
(gravitational force) of
the object.
Ff = μFn
Coefficient of friction (μ) – indicates
the “roughness” of the surface.
Unique to each surface.
Typically, the normal force of the
object is just equal and opposite to
the gravitational force (but not
always).
Two Types of Friction
Static (Stationary Object)
If an object is at rest, an applied force has to exceed the
maximum static frictional force of the object for it to move
static
friction
applied
box at
rest
Ffs = μsFn
Kinetic (Moving Object)
If an object is moving, there is a kinetic frictional force
that opposes motion (always less than static frictional
force)
applied
box in
motion
kinetic
friction
Ffk = μkFn
A 10.0-kg box rests on a horizontal floor. The coefficient of static
friction is 0.40 and the coefficient of kinetic friction is 0.30.
Determine the maximum static frictional force and the kinetic
frictional force. Would the box move if a 10 N force was applied? If
so, what would be its acceleration?
Would the box move if a 40 N force was applied? If so, what would be its
acceleration?