Download Force

Chapter 4 : Forces and
the laws of motion
Definition
Force –describes the interaction between and object and
its environment and it’s a vector quantity





Forces can act on an
object in three ways
They can cause objects
to…
Start moving-golf ball
Stop moving -soccer
Change directionvolleyball
Units

The SI unit for force is the Newton
 This was named after Sir Isaac Newton
because of the work he contributed to the
understanding of force.
 The Newton is the amount of force that when
it is acting on an object of 1 kg it produces
and acceleration of 1 meter/second squared
 This is why one Newton equals 1 kg times 1
m/s ²
The way forces act



Forces are able to act
through contact or at a
distance
When forces touch
each other the motion
the object previously
had is changed
When a soccer ball is
kicked the object
changes direction
because of the physical
contact between the
forces
Field Forces




The second way that forces
act is called force fields
This could be a gravitational
force- happens as objects
are falling
The object will accelerate
due to the earth’s gravity
even thought the earth is on
in contact with the object.
Another example is the
attraction or repulsion
between electric charges
Force Diagrams




The acceleration of an object
depends on the magnitude
of the force acting on it and
the direction
Force is a vector quantity
Force diagrams show force
vectors and the direction
they act in
The force vector will point in
the direction of the force and
its length is proportional to
the magnitude of the force
Free body diagrams


A free-body diagram
is a way to show just
the forces acting on
the object
The free body
diagram is used to
find only the forces
that are affecting the
motion of the object

Inertia- the objects nature
to maintain its state of rest
 Mass is the measure of
inertia
 The greater the mass of a
body the less the object
will accelerate under an
applied fore
 Example: What this
means for a racecar. The
greater the mass of the
car, the more energy
required to get the
stationary car moving. If
two cars use the same
amount of propulsive
energy, the car with the
lower mass will accelerate
faster.
Section 2
Net Force





The sum of all forces
acting on an object is the
net force: add up normal,
friction, gravity and
applied forces
F-friction: the resistance
force, opposite force
applied
F-applied: the direction
the force is acting
F-normal upward force
exerted
F-gravity the force gravity
has on objects
Newton’s first Law





Newton’s first Law: an object
will continue the motion it is in
unless acted on by a force
An object will stay in rest unless
acted on by a force
the same is true for an object in
motion it will continue in the
same speed and direction
unless acted on by a force-two
cars crashing
Other definition: when the net
external force on an object is
zero the net acceleration of the
object is zero
External forces-single forces
acting on the object and they
result from the interacting
between an object and its
environment
Equilibrium

Objects that are either
at rest or moving at a
constant velocity are
said to be at equilibrium
 This happens when the
vector sum of the forces
acting on the object
equals zero
 When you find the net
force acting on an
object and it equals
zero then the object is
at equilibrium
Section 3
Newton’s 2nd Law:
the acceleration
(change in velocity)
of an object is
directly proportional
to the net force
acting on the object
 It is also inversely
proportional to the
objects mass


According to this law
if equal forces are
applied to objects of
different masses the
one with the greater
mass will
experience a larger
acceleration
Net Force




A net force acting on an
object causes the object
to accelerate and the
speed to increase
ΣF =ma
Net Force = mass x
acceleration
Σ is the Greek symbol
sigma, which means the
sum
Practice
Working out how much force is used
to push a F1 car when broken down or run out of fuel
If the car has a mass of 700kg
And a driver pushes the car with an acceleration of 0.05m/s/s
What’s the force needed?
Answers

Race Car
F = ma
Force = 700kg x
0.05m/s/s
Force = 35kN (kilo
Newtons)

Bus
F=ma
F= 2000kg x .05 m/s²
F= 100 Newtons
Newton’s third Law



Forces always exist in
pairs
For every action there is
an equal and opposite
reaction
When two objects
interact, the forces that
act on each other are
called an actionreaction pair
Action-Reaction

The action force is
equal in magnitude and
opposite in direction to
the reaction force
 These two forces occur
at the exact same time
 Because they coexist
either force can be
called the action or
reaction depending on
which way you are
looking at the situation
More action and reaction



Action and reaction
forces act on different
objects
Because of this an
action-reaction pair
does not mean the net
force is zero
This can best be
described by the act of
driving a nail into wood
with a hammer

The hammer exerts a
for on the nail and the
nail exerts a force back
on the hammer
 However the nail is only
affected but the forces
acting on it
 It is not affected by the
force on the hammer
 Therefore they don’t
balance out and
acceleration is able to
occur on the nail
Field Forces


Newton's third law
applies to field forces
too
Newton’s third law says
that as an object falls to
the Earth and
accelerates the Earth
also accelerates
towards this object

However if you think back
to Newton’s second law
you will remember the
greater the mass the less
acceleration occurs
 Therefore in the case of a
falling object such as an
apple or a book, the
object will have a large
acceleration due to gravity
 The Earth will have an
acceleration towards the
object, but it will be
negligibly small because
of the Earth’s large mass
Section 4: Everyday forces




Fg is a vector quantity of the
force exerted toward the
Earth
Weight is the magnitude of
this force Fg is a scalar
quality
Weight is not always the
same for the object
It depends on location of
the object
Weight
•You can calculate the weight of
an object using Fg=mag
•ag= the magnitude of acceleration
due to gravity or free-fall
acceleration
•In this book ag= 9.81m/s²
•On the moon ag= 1.6m/s²
•Because of this the object weighs
less on the moon
The Normal Force



The normal force is used to
explain how if gravity is
action on all objects, they
don’t continue to fall into the
earth
When objects are at
Equilibrium they don’t move
anymore, much like a desk
or dresser in your room or
the tires on your car
Because we know gravity
acts on everything thing
there must be a force acting
opposite to it so all these
objects remain at equilibrium
More normal force



The definition of the
word normal is
“perpendicular”
The normal force is
always perpendicular to
the contact surface, but
not necessarily opposite
the force due to gravity
In the absence of other
forces the normal force
is opposite the Fg
How to find Normal Force


The magnitude of the
normal force can be
calculated by mgcosθ
the angle θ is the angle
between the normal
force and a vertical line
And is also the angle
between the contact
force surface and a
horizontal line
The force of friction
Friction opposes the applied force
 The static friction is the resistive force
that keeps an object from moving
 This happens when you push on an
object but you don’t apply enough force
to move the object and it remains at
equilibrium
 Fs= -Fapplied

Kinetic friction


Kinetic friction is less
than static friction
The magnitude of the
net force acting on an
object is equal to the
difference between the
applied force and the
forces of kinetic friction
(Fapplied= –Fk)

In free-body
diagrams the force
of friction is always
parallel to the
surface of contact.
 The force of kinetic
friction is always
opposite the
direction of motion.
The Force of Friction
The force of friction is proportional to
the normal force
 Heavier objects also experience greater
normal force and more friction than
lighter objects
 The relationship between normal force
and the force of friction is one factor that
affects friction

More Friction



The force of friction
depends on the
composition and
qualities of the surfaces
in contact
One example of this
would be skiing or
snowboarding
When the snow is more
compact there would be
less friction present and
more acceleration will
take place and you will
be able to go much
faster
Coefficient of friction
μk = Fk / Fn
 μk Stands for the coefficient of kinetic
friction
 The coefficient of friction is a ratio of the
force due to friction, and the normal
force between two surfaces

Coefficient of static and the
force of friction

μ = Fs,max / FN this equation represents the ratio
s
of maximum value of the forces of static
friction

Ff = μFN
 If you know both the value of μ and the
normal force you can find the magnitude of
the force of friction
Example problems

A desk has a mass of 52.5 kilograms. If the
coefficient of static friction between the desk
and the floor is 0.84, what force must be used
to move the desk from rest?

A dresser has a mass of 61.25 kilograms Once
is in motion, what force must be used to keep
it moving at a constant velocity if the
coefficient of kinetic friction is
0.6533333333333333?
Answers

432.621 N
 You must do 52.5 times 9.81m/s² to get the
FN which equals 515.025n then you multiply
that by the .84 and you get the Fs,max
 Second answer =392.5635 N
 You need to first change Kg’s into Newtons
by multiplying by 9.81m/s² you will get around
600.9N then you multiply this by the
coefficient of kinetic friction which gives you
the answer
Air resistance


•Whenever an object moves
through a fluid medium (like air or
water) the fluid provides a
resistance to the objects motion
this is called FR


When an object is in free fall
its velocity increases
As the velocity increase so
does the the resistance on
the object when the upward
force of air resistance
balances out the downward
gravitational force the net
force on the object will be
zero
When it reaches zero the
object will continue
downward at a constant
maximum velocity
This is called terminal speed
The 4 fundamental forces








Electromagnetic forces-interaction between
protons and electrons
Gravitational force-due to gravity
Strong Nuclear forces
Weak Nuclear forces
All four of these forces are field forces
The nuclear forces have small ranges so you
can not directly see their effects
Any force you can see at a macroscopic level
is either gravitational or electromagnetic
Out of these four forces Gravity is the
weakest and strong Nuclear is the strongest