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FORCES
What is a force?
A force is an influence on a
system or object which, acting
alone, will cause the motion of the
system or object to change. If a
system or object at rest is
subjected to a non-zero force it
will start to move.
Weight
Weight is a force caused (on Earth)
by the gravitational attraction of a
mass to the Earth’s centre.
The weight of a body, of mass m, is
defined to be the force, W, with
which it is attracted to the Earth.
On Earth, W = mg, where g is the
acceleration due to gravity (g ≈ 9.81
m s−2 on Earth).
Tension
Many mechanics problems involve
objects being pulled, pushed or
suspended from a string, spring, rod
or something similar.
The force that the string (or similar)
exerts on the object in these types
of problems is called tension.
Friction
A force that prevents, or tries to
prevent, the slipping or sliding of
two surfaces in contact.
Air Resistance
Acts in the opposite direction to the
motion.
Normal reaction force
A mass lies on a horizontal surface.
The weight of the mass pulls it
downwards.
The reason it does not fall is
because the horizontal surface
exerts an equal and opposite force
on the mass called the normal
reaction force.
The normal reaction force always
acts perpendicularly to the surface
that is causing it.
Example
A block lies at rest (not moving) on
a rough inclined slope; what forces
act on the block?
Example
A ball, attached to the ceiling, is
swinging on the end of a string in a
simple pendulum motion; modelling
the ball as a particle, what forces
act on it?
Example
What forces act on a trailer
attached to a tow rope being pulled
by a car?
Force Diagrams
Forces
• We know that a force can be a
push or a pull acting on an
object
• There is a good chance that 2
forces can be acting on an
object at any one time
• Examples:
– Lifting something
– Dragging something
– Floating Globe
Balanced Forces
• If two equal forces are
applied to an object in
opposite directions, the
object does not move /
or moves as a constant
speed. This is called
balanced forces.
Unbalanced forces
• If two unequal forces
are applied to an
object in opposite
directions, the object
does move. This is
called unbalanced
forces.
• And someone will win!!
Unbalanced Forces
• If the forces acting on an
object are not balanced then
the object with either:
–
–
–
–
Speed up
Slow down
Change direction
Chance its shape
We can use force diagrams
to show this:
4N
7N
The block has 2 opposing forces being applied to it:
7N to the right and 4N to the left. They are Unbalanced Forces.
To Calculate the Resultant Force (Fr),
subtract one from the other: 7N – 4N = 3N to the right
3N
Problem:
• A man of mass
75kg has a weight
of 750N. This
750N will act
downward on the
table and the table
will exert a 750N
force upwards on
the man. The
forces are
balanced.
• What would
happen if the
upward force were
less than 750N?
Force down 750N
Force up 750N
Newton’s Laws
Newton’s Laws
• Newton’s First Law
• Law of Inertia
• Newton’s Second
Law
• F = ma
• Newton’s Third Law
• Action
Reaction
• Law of Universal F
Gravitation
Gmm'
= 2
r
Mass
• …is measured in kilograms.
• …is the measure of the inertia of an object.
• Inertia is the natural tendency of a body resist
changes in motion.
Force
• …the agency of change.
• …changes the velocity.
• …is a vector quantity.
• ...measured in Newton’s.
Newton’s First
• Law of Inertia
Law
• “A body remains at rest
or moves in a straight
line at a constant speed
unless acted upon by a
force.”
Newton’s First Law
No mention of chemical composition
No mention of terrestrial or celestial
realms
Force required when object changes
motion
Acceleration is the observable
consequence of forces acting
Newton’s Second
Law
The Sum of the Forces
acting on a body is
proportional to the
acceleration that the body
experiences
F  a
 F = (mass) a


F  ma
Net Force
 Fx  max
 Fy  may
 Fz  maz
Newton’s Third
Law
• Action-Reaction
• For every action force
there is an equal and
opposite reaction force
The Law of
•Gravity
Every mass exerts a force of attraction on
every other mass.
• The math…
Gmm'
F 2
r
G = 6.67  10-11 N·m2/kg2
Gravity Questions
• Did the Moon exert a gravitational force on
the Apollo astronauts?
• What kind of objects can exert a gravitational
force on other objects?
Gravity Questions
• The constant G is a rather small number.
What kind of objects can exert strong
gravitational forces?
• If the distance between two objects in space
is doubled, then what happens to the
gravitational force between them?
Weight
• The weight of an object FW is the gravitational
force acting downward on the object.
• FW = m g
Tension
(Tensile Force)
• Tension is the force in a string, chain or
tendon that is applied tending to stretch it.
• FT
Normal Force
• The normal force on an object that is being
supported by a surface is the component of
the supporting force that is perpendicular to
the surface.
• FN
Coefficient of
Friction
Kinetic Friction
• Ff = mk FN
• Static Friction
• Ff  ms FN
• In most cases, mk < ms.