Download Force - Eastside Physics

Force
• Force is a push or pull on an object
• The object is called the System
• Force on a system in motion causes change
in velocity = acceleration
• Force is a vector, it has direction
• External World = everything around the
system that exerts a force
Types of Forces
Forces Contact
• When the external world is in contact with
the system and exerts a force ex. Hand that
pushes, Rope that pulls
Field Forces
• A force exerted on a system without
touching the system ex. Gravity, Magnetic
force, Interaction between charged particles
Agent
• An agent is the item in the external world
that causes the force to act on the system
• ex. The hand that holds
The rope that pulls
The earth
The magnet
Free Body Diagram
• A diagram representing the System, Agent
and the Forces acting on the system
• System is represented by a dot
• Forces represented by arrows pointing in
the direction of the force, away from system
• Label each force with a subscript
• Choose direction of +ve usually towards
stronger force
String
Positive Direction
Ball
Ft (force of tension)
Fg (force of gravity)
Combining Forces
• As forces are vectors they can be added just
as vectors
ex.1 Fa
+ Fb
=
F net
ex.2 Fa
+
Fb
=
Fnet
Nonlinear non perpendicular forces are added by adding
x and y components of the force vectors
Newton’s First Law
• An object at rest will remain at rest and an
object in motion will remain in motion
unless acted on by an external net force
• Also known as the Law of Inertia
• Inertia is the tendency of an object to resist
change in motion
• Equilibrium = if the net force on a system is
zero the system is in equilibrium speed and
direction is unchanged.
Common Types of Forces
• Friction F = Contact force opposing motion
of two surfaces, parallel to the surface and
opposite to the direction of motion
• Normal F = Contact force exerted by
system’s surface perpendicular to and away
from surface
• Spring F = A restoring force- push or pull
of spring exerted on system opposite to
displacement
f
N
sp
• Tension F = pull exerted by a rope on a
system, away from the system and parallel
to the rope
• Thrust F
= a force that moves a system
in the same direction as the acceleration
• Weight F = a field force on a system due to
gravity directed to the center of the earth
T
THRUST
g
Newton’s Second Law
• The acceleration of an object is equal to the
net force acting on it divided by the mass of
the object
a = F/m or F = ma
• The larger the force the greater the
acceleration The greater the mass with the
same force the lower the acceleration
• Force of 1N = 1kg * 1m/s.s
Newton’s Third Law
• All forces come in pairs equal and opposite
in direction
• ex. A ball on a table and the table on the
earth. Forces-ball on table / table on ball
-table on earth/ earth on table
- ball on earth/ earth on ball
Net free body diagram
ball
table
earth
F table on ball
F earth on ball
Drag Force and Terminal
Velocity
• Drag Force is the force exerted on an object
as it moves the a fluid ex. Air and water
• as the speed of the object through the fluid
increases so does the drag
• Drag is effected by the fluids viscosity and
temperature
• Terminal velocity is when the drag force
equals the force due to gravity no
acceleration constant velocity app.(60m/s)
Tension
• Is a force exerted by a string or rope on a
system
• It is assumed the rope has no mass
• Tension within all points of the rope is equal
and opposite to the force exerted by the
system’s weight
Example of bucket on rope
F
T
F
g
=mg
Normal Force
• Is a contact force exerted by a surface on an
object. This force is perpendicular to the
surface of contact
N
Fg
Gravitational Force
• Gravitational force is the mutual attraction
between any two bodies in the universe
• Newton’s Law of Universal Gravitation =every
particle in the universe attracts every other particle
with a force that is directly proportional to the
product of their masses and inversely proportional
to the square of the distance between them
• F g = Gm1m2/r2 where G = the universal
gravitational constant = 6.67*10-11N.m2/kg2
Gravity
• As weight = mg then w = GMEm/r2
• Then g = GME/r2
• Therefore the larger the mass of an object
the larger the effective gravity it generates
Forces of Friction fs and fk
• An object moving on a surface or passing
through a fluid experiences resistance to
motion = friction
• F f = forces of static friction = the force that
prevents movement of an object that is
being subjected to an external force
• When movement is about to occur fs is at
max.
s
fs and f k
• When a force F exceeds Ffs max then movement will
occur and the new friction force is less. This new
friction force is called the Force of kinetic friction
fk
• When F-fk= positive value there is acceleration
• Both fs and fk are proportional to the normal force;
fs=usn and fk=ukn
• us is the coefficient of static friction
• Uk is the coefficient of kinetic friction
Solving Friction Problems
• Draw a free body diagram making sure to
label all forces