Download Concept of a Force

Concept of a Force
What is a Force?
• Usually think of a force as a push or pull. A force
can deform, stretch, rotate, or compress a body.
• It makes an object accelerate.
• A force is the result of an interaction between
two objects
– An object doesn’t possess a force
– An object cannot produce a force on its own. It needs
a partner
• Vector quantity
• May be contact or field force
Contact and Field forces
• Contact forces
require contact
between the 2
objects interacting
together.
• Field forces don’t
require contact.
They act at a
distance.
The Four Fundamental Forces
• There are four kinds of forces in nature. These forces are called fundamental
forces:
– Gravitational force: This is the force that holds us onto the Earth.
• The weakest force among the four fundamental forces.
• Infinite range
– Electromagnetic force: This is the principle that keeps atoms together. This is the force
which exists between all particles which have an electric charge.
• Many everyday experiences such as friction and air resistance are due to this force. This is also the
resistant force that we feel, for example, when pressing our palm against a wall.
• strong force
• Infinite range
– Weak nuclear force: responsible for the radioactive decay processes inside the atomic
nuclei
– Strong nuclear force: responsible for keeping quarks together inside protons and
neutrons, and keeping protons and neutrons inside atomic nuclei.
• Very strong but short range
– The gravitational and electromagnetic forces can be experienced in our daily life. They
are also called the familiar forces which are the electromagnetic and gravitaional
forces. Similarly, the strong force and the weak force are called the unfamiliar forces.
Common Examples of Forces
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Weight
Tension
Normal reaction forces
Drag forces
Upthrust forces
Frictional forces
Weight
• The weight of an object is the result of the interaction
between the object and the planet earth.
• So, the weight of an object depends not only on the object
but also on the planet it is interacting with.
– Your weight on Earth is different that your weight on the Moon or
Jupiter.
• Symbol: w or Fg
• Weight is vector quantity. Its direction is always vertically
downward, towards the center of earth.
• W= m.g
– m is the mass of the object. SI Unit is Kg.
– g is the gravity of the planet (also called gravitational field
strength). SI unit is Nkg-1 or m.s-2
– Unit of weight is N or Kgms-2
Weight versus Mass
• Mass is the property of the object, Weight is
not.
• Mass of an object is constant everywhere in
the universe, weight varies according to the
location of the object.
Tension
• Exists in ropes, strings, springs, etc.
• Tension force in a string is the result of
electromagnetic interactions between the
molecules of the material making up the
string.
• A tension force in a string is created when two
forces are applied in opposite directions at the
ends of the string.
More on Tension
• Consider the string hanging from the
ceiling from one end and attached to a
mass M at the other end.
• At the point of support at the ceiling,
the tension force pulls down on the
ceiling.
– at the same time, the ceiling pulls
upwards on the string by the same
amount of force.
• At the point of contact with the mass,
the tension force acts upwards on the
mass.
– at the same time, the mass pulls down on
the rope by the same amount of force.
More on Tension
• Most of the time, we neglect the mass of the
string.
– The tension is the same at all points in the string.
• The direction of the tension force is always
along the string.
• Non-stretched string has zero tension force.
More on Tension
Normal Reaction Force
• The normal force is the
support force exerted upon an
object that is in contact with
another stable object/support.
• if a book is resting upon a
surface, then the surface is
exerting an upward force upon
the book in order to support
the weight of the book.
• The direction of the force is
always perpendicular to the
surface exerting the force.
• The origin of the force is
electromagnetic force.
Drag Forces
• Drag forces are forces that oppose the motion of a
body through a fluid (gas or fluid). Its source is also
electromagnetic force.
• Air resistance forces acting on a car are examples of
drag forces.
• A marble thrown in oil or in honey experiences drag
forces.
• Drag forces depend on:
– The shape and size of the object,
– velocity and inclination
– Properties of the fluid: viscosity
http://www.grc.nasa.gov/WWW/k-12/airplane/drag1.html
Upthrust
• Any object placed in a fluid
experiences an upward force
called upthrsut, buoyant
force, or lift.
• If an object floats in water,
the upthrust will be equal to
the weight of the object.
• If upthrust is less than weight,
the object sinks.
• If upthrust is more than
weight, the object rises: the
example of hot air balloon or
helium balloon.
• Formula 1Formula for Success: Aerodynamics
• http://www.bbc.co.uk/sport/0/formula1/2025
7227
Frictional Forces
• Frictional forces are also
electromagnetic in origin.
• The oppose the motion of
the object.
• Friction arises when two
surfaces move past each
other. This type of friction is
called kinetic friction.
• Friction forces can also exist
when the two surfaces are
not sliding across each
other: static friction.
Static Friction
• A heavy box rests on the floor: static friction force exists between the surfaces of
the floor and the box to prevent the box from being set into motion.
• The static friction force balances the force that you exert on the box such that the
stationary box remains at rest.
• When exerting 5 Newton of applied force on the box, the static friction force has a
magnitude of 5 Newton.
• Suppose that you were to push with 25 Newton of force on the large box and the
box were to still remain in place. Static friction now has a magnitude of 25 Newton.
• Then suppose that you were to increase the force to 26 Newton and the box finally
budged from its resting position and was set into motion across the floor.
• The box-floor surfaces were able to provide up to 25 Newton of static friction force
to match your applied force. Yet the two surfaces were not able to provide 26
Newton of static friction force.
• The amount of static friction resulting from the adhesion of any two surfaces has
an upper limit. In this case, the static friction force spans the range from 0 Newton
(if there is no force upon the box) to 25 Newton (if you push on the box with 25
Newton of force). 25 N is the maximum static friction force.
• Once in motion, pushing the box becomes much easier. The type of friction that
you need now to overcome is kinetic friction which is less than the maximum static
friction.
http://www.physicsclassroom.com/class/newtlaws/u2l2b.cfm#friction
Free-Body Diagram
• Free-body diagrams are diagrams used to
show the relative magnitude and direction
of all forces acting upon an object in a given
situation.
• The term “free” is because the body is
shown on its own, free of its surroundings
• The object is represented by a box.
• The forces are represented by arrows from
the center of the box outward in the
direction that the force is acting.
• The size of the arrow in a free-body
diagram reflects the magnitude of the
force.
• Each force arrow in the diagram is labeled
to indicate the exact type of force.