When you hold a pen with your fingers with pen being vertical. Then friction force is the contact force. If you keep a pen on your palm then it is the normal force acting on it through the contact. If you leave the pen, then the pen falls on ground. That is due to the gravity of EARTH. This is a non-contact force.
Friction force : it acts
through the contact between two bodies. It acts along the surface in the
direction opposite to the relative speed between the surfaces. It is equal
to the coefficient of friction multiplied by the normal force between the surfaces.
force: it is the contact force (it is the push between the two bodies). It acts
perpendicular to the surface. If a body is lying on a floor. The
body exerts a force equal to its weight on the floor. Floor exerts a
force on the bottom surface of the body equal to the weight of the body,
force , push or pull force or applied force : The force you exert on a
door, when you push the door. The applied force can be at any angle.
Spring force, - you have
to pull the string by touching , then
only spring force acts. It is proportional to the extension and opposite to the
direction of extension.
Tension force: It is the
force developed in a string or thread or a thin material, when it is subjected
to a push or pull at its ends. This
force is same all along the thread or rope or string. Tension makes the string
or thread tight. It always acts inward from the ends to the center of the
Air resistance force or
drag force : it is proportional to the velocity and is opposite to the movement
of the body.
force in a fluid: it is equal to the weight of the fluid displaced by the body.
It acts through the surface of the body immersed in a liquid. This is how
ships float on water.
NON-CONTACT FORCES or FORCES AT A DISTANCE:
These are also called fundamental forces. They are acting even if there is not contact betwen bodies and even if bodies are in vacuum.
Gravitational forces : our weight for example is due to gravity. Earth revolves around Sun due to gravity. Newtons' law gives the formula. Force = G M1 M2 / d²
G = universal gravitational constant M1, M2 are masses
d = distance between the bodies.
These forces are always attractive.
Electrical forces: These are the forces which act between bodies having electric charges : positive or negative. The formula is = F = (1/4 π ∈ ) Q1 Q2 / d²
∈ = electric permittivity of medium Q1, Q2 are charges on the body
d = distance between bodies/charges
Opposite charges attract and similar charges repel.
Magnetic forces: This is the force of attraction or repulsion between two opposite poles or similar poles, respectively. Force is F = (1/4π μ ) m1 m2 / d² ,
μ = magnetic permeability of the medium m1 and m2 are pole strengths
d = distance between the poles
Nuclear forces: These are the forces among the particles at nucleus level.