All about Force?

Force definition

The push or pull of a body by an object is called force on the body.

or

Force is an external influence acting on a body to change its state of motion.

Types of Forces

Contact Force 

Force which results when there is a direct physical contact between two interacting objects is known as contact force. 

Types of Contact Forces

Muscular Force

The force which we exert by using our muscles is called as muscular force.

Friction Force

The force which resists the motion of an object over the surface of other.  The direction of friction force is always opposite to the direction of the motion relative to the surface.

Normal Force

The friction force, on an object, which acts perpendicular to the surface of  its plane is called as Normal Force. 

Tension

Tension is a pulling force acting on a object by means of a string against the gravitational force.  

Field Force

The force which occurs without any physical contact is known as force at a distance or field force.

Field is region of imaginary lines around an object. When an other body is placed in this region, it will experience the force. The greater the density of these imaginary lines, the stronger the forces in that region.

Types of Field Forces

Magnetic Force

Magnetic force is a field force responsible for attraction of like poles and repulsion of unlike poles.

Electrostatic Force

Force exerted by a charged body on another charged body is known electrostatic force. This force is due to electric field from the charge.

Gravitational Force

The force of attraction that exists between any two masses n the universe is known as Gravitational force. This force is due to electric field from the charge. Gravitational force is also non contact force and hence it is also a field force.

Net Force

Multiple forces can act on an object simultaneously at a time. When the forces act in same direction then net force will be addition of all forces. When the forces are in opposite direction, then net force will be difference of the forces. When the Net force is zero, there is no change in state of object. 

The direction of motion of the object will be in the direction of Net Force. If the Net Force acts in direction of motion, the speed of the an object moving with constant speed also increase. If the Net Force acts in a direction opposite to the motion, then it either slows down the object or brings it to rest or it may change the direct of motion. 

The unit of force in SI system is Newton(N). 

Pressure

The force acting perpendicularly on unit area of a surface is called as pressure.

Pressure = Force/Area

The unit of pressure in SI system is Newton/Meter² or N/m². 

  

FRICTION - Important Points to be remembered

1.  The force which opposes the relative motion of two surfaces of bodies in contact, is called as "frictional force".

2. Friction is tangential force between the contact surfaces of two bodies.

3. Friction is due to Electromagnetic Forces between the surfaces in contact.

4. Friction is due to molecular interaction at the surfaces in contact.Friction is due to adhesive forces between molecules of two surfaces in contact.

5. Friction depends on nature of surfaces in contact and on the impurities present on these surfaces.

6. Normal Force: When two bodies are in contact or when one body is placed over another body, the contact force which either body exerts on other normal to contact surface is called Normal Force or Normal Reaction.

7. Friction is proportional to Normal Force.

8. Limiting Friction is least force necessary to set a body into motion.

9. Sliding Friction is the friction which comes into play when the surface of an object moves relative to the surface of another object.

10. Static friction is the friction which comes into play when surfaces of the objects are at rest relative to each other even there is an external force acted upon. 

11. Static friction is a self adjusting force.

12. Kinetic Friction is not a self adjusting friction.

13. The substances which reduce friction are called as lubricants.

14. Generally coefficient of static friction is less than 1 but in some cases it may exceed 1.

15. Frictional force is a "Non-Conservative" force.

16. If a body of mass 'm' is on the floor of a lift which is moving with uniform acceleration 'a', Normal force on body or its apparent weight is

N = mg ±ma = m(g±a)

a) If the lift moves up, then N = m(g+a)
b) If the lift moves down, then N = m(g-a)
c) If the lift falls freely, then N=0
d) If the lift moves with uniform velocity, then  a =0, and N=mg

17. When a person falls on a rough road, the frictional force exerted by road on him is along his direction of motion.

18. The angle made by resultant of Normal force and Limiting friction with Normal force is called angle of friction. The tangent of this angle gives coefficient of static friction.

19. The substances which reduce friction are called Lubricants. 

20. A good lubricant must be highly viscous and low volatile in nature.

21. The frictional force exerted by fluids is also called as "drag".

22. Frictional force on an object in a fluid depend on its speed with respect to fluid, on the shape of the object and on the nature of fluid.

23. Friction can produce heat.