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## Monday, April 16, 2012

### CBSE PHYSICS: Force Fiction and Pressure for VIII Physics-Notes a...

FORCE AND PRESSURE
A force is a push or pulls acting on an object which changes or tends to change the state of the object.
In the international system of units (SI System), the unit of force is Newton (N) that is named after Sir Isaac Newton.

There are also other units like dyne, kilogram weight and pound.

Action of force and its effects:

Contact and non-contact forces

A force forces which act only when there is physical contact between two interacting objects are known as contact forces.

Forces which can act without physical contact between objects, i.e. those that can act from a distance, are called non-contact forces or field forces.

Types of forces

1. Muscular force: This is the force we can exert with our bodies by using our muscles, e.g. pull, push, kick etc. These are contact forces

2. Magnetic force : Magnets exert forces of attraction or repulsion on other magnets. An important feature of magnetic force is that it can act from a distance, and is therefore of a non-contact force.

3. Electrostatic force: The force exerted by a charged body on another charged or uncharged body is known as electrostatic force. Electrostatic force is used to separate solid pollutant particles from smoke given out from factories.

4. Gravitational force : All objects in the universe exert a force on all other objects. This is called gravitational force. The  gravitational force exerted by the Earth on all the bodies on its surface is called gravity.

5. Frictional force : The fact that the rolling ball comes to rest after some time shows that there must be a force acting on it which tends to slow it down. This force seems to be more on rough surfaces than on smooth surfaces. The force acting against the relative motion of surfaces in contact is called frictional force or friction.

6. Tension Force:Strings, ropes and chains can only pull on things!  The force of pull supplied by strings, ropes or chains is called the tension force. The tension force is always directed along the length of the thing doing the pulling (string, rope, chain).

7. Spring Force‎: is the force exerted by a compressed or stretched spring upon any object that is attached to it.

8. Applied Force:  An applied force is a force that is applied to an object by a person or another object. If a person is pushing a desk across the room, then there is an applied force acting upon the object. The applied force is the force exerted on the desk by the person.

Mass and Weight : The mass of an object is the amount of matter that is contained by the object; the weight of an object is the force of gravity acting upon that object.
The mass of an object (measured in kg) by Pan Balance and  will be the same no matter where in the universe that object is located. On the other hand, the weight of an object (measured in Newton) by Spring balance and  will vary according to where in the universe the object is

Type of friction: