Force and laws of motion class 9th notes

Force and laws of motion class 9th notes

FORCE

A force is defined as any influence that can produce a change in the velocity of an object. It is a vector quantity.                                                    

• Force is an external effort in the form of pushing, pulling, stretching, compressing etc., which may move a body at rest or stop a moving body or change the speed of a body or change the direction of a moving body or change the size and shape of a body.

Effects of force

• A force can make a stationary object move
• A force can make a moving object stop
• A force can change the speed of a moving object
• A force can change the direction of a moving object
• A force can change the shape of an object
• A force can change the size of an object

Forces can be classified into two categories

Contact Forces 

1. Applied force: Force which is applied to the object by another object.  A person pushing his car is an example of applied force. When the person pushes the car then there is applied force acting upon the car.
2. Frictional force:  Friction force is the result of two surfaces being pressed together closely. The frictional forces depend upon the nature of the surfaces. The rougher the surface the more will be the frictional force. 
3. Air Resistance Force:  The air resistance force acts upon objects as they travel through the air. This force opposes the motion of the object in the air.  Example: A skydiver diving in the sky.
4. Spring force: The spring force is the force exerted by a compressed or stretched spring.  Here the magnitude of the spring force is directly proportional to the amount of stretch or compression of the spring.
5. Tension Force:  The tension force is transmitted through a string, rope or cable. The tension force is directed along the length of the wire.  This force pulls equally, the objects that are placed at opposite ends of the wire.
6. Normal force:  This is also called the support force. If an object is resting upon a table then the table is exerting an upward force upon the object in order to support the weight of the object.  If a person leans against a wall, the wall pushes horizontally on the person.

Non-contact Force

1. Gravitational force: Gravity is a non-contact force between of two objects. Such as:  An object thrown up is pulled back to the earth by the gravitational force of the earth
2. Electric forces:  Electric charges are able to jump between objects that are within a close enough range but do not have to actually be in physical contact.
3. Magnetic forces:  Magnetic forces act over a distance. Objects need not be in physical contact with each other for magnetic field to pass through them.

Types of forces acting on a body

1. Balanced forces

If the resultant of all the forces acting on a body is zero, the force is called balanced forces. Example: When we hold a suitcase steady at some height, the suitcase is under the action of two forces.

1. Weight mg of the suitcase due to gravitational pull of earth, acting vertically downwards.
2. Force applied P by our hand in the vertically upward direction. As mg = P, therefore, the forces are balanced, and the suitcase is steady.
3. Unbalanced forces

If the resultant of all the forces acting on a body is not zero, the forces are called unbalanced forces. Example: A ball set rolling on the ground stops after some time. This is due to unbalanced force of friction between the ball and the ground.