lecture-3 engineering mechanics force part -2

Principle of transmissibility

The principle of transmissibility states that the point of application of a force can be moved anywhere along its line of action without changing the external reaction forces on a rigid body. .

Let a force F acting on a point A of the body . 

Proof - 

principle of transmissibility only relevant in case of external force and net acceleration.

the principle of transmissibility does not consider internal forces and stresses. 

 Let force act on one side of object 

Principle of superposition of forces - 

The combined effect of force system acting on a rigid body of the sum of the of individual forces .

Let a boat is pulled by two rope . Each rope exert a force P and Q respectively. As we know a body always moves in the direction of forces i.e. line of action of forces. But here boat is not moving in the direction of either of forces. The boat is moving under action of a unknown force R . 

The effect of the unknown force R is equal to the combined effort of two forces P and Q .

Action and reaction forces

let two bodies A and B contact each other. 

to counter the force applied a

The force which body A is applied on body B is called action force.

The force which body B applied on body A called a reaction force .

according to newton's third law

action and reaction have equal and opposite magnitude. 

Concept of free body diagram

A a diagram said to be free body diagram if all the forces and support and stresses are removed and replaced from a body under forces with their respective reaction forces . 

Let a body lying on the ground . As body has weight W , it always acting downward . 

To balance this action force another reaction force act on the body from ground. 

That reaction force is the normal reaction R to the body . A is point of contact through which normal reaction force act on the body.