Class 9 : Definition of work and it's Type.

In our daily life , the word "work" means any kind of mental and Physical activities , But in physics , the terms work has entirely a different meaning. 

Scientific Conception of work : 

Definition: Work is said to be done by a force on a body or an object of the force applied causes a displacement in the body or the object.

Factors on which work done depends :

(1). The magnitude of the applied force (F).

W ∝ F

(2). The distance travelled by the body on the application of force . 

W ∝ S ( displacement)

Work done by a contant force : 

(i) When a contant force is applied in the horizontal direction.

W = F.S

(ii) When force is applied at an angle with the horizontal direction.

W = FScosΘ

# Dot product of two Vector quantities gives a scalar quantity. So work is a scalar quantity.

Thus , Work done on a body by a force is defined as the product of the magnitude of the displacement and the force in the direction of the displacement of the body.

Type of work done : 

1. Positive work done : if Θ is 0°

W = FScos0° = FS 

Such work done is known as Positive work done. 

# if Θ < 90° ( Acute angle ) , then work is positive .

# Work done by the force of gravity on a falling body is positive.

# When a coolie lifts a box from the ground to put it on his head.

2. No work done or Zero work done : if Θ = 90° the force F acts at right angles to the displacement ( S ) of the body , then 

W = FScos90° = 0

# When a stone attached to a string is rotated in a horizontal circular path , the force acting on the stone is always towards the centre of the circular path. This force is called Centripetal force F.

# if S = 0 , the body does not move from it's position , then W = 0.

# When a person pushes a wall but fail to move the wall , then work done by the force on the wall is zero.

3. Negative work done : if Θ = 180° , force (F) acts just opposite to the displacement ( S ) of the body .

W = FScos180° = -FS

# When Θ is  90<Θ<180 , then work also negative.

# When an object is lifted upward to a certain height , then the work done by the force of gravity on the object is negative.

# In a tug of war , the work done by the losing team is negative.




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