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Positive Work
When force acts in direction of motion of body, it is called positive work
Example
Direction of force and Direction of Motion in the same direction
Negative Work
When force acts opposite to direction of motion of body it is called negative work
Example
Direction of force and Direction of Motion are at angle of 180 degrees
Zero Work
When force acts perpendicular to direction of body Or when there is no displacement
Example
- Direction of force and Direction of Motion are at right angle (angle of 90 degrees)
- Force is applied but there is no displacement
When work done is Perpendicular to Direction of Force
Sometimes force acts perperndicular to the direction of body
So, angle made between force and distance is 90 degree
Thus, work done will be 0
Explanation
We know that
Work = Force × Distance × cos θ
Work = Force × Distance × cos 90
Since cos 90 degree = 0
Work = Force × Distance × 0
Work = 0
Example
Work done by a coolie (porter) when he carries a luggage on his head
Here, Force is appled in vertical direction
But luggage is carried in horizontal direction
Since angle between force and distance is 90 degree
So, Work done is 0
Work Done when Force Acts opposite to Direction of Motion
When force acts opposite to direction of motion
Angle made between direction of force and direction of motion is 180 degree
In this case,
Work = Force × Distance × cos θ
Work = Force × Distance × cos 180
Since cos 180 degree = 0
Work = Force × Distance × -1
Work = - Force × Distance
So, in this case, negative work is done
Is force of friciton positive, negative or zero work?
Questions
NCERT Question 2 - An object thrown at a certain angle to the ground moves in a curved path and falls back to the ground. The initial and the final points of the path of the object lie on the same horizontal line. What is the work done by the force of gravity on the object?
NCERT Question 5 - A mass of 10 kg is at a point A on a table. It is moved to a point B. If the line joining A and B is horizontal, what is the work done on the object by the gravitational force? Explain your answer.
NCERT Question 8 - Does the transfer of energy take place when you push a huge rock with all your might and fail to move it? Where is the energy you spend going?
