There are some words whose meaning in everyday life is not the same as in physics, one of which is the concept of work.
In everyday use the word “work” is found in various ways, for example:
- The research you did for geography class was really hard.
- It took a lot of work pushing a truck.
- That the masons who are building the house every weekend and receive the same salary, however, the work that each does is different.
Physics scientists agreed on a working definition:
“Work is the product of force by the displacement through which the force acts.”
Note: Work is represented by the letter W (from the English word work).
The displacement (which is a vector) is carried out in a straight line, in this case coincides with the distance (which is a scalar quantity).
W = F · d
F = force in newtons (N)
d = distance in meters (m)
W = work in newtons per meter (N · m)
The unit (N · m) is called the joule (j), named after the English physicist James P. Joule, who did research related to heat energy and work.
(1N) (1m) = 1J
(1 newton) (1 meter) = 1 joule
If you apply a force of 100 N to push a car and travels a distance of 5 m, the work done is:
W = (100 N) (5 m) = 500 N. m
= 500 J
Calculating the work done in raising a body
To calculate the work done in lifting a book from the floor to the table: The weight of the book (P) is the force of gravity pulls straight down and this force must be overcome by applying one of equal magnitude but opposite direction. That is: F = P.
As you may recall, the weight of a body is calculated by multiplying its mass (m) by gravity (g) of the Earth: P = m · g.
The distance over which the force acts required to raise the book is the height (h) between the floor and the surface of the table: d = H.
W = F · d
W = P · h
If the weight of the book is 3 newtons and the table height is 0.7 meters, the work being done to rise is:
W = (3 N) (0.7 m)
= 2.1 N · m
= 2.1 J