The work | POINT classical mechanics
The work
The physical meaning of work differs from its meaning in everyday life. Work in physics does not mean doing strenuous mental or physical work. In order to do work on an object, the object must move a certain displacement as a result of the effect of your force. If the object does not move, you have not done work, regardless of the amount of force you apply to the object.
Therefore, work is related to two interrelated factors (conditions for doing work):
(1) That a certain force acts on the object.
(2) That the object moves a certain displacement in the same direction as the action of the force.
This is clear from the following two examples:
1- The player who lifts weights upwards does work
Because the force acting on the weights moves them upward a certain distance in the direction of the force.
2- the person who pulls the wall does not do work.
because the force acting on the wall does not move it (the wall remains stationary).
Conclusion: When a force acts on an object and moves it a certain displacement in the direction of the force's line of action, the force is said to be doing work.
The work (w) is determined from this equation: W = F.d = Fd cos(θ)
[F is the applied force
d is the displacement that the body moves in the direction of the line of action of the force
θ is the angle between the direction of the force and the direction of the displacement]
The unit of work is kg.m²/s² equivalent to N.m or joule (J)
and its dimensional formula is M.L/T².
Work: is the product of the force acting on an object and its displacement in the direction of the line of action of the force.
A joule: is the work done by a force of 1N to move an object with a displacement of 1m in the direction of the line of action of the force.
*Although force and displacement are vector quantities, work is a scalar quantity because work is the scalar product of force and displacement vectors.
The effect of the angle of inclination (θ) on the value of the work done
1) Angle value (θ) equal 0°
- The work done is: that is, when the direction of the force is in the same direction as the displacement, the work done becomes a maximum positive value.
Example: A person pulls a horizontally flat object and moves it a distance.
W = Fd cos(0) = Fd
2) Angle value (θ) equal 0°<θ<90°
The work done is a positive value. This is because: The angle between the direction of both the force acting on the body and the displacement is less than 90°, so the cosine of the angle is a positive value (the person is the one who does the work on the body).
Example: A person pulling a suitcase on wheels.
3) Angle value (θ) equal 90°
The work done is: That is, when the direction of the force acting on the body is perpendicular to the direction of the object's displacement, the work done on the body becomes zero.
Example: A girl carrying a bucket and walking a horizontal distance, where the direction of the girl's horizontal movement is perpendicular to the direction of the force exerted by the girl's hand on the bucket.
W = Fd cos 90 = 0
4) Angle value (θ) equal 180°>θ>90°
The work done is a negative value and this is due to: the angle between the direction of each of the force acting on the body and displacement are greater than 90° and less than 180°, so the cosine of the angle is a negative value.
Example: A person is trying to pull a horizontally flat object while it is moving against the direction of the line of action of the force.
5) Angle value (θ) equal 180°
The work done is: That is, when the direction of the force acting on the body is opposite to the direction of its displacement, the work done becomes a negative maximum value.
Example: Work done by frictional forces (such as brake force).
W = Fd cos 180 = -Fd
Work calculation graphically
Work can be calculated graphically using the force-displacement graph, as follows: If a force F acts on an object, causing it to displace d in the same direction as the force acting, then (θ = 0°),
When representing the (force - displacement) relationship graphically, we get the corresponding figure: Work = Force × Displacement
Then Work (graphically) = Area under the (force-displacement graph)
Scientists who benefited humanity:
James Joule (1818-1889 AD):
The English scientist James Joule is considered one of the first to realize that work generates heat. In one of his experiments, he found that the temperature of the water at the bottom of the waterfall was greater than at the top of the waterfall, proving that part of the energy of the falling water
was converted into heat.