Generally, anything we put action into is work. For example, eating, walking, reading, writing, running and so on. Work is energy put into action. Work can be categorised into three types: positive work, negative work and zero work. This article will cover the concepts of work and energy, positive and negative work done examples to make the topics clear.
Work
Doing work requires mental and physical effort. Work is said to be done when force is applied to an object and there is a change in its position. Scientifically, work is only done when a certain distance is covered or when force is applied. If the object against which the force is applied does not move, then no work is done. If a man carrying a heavy bag is standing still, no work is done according to the scientific definition of work.
For example, if a man lifts a heavy bag from the floor and moves in a specific direction, work is said to be done. If a man carrying a heavy bag is standing still, no work is done according to the scientific definition of work.
To define work, we can say – force applied to an object causing it to move in any direction is known as work. Work is done so that both the force applied and the distance an object covers are included in the work done; that is, work is the measure of energy used in moving an object.
SI Unit of Work is the unit of energy = Joule (J).
1 J = 1 N m; since force is measured in Newton (N) and distance is measured in meter (m), the product of force and displacement gives N m, which equates to 1 J.
The formula of work done (W) = Force (F) * distance (d)
Force and distance are the vector quantities because both of them have magnitude and direction. Work is the product of force and displacement/distance, so it has only magnitude and no direction. Hence, work is a scalar quantity.
Work Done Against Gravity
Work done against gravity is simply the measure of energy expended when the force is applied against the gravity. Gravity is the force of attraction that pulls the body towards the earth.
An object falling on the ground is because of the gravitational pull of the earth. If we pick up that object from the ground, it is called work done against gravity.
We know that,
Work (W) = Force (F) * Displacement (d)
And, gravitational force (F) = mass (m) * acceleration due to gravity (g)
In calculating work done against gravity, the object’s displacement is the height of the object falling to earth. So, distance (d) = height of the object from the earth (h)
This concludes, Work (W) = Force (F) * Displacement (d)
Work (W) = mass (m) * acceleration due to gravity (g) * height (h)
W = m * g * h
The SI Unit for work done against gravity will be Joule (J).
Positive Work Done
Positive work done is the work done in the direction of movement of the object. If the direction of force applied is the same as the direction of motion of the object, then the work done is positive work done.
For example, the object falling on the earth is in the direction of the force of gravity. Since the force of gravity is acting downwards in the direction of the falling object, the work done is said to be positive work done.
W = ( + ) F * d
Here are a few examples of positive work to clearly explain the positive work done:
- Players kicking the football in the direction of motion.
- A nurse moving the patient in the wheelchair.
- A person riding a skateboard.
- Vehicles on the road, moving forward.
- Cutting the vegetables using the knife.
- Lifting the chair and moving it in another direction.
- Moving a box across the table.
- Two children throwing a ball at each other.
Negative Work Done
Negative work done is the work done the opposite to the direction of movement of the object. If the direction of force applied is opposite to the direction of motion of the object, then the work done is negative work done.
For example, if the object is thrown in the upward direction, the motion of the object is opposite to the gravitational force of the earth acting downwards. Hence, the work done is the negative work done.
W = (-) F * d
Here are a few examples to clearly explain the negative work done:
- When a ball is thrown up in the air, it moves against the force of gravity.
- While walking on the road, the frictional force acting against the movement of the feet is negative work.
- When the water is being pulled out of the well, the force applied on the rope is downwards, but the bucket comes upward.
- The case of the see-saw is also an example of negative work done because the force is applied downwards, and the person sitting opposite moves in the upward direction.
- When one jumps, then the work done by the gravitational force is negative work done.
- When one catches an object coming from a distance with some force, the hands move downward.
- A game of tug of war is an example of negative work done where the opposite team applies force in the opposite direction.
- In a game of wrestling, the negative work is when the person is applying force to defeat the opponent.
Zero Work Done
Zero work done is either when there is no force applied to the object or when force is applied, but the displacement is zero.
For example, if the force is applied hard against the wall, the applied force will not cause any displacement. Hence, the work done is zero. In this case, the measure of energy used does not matter because the energy causes no object displacement.
Conclusion
Any force causing an object to move in a direction is work done. Depending on the movement of an object toward or against the force, we define positive and negative work done. If there is no displacement at all, then work done is said to be zero. It is important to remember that work done is a scalar quantity while both force and displacement are vector quantities.
By: Sana Gupta
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