What is work?
I have often said that in science we use words for different meanings than we do in real life. This is not entirely true.
What I really mean is that in science we often use words for more specific meanings than we do in the rest of our lives.
“Work” is a perfect example.
The meaning of work.
Scientist use work to describe the act of exerting a force on an object that makes it move.
When we push a swing we exert a force onto the swing, this make the swing move away from us: We have done work on the swing.
When you pick up a stack of books from your table, you are exerting a force onto those books and making them move: You are doing work on the books.
Work must contain the following things:
A force must be exerted.
The force must be onto an object.
The object must move.
The movement must be in the same direction as the force.
Let’s look at those one by one.
A force must be exerted.
This is obvious, to be work there must be a force exerted. That means work requires energy (more on energy later in the course.) So the ability to do work must be connected to our ability to use energy to create a force. There are many types of energy, the two that will be most relevant to this topic are potential energy and kinetic energy.
Potential energy: This is energy that has the potential to be used to move something. A spring when it is pressed down has potential energy, a rock balanced on top of a cliff has the potential energy to fall down and crush a car.
Kinetic energy: This is the energy of movement, when an object is moving it has kinetic energy. In the example above when the rock falls it turns potential energy into kinetic energy.
More on energy later.
The force must be onto an object.
This one is obvious. In order for an object to move a force must act upon it (Newton’s first law) so in order for us to do work we have to have an object to make move.
See how Newton’s first law and work are tied together? We are using our ability to do work to overcome the inertia of the object.
If I want to push a desk across the room I have to overcome the inertia of the object to get it moving, then I also have to overcome friction and gravity to move it along.
Knowing how lazy I am, you are probably thinking “that sounds like a lot of work”
The object must move.
This one seems the most odd, but it is usually the most important for deciding if something is, or is not work.
In order for it to be work the object must move. Imagine my previous example of me pushing a desk. If the desk is bolted down to the floor then no matter how hard I push it the desk will not move. I could push and push for hours, getting tired and frustrated, but no matter how hard I pushed the desk would not move an inch.
If this was to happen, no matter how tired I got trying, I would have done no work.
Another example would be holding a heavy weight. Suppose I make you hold your book bag out at arm’s length. You have to hold it there and not let it move. You’d get pretty tired right? But according to the physics definition you would have done no work at all.You are exerting a force upwards to hold up the bag, but the bag is not moving.
The movement must be in the same direction as the force.
Let’s think about that book bag again. If you have a backpack style book bag then when you carry it you must exert force to hold it up, and when you are walking along the hallway the bag is in motion. However, the motion is horizontal (along the hallway) and the force is upwards towards the ceiling. Because of this you are doing no work on the bag (no matter how many heavy books are in it.) That is because the force and motion are not in the same direction.
But what about when only some of your force is in the same direction?
Imagine the book bag you use is the suitcase style with the wheels on it, then not all of your force is in the direction of the bag’s motion. You are pulling the arm of the bag at an angle to its motion. In this case you are still doing some work, but not as much as you’d probably like to claim. In fact you are only using some of the force for work, the other force is not. The angle at which you pull the bag will help decide how much force is working.
Calculating work.
Once you have the rules about what is, and is not work, calculating the amount of work being done is very easy. We know it is more work to move a heavier object than a lighter one – because the heavier object takes more force to move it. We also know it is going to be more work to move an object a long way than moving it a short way. So we can use this simple formula:
Work = Force x Distance
But what do we measure it in? Well the SI unit of work is the N/M (Newton per Meter) and in honor of the physicist who worked out most of the stuff we know about work we call this unit a Joule (after James Prescott Joule.) One Joule (J) is the amount of work you do when you exert 1 Newton of Force to move an object 1 Meter.
Imagine you are trying to lift a very heavy set of weights at the gym. The force required to lift the heavy weights is 200N and you need to lift them 2 meters to raise them above your head. So the total work needed is
200N x 2m = 400J
This seems like a lot of work, what if you shared it? Well then each of you would only have to put in half the force.
100N x 2m = 200J (each)
The total work would be the same, but done between two people. Think of that next time you need to move something heavy, and get some help.
Of course when our weightlifter finally gets that set of weights above his head he stops doing work, even though it looks like very hard work holding those weights above his head, he is doing no work at all because the object is no longer moving.