Pushing a wall all day may feel like work, but unless you get that wall moving you're not doing any work according to the rules of physics. In this video lesson, you'll learn how work is defined as well as how to calculate the amount of work done on an object.
What Is Work?
On a typical day, you probably wake up, get dressed, eat breakfast, and head off to work. After you spend all day at your job, you go home, eat dinner, walk the dog, maybe watch some TV, and then go to bed. In this sense, work can be just about anything - construction, typing on a keyboard, driving a bus, teaching a class, cooking food, treating patients, and so much more.
But in physics, work is more specific. This is the displacement of an object due to force. How much work is done depends on the distance the object is moved. This makes it easy to put work into a solvable equation: work = force * distance. While this equation is fairly straightforward, there are three important things to note. First, the object must move over some distance in order for work to be done. Second, the force and the distance of movement must be in the same direction. And finally, the force must be constant.
The units we use for work are joules (J), named for James Prescott Joule. Though he is now known for his work in science, he actually preferred brewing beer… until he realized how science could help him be a better brewer!
The joule is a combination of both of the components on the right side of our work equation: force and distance. Quite simply, a joule is a Newton-meter (N*m), and one joule of work is done when a force of 1 N is exerted over a distance of 1 m.
This amount of work is on par with lifting an apple over your head. I bet you didn't realize that counted as work, but in the world of physics, it does! You can probably see that 1 J isn't really practical to use for larger amounts of work, so instead we use kilojoule (kJ), which is 1000 J, or megajoule (MJ), which is 1,000,000 J.
Regardless of the amount of work done, it involves three key components: the amount of force, the distance displaced, and the cause of the displacement, which is the force itself.
Calculating Work Done
Work is an interesting concept because the same amount of work can be done in different situations. For example, if you lift a 5 N load 10 m in the air, the amount of work done is: 5 N * 10 m, or 50 J.
But say you lift twice the weight over half the distance. In this case, the amount of work done is: 10 N * 5 m…also 50 J! The work done is the same because even though the displacement distance is less, the weight of the object is more, so it takes more force to displace it.
But if you were to lift the 10 N object the original distance of 10 m, the amount of work done in this case is twice as much because now 10 N * 10 m = 100 J.
Can you see how the amount of work done increases if you increase either the force (the weight), the distance, or both? Lifting the same amount of weight twice as high means twice the amount of work is done. Likewise, lifting twice the weight over the same distance also doubles the amount of work.
However, simply holding an object in the air doesn't count as work. The object must move over some distance in order for work to be done, the force and the distance of movement must be in the same direction, and that force must be constant. So if you move that apple up in the air, you're doing work on the apple, but holding it in place is not doing any work because the apple isn't moving. Likewise, you can push on a wall all day, but if that wall doesn't move, no work is done on it!
Our everyday work is different for each individual. But in physics, work is only one thing: the displacement of an object due to force. Anytime work is done there are three components involved: the amount of force (in Newtons), the distance of the displacement (in meters), and the cause of the displacement (the force).
In multiplying force and distance for work, we end up with the unit of joules. 1 J of work is equal to a force of 1 N exerted over a distance of 1 m, about the same as lifting an apple over your head. Because of this, we often work with kJ or MJ for very large amounts of work.
The amount of work done can be increased by increasing the amount of force, the distance of displacement, or both. But in order for work to be done, there must be a displacement of the object. Pushing on a wall may be difficult and tiring, but if you don't move that wall, no work is done on it.
Review this lesson so that you can:
- Assess the components of 'work'
- Define work in physics
- Write the formula that measures the amount of work that has been done