Hi Folks, I am having a devil of a time understand the concept of input work vs input force (relating to machines). OK -- I get that you push down a car's pedal and the car moves. Your force is vastly increased by the car's engine. The input force is much much less than the output force. The car has a mechanical advantage of some factor. But I don't understand how this isn't the same terminology as work. If my input work is pushing the pedal, the car still moves the same amount. The amount of work done by the car is a huge amount more than mine. Sure, my work is the same, but the ouput work is much more than the input work. What am I missing here?!?!! Thanks! WT
You are trying to mix mechanical energy with human energy and those don't mix well together. Trying to figure out why mechanical energy works is one thing by itself. "Mechanical energy is the sum of energy in a mechanical system. This energy includes both kinetic energy, the energy of motion, and potential energy, the stored energy of position. A mechanical system is any group of objects that interact based on basic mechanical principles. Typically, in a mechanical system, gravity is the only major outside force that needs to be considered. In a chemical system, on the contrary, the forces between individual molecules and atoms must all be considered." http://www.wisegeek.com/what-is-mechanical-energy.htm Then human energy which is derived from food. Like other forms of energy, food energy is expressed in calories or joules. The food calorie is also (confusingly) called the kilocalorie (kcal), and is equal to 4.182 kilojoules (kJ). The kilojoule is the unit officially recommended by the World Health Organization[1] and other international organizations. In some countries only the kilojoule is normally used on food packaging, while in others the calorie is the most common unit. Carbohydrates, fiber, fats, proteins, organic acids, polyols, and ethanol all release energy during respiration — this is often called 'food energy'.[2] When the food (providing fuel) reacts with oxygen in the cells of living things energy is released. A small amount of energy is available through anaerobic respiration. Nutritionists usually talk about the number of calories in a gram of a nutrient, but this implies that the food actually 'contains' energy. It's better to say that each gram of food (fuel) is associated with a particular amount of energy (released when the food is respired). Fats and ethanol have the greatest amount of food energy per mass, 9 and 7 kcal/g (38 and 30 kJ/g) respectively. Proteins and most carbohydrates have about 4 kcal/g (17 kJ/g). Carbohydrates that are not easily absorbed, such as fiber or lactose in lactose-intolerant individuals, contribute less food energy. Polyols (including sugar alcohols) and organic acids have less than 4 kcal/g. http://en.wikipedia.org/wiki/Food_energy
I'm sure you hear this all the time, but I'm not trying to get someone to do my homework or cheat on anything. I just don't understand what seems to be a logical fallacy: I'm being introduced to the concept of machines with the sentences: 1. Machines make work easier. 2. Work is done when a force causes an object to move in the same direction that the force is applied. 3. The output work of a machine cannot be greater than the input work. This makes me think that two opposite things are being said. First, that a machine will make it easier for me to exert a force, but also that that force won't be greater than what I've exerted. How can something not be greater, yet be easier? GDT
Because you sacrifice (trade off) something in the process. Here's an example: You want to raise a heavy weight to some height. By using a winch, hoist or windlass (a series of pulleys), you can multiply the force - making the job easier - but taking extra energy to do so. You will have to pull on the rope so much that you will wind up pulling a much greater length of it than the actual distance the weight is raised. So in the end you have traded the use of extra energy to make the job easier to accomplish. Once you have studied the subject a little more you will come to understand that the foot-pounds of work you've done (your effort) is more than the foot-pounds of actual work accomplished in raising the weight. Please Register or Log in to view the hidden image!
You're right, it seems like a contradiction. The first answer has already been explained by Steve100 and Read-Only, which is that leverage allows us to achieve things that we could not normally do even though the actual work involved is no different (think of a 10-speed bicycle). The second answer is that the input work is not restricted to human effort - machines make work easier because they allow us to use energy sources such as gasoline...
More force = push harder More work = more total energy Think of simple tools. Hammer. Screwdriver. Spanner. Each of those allow you to push harder on something (exert a greater force) than you could with only your hands - but all the work done (energy spent) comes from you. The tools don't add any energy (unless they are powered from mains or battery).
