Input Work vs Input Force

[WriterTyper]

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

[TonyMcC]

All you are doing with the accelerator pedal is controlling the amount of fuel used by the cars engine. The engine does the work and gets its energy from the fuel. How much work does the engine do if you run out of fuel?

[swansont]

Depends on the kind of machine. For a lever or pulley system, you do all the work, but the force is reduced by the mechanical advantage. e.g. a machine where you have a 2:1 mechanical advantage means you can lift a 100N object by exerting 50N of force. But lifting it 1m still requires you to do 1kJ of work, same as if there were no machine. You'll have to push 50N for 2m in order to do this.

[WriterTyper]

So...at what point does my work no longer become my work? Is it when my direct force is spent? Or rather, if there is a new force that is no directly caused by my action then that is no longer work?

[TonyMcC]

Just realised this is the Homework Help so perhaps a little too much has been said already. I will just say YOUR work is what YOU do.

Edited January 11, 2011 by TonyMcC

[WriterTyper]

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?

 

WT

[A Tripolation]

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?

 

The machine does not make it easier to exert the force. You always exert the amount of work that is put into it. When you push down on the gas pedal, the throttle responds proportionally. The chemical energy in the bonds of gasoline are what makes the car move.

 

Think about what you're saying. You seem to be confusing yourself.

[WriterTyper]

Ok, I think I may have come up with a metaphor to help me understand (it's how I understand the best).

 

If I pour 5 liters of water into a bucket, then I make six holes, only 5 liters of water can come out of this bucket.

 

But if I take all day, drip by drip, to fill up the bucket, then I make six holes, the water will spray out much more quickly than it took me to fill the bucket.

 

In other words, the amount of energy I put into the system remains constant, but the machine increases the force applied to the object.

 

Is that a good metaphor?

Edited January 12, 2011 by WriterTyper

[TonyMcC]

Ok, I think I may have come up with a metaphor to help me understand (it's how I understand the best).

 

If I pour 5 liters of water into a bucket, then I make six holes, only 5 liters of water can come out of this bucket.

 

But if I take all day, drip by drip, to fill up the bucket, then I make six holes, the water will spray out much more quickly than it took me to fill the bucket.

 

In other words, the amount of energy I put into the system remains constant, but the machine increases the force applied to the object.

 

Is that a good metaphor?

I don't think you have quite got it. I think the confusion arises because you are not dealing with a machine in what you might call the classical sense. In other words a machine consisting of things like pulleys, cogs and levers. You are dealing with a system where the output power and capacity to do work is much greater than you personally put in. Consider this example - you are a farmer and own a plough. You want to plough a field but pulling the plough up and down the field is beyond your strength. You could use a system of ropes, pulleys etc to allow you to do it, but your progress would be very slow. Alternatively you could use a couple of horses to pull the plough. This would enable you to plough the field in a much faster time and require much less work from you personally. Hope this helps.

Edited January 12, 2011 by TonyMcC

[swansont]

If you want to consider an engine, you have to account for the energy in the gasoline. The amount of work it can do cannot be greater than the energy content of the gas. Energy cannot be created (or destroyed)

[TonyMcC]

Just in case my post was open to misinterpretation I have added the word "personally" after the word "you" in two instances. I have previously indicated in earlier posts that the energy required for the output power comes from the fuel and this process ceases when the fuel runs out. I don't think there is any disagreement.

Edited January 12, 2011 by TonyMcC