# Calculate power using energy and work concepts

1. Mar 22, 2014

### Bittencourt

Hi,
I was reading a physics book and started to make some links, but i don't know if it's correct.
For example:
I need to calculate the power to move a car in a slide with friction. But as far as it's a car pulled by an motor, the motor provides the needes force to keep the speed.

I made the relations using moment:

Work = Moment and Work = Energy

M = F x wheel radius (this relation is between moment, pulling force and wheel radius of the car)

K = 1/2 m x v^2

Friction = m x μ x cos α

Horizontal component of weight: Px = m x sen α

So, I applied the forces (friction, horizontal component of weight and resultant force) in the wheel radius:

wheel radius x F - Friction x wheel radius - Px x radius = 1/2 x m x v^2

With this I found the force

W = F x d but I need to convert to [Newtons]

P = W/t

I would like to know if this relations are correct. As far as it isn't a conservative system, I got a bit lost.
I tried to get some numeric examples from the books to clarify, but I didn't find any example close to this. Only hypothetical systems that don't fit to reality.

What about the potencial energy, should I take in consider too?

Thank you very much!

2. Mar 22, 2014

### Simon Bridge

Welcome to PF;
... I think you need a clearer problem statement - in science, most of the work is in asking the right question.
As it stands, it's a good effort but I suspect you are over-thinking the situation.

If the car is moving forward on a level road and sliding, it slows down unless you give it some power.
Work out the rate it would lose energy at the speed you want to maintain - this is the additional power you want to supply in order to maintain that speed.

At constant velocity - you are converting energy in the fuel tank into heat and noise in the tires and the road and the air. What you consider important depends on the question you ask.
i.e. maybe you want to know how big-a power plant you need to keep a constant speed?
Then you don't care about the PE in the fuel tank.
But if you want to know how long a vehicle can keep this up, then the amount of fuel is very important.
If the road is not level then gravity becomes important.
If you are going around a corner, then you are accelerating, so you need more power etc.

3. Mar 23, 2014

### Bittencourt

An explosion engine is way beyond my knowledges. I was thinking in a simplier thing, like an electric motor.

In the company I work, there is a small car that brings any kind of stuff from one plant to the other, and it passes in fornt of me 1000 times per day.
It has a plate telling that the load can't exceed 1000 kg and it moves over a small rail. As I was reading about it, I started to think how the forces, moments and energy work in this situation.
The project of the car looks so simple, that I was curious.

4. Mar 23, 2014

### Simon Bridge

... at this level it makes no difference: the basic question that I answered does not depend on the way the power is produced.

You found yourself musing about how the World around you works. This is a horrible disease called "a scientific tendency"; it will only get worse and there is no cure, but you can get help. Basically, you are a latent scientist :)

There is nothing for it but to continue as you have been doing - learning how other scientists have been describing things and making connections, talking to other afflicted people, and getting some guidance. You are doing fine.

The big thing in science is asking questions - you usually start with a fairly general question, and then narrow it down to what you need to know. This is the first, and most important, stage in the scientific method. You've just started this journey and you've learned that you can get an answer about how much power is needed quite simply ... but you need a lot of detail if you want to know about how the power gets from the engine, via transmission and wheels, to motion in the car.

A good take-away for you is that you don't always need to talk about "forces" ;)

Last edited: Mar 23, 2014