Landracing Forum Home
October 17, 2017, 07:25:24 PM *
Welcome, Guest. Please login or register.
Did you miss your activation email?

Login with username, password and session length
News:
BACK TO LANDRACING.COM HOMEPAGE
 
   Home   Help Search Calendar Login Register  


(Note: Donations are not tax deductible)







Live Audio Streaming and Archives of Past Events
Next Live Event: TBD
Pages: [1]   Go Down
  Print  
Author Topic: Relating Force, Work and Power  (Read 107846 times)
0 Members and 1 Guest are viewing this topic.
DaveB
Full Member
***
Offline Offline

Age: 59
Location: Ohio
Posts: 70




Ignore
« on: January 07, 2014, 04:48:28 PM »

Relating Force, Work and Power

I thought it might be beneficial to discuss the relationship between Force, Work and Power. Also we might look at basic versions of the equations that define the three.

Why important

This might seem unneeded, but there are at least a couple good reasons to have a basic understanding of them. One reason is to better understand the relationship between aerodynamic drag (Force) and Horsepower (Power). Another reason is these simple equations express why the relationship between Horsepower and speed is a cubed relationship. These are things land speed racers can put to good use!

Force is just like when you push on something but it does not move. When you apply a force to something and it moves a certain distance you have performed Work. Another way to put it is, force over a distance is work. Now let’s talk about power. Power is work performed at a certain rate. It’s like these three things progress from one to the other. First there is Force, factor in distance and you have Work, then factor in time and you have Power.

Equations and their use

Since Work is Force over a distance:  Work (ft lbs) = Force (lbs) x distance (ft).
Work in "ft lbs" should not be confused with torque in "lb ft". Torque "lb ft" is just describing the force at a radius like a torque wrench, whereas work in "ft lbs" is a force applied over a distance.
The term Work is a special case of Energy. Energy is often described in BTU whereas work here is described as "ft lbs", but it is possible to convert between the two. You can see the usefulness of using BTU if we started getting into how much energy was stored in our fuel before we burned it in our engines since fuel energy is usually described in BTU. But enough about energy and back to Work.

Power is work at a certain rate: Power (ft lbs per sec) = Work (ft lbs)/ Time (sec). Since Work = Force x Distance, lets make that substitution in the above equation. Now we can say: Power = Force x Distance / Time. Many substitutions (similar to the one we just did) can be made that put the power equation in different forms. Constants can be added that put the variables in different units such as using Watts instead of “ft lbs per sec” when we are talking about Power. We can see it is easier to use power units with simple names like Watts or horsepower instead of “ft lbs per sec”.
It would be very useful to have an equation that gave us power in the units horsepower (hp) and used common inputs that engine people used such as torque and RPM. This special case of the power equation is one we will all recognize. It is:
Power(hp) = Torque(lb ft) x Rotation speed(RPM) / 5252.

Close

I have always found that a little background in the principals I am applying is helpful and that's why I've opened this discussion.

Aerodynamic drag is nothing but a force so we see that by using the drag equation and one of the above power equations we can relate drag with horsepower. If this kind of discussion is helpful and there is interest, I will present that as a future topic.

Regards, DaveB
« Last Edit: January 07, 2014, 08:42:36 PM by DaveB » Logged

Everything I know about opera, I learned from Bugs Bunny.
Interested Observer
Sr. Member
****
Offline Offline

Posts: 327




Ignore
« Reply #1 on: January 07, 2014, 07:35:49 PM »

In the more technical circles, energy and torque are differentiated by denoting energy as ft-lb and torque as lb-ft.
Logged
manta22
Hero Member
*****
Offline Offline

Age: 78
Location: Tucson, AZ
Posts: 2983


What, me worry?




Ignore
« Reply #2 on: January 07, 2014, 07:51:01 PM »

In the more technical circles, energy and torque are differentiated by denoting energy as ft-lb and torque as lb-ft.

Back when I took Physics (in the late Pleistocene Era) it was as you say. More times than not that convention is overlooked.

Regards, Neil  Tucson, AZ
Logged

Regards, Neil  Tucson, AZ
DaveB
Full Member
***
Offline Offline

Age: 59
Location: Ohio
Posts: 70




Ignore
« Reply #3 on: January 07, 2014, 08:23:10 PM »

Thanks for bringing it back to my mind guys. It is as you say in my old Statics textbook. shocked At least I pointed out the difference in energy and a moment. rolleyes If the original post lets me, I will change the units. smiley You maybe also noticed that when I used lb I did not add the f subscript to designate lb force.
« Last Edit: January 07, 2014, 08:31:37 PM by DaveB » Logged

Everything I know about opera, I learned from Bugs Bunny.
WOODY@DDLLC
Hero Member
*****
Offline Offline

Age: 72
Location: Cincinnati
Posts: 1120


It's GONE ......... the Ohio Mile! :-(


WWW

Ignore
« Reply #4 on: January 08, 2014, 01:17:04 PM »

http://www.eaa.org/experimenter/articles/2011-02_howto.asp
Logged

All models are wrong, but some are useful! G.E. Box (1967) www.designdreams.biz
rbeikmann
New folks

Offline Offline

Location: Brighton, MI
Posts: 9


Don't be slow!


WWW

Ignore
« Reply #5 on: June 07, 2015, 08:32:08 PM »

I've seen where energy is in ft-lb and torque in lb-ft, but in both cases you are multiplying feet by pounds, so it really shouldn't matter the order. To me it's like saying 2 times 3 is different than 3 times 2.

The more important distinction IMHO is this:

For torque, the force (applied at the end of a wrench, say) is perpendicular to the distance in feet (the length of the wrench handle).

For energy, such as work, the force (pushing the car, say) is parallel to the distance (how far the car has been pushed).
Logged

Author - Physics for Gearheads

"A problem well stated is a problem half solved." - Charles Kettering.
"Whether you think you can, or you think you can't, you're right." - Henry Ford
Pickle
Jr. Member
**
Offline Offline

Age: 40
Location: redondo beach
Posts: 34




Ignore
« Reply #6 on: December 24, 2015, 12:07:28 PM »

DaveB's original post is correct. And rbeikmann's description is also correct. But to really visualize this these quantities should be seen as two different things. Torque is a vector quantity where work is a scalar.

Torque is the cross product of two vectors the force vector and the position vector. It creates a new vector the torque vector that is perpendicular to the force and position vectors and its orientation follows the right hand rule. Interestingly this vector is responsible for all sorts of fun stuff like the precession of the spinning axis of a top for instance.

Work is just as DaveB described, its the scalar quantity of force applied over a distance. If a one lb block is moved (with no friction) one foot you have done one lb-ft of work. Likewise if a 1/4 lb block is moved 4 feet you have done one lb-ft of work. From this you can back out different quantities. If lets say you know the force and the distance the block moves but it moves less than described above you can calculate the friction force from surface interaction and back out the friction coefficient for that surface and block material.

I bet there are a few on here that can come up with an interesting way to find the coefficient of static friction on the salt from this.

James
Logged
Interested Observer
Sr. Member
****
Offline Offline

Posts: 327




Ignore
« Reply #7 on: December 24, 2015, 12:58:40 PM »

Quote
If a one lb block is moved (with no friction) one foot you have done one lb-ft of work. Likewise if a 1/4 lb block is moved 4 feet you have done one lb-ft of work.

This is an incorrect illustration of the matter.  A 1 lb block moved frictionlessly (horizontally) one foot does NO work.  If it is moved vertically upward one foot, it required 1ft-lb of energy.  If moved downward, the block performed 1 ft-lb of work.  W = E = force x distance.
Logged
SPARKY
Global Moderator
Hero Member
***
Offline Offline

Age: 75
Location: Phoenix
Posts: 6235




« Reply #8 on: December 24, 2015, 04:34:40 PM »

I can tell already you guys are going to make my head spin and hurt   but I am in for the journey

--- not being an End -ga -neer  I did not know the difference in ft # and #s Feet --- thanks guys
Logged

Miss LIBERTY,  changing TKI  to noise, dust and RUST!!!

The # 1 issue is: TO KEEP THE REPUBLIC      
   Center for Self Governance            tncsg.org     mrspowell.org

"Security is mostly a superstition. It does not exist in nature, nor do the children of men as a whole experience it. Avoiding danger is no safer in the long run than outright exposure. Life is either a daring adventure or nothing."   Helen Keller
tallguy
Full Member
***
Offline Offline

Location: Northern California
Posts: 157




Ignore
« Reply #9 on: December 26, 2015, 06:20:56 PM »

No offense to anyone intended . . . I think some clarification is in order.

A scalar has magnitude but not direction.  An example of this is a temperature (say 212 degrees F).
If something is 212 degrees F, there should be no misunderstanding of what this means, with no
further description.  You don't need to specify the direction of temperature, as temperature itself has no
direction.

Speed is also a scalar.  For example, "I am going 100 miles per hour."  This can give an idea of related
scalar things like "I'm exceeding the 65 mph speed limit by 35 mph.  There is a lot of momentum
represented in my motion right now.  Also kinetic energy.  I don't want to hit a fire hydrant."

Velocity is a vector and has magnitude and direction.  An example of this is described by saying "I am
going 100 miles per hour southward.  In many cases, the direction is quite important.  When you are
navigating, you should be aware of direction as well as speed.

Mass is a scalar.  An example of this is illustrated by a bowling ball having a mass of 16 pounds.  On earth,
the bowling ball would typically weigh 16 pounds of force (a vector having magnitude and downward direction).
But on the moon, the same bowling ball would have a weight of only about 3 pounds (of force).  It is important
to indicate whether one is talking about force or mass, which are not interchangeable.  

A common unit of force is "pound of force", or lbf, while a common unit of mass is "pound of mass", or lbm.  
It may be convenient to remember that a one-lbm item (such as a small bag of dried beans) weighs one lbf on earth.

« Last Edit: December 26, 2015, 06:26:25 PM by tallguy » Logged
Pickle
Jr. Member
**
Offline Offline

Age: 40
Location: redondo beach
Posts: 34




Ignore
« Reply #10 on: December 27, 2015, 12:13:48 PM »

Interested observer you are correct. I did not specify direction to keep things simple and to keep sparky's head intact. I was addressing purely the view that ft-lb and lb-ft were not the same units in this context as rbeikmann had said earlier. But, technically work does not equal energy. Work equals the total change of energy in the system, W = dE = dKE + dU, where KE is the kinetic energy and U is the potential energy. Some changes are path dependent and others are not.

Tallguy, the distinction between lbm and lbf is always misunderstood. Interestingly lbm is a misnomer as well. The imperial unit for mass is slugs, but people got so used to pounds that lbm is used. It has caused so many problems for people even in graduate and professional levels. English units are fine for certain things like miles per hour and such but it can also lead to very weird things like slugs fathom per fortnight. This is why I love the metric system. Also mass is a scaler but weight is a vector, once a mass is in a potential energy field such as gravity it creates a force called weight just as you described, and all forces are vectors.

Logged
tortoise
Sr. Member
****
Offline Offline

Age: 75
Location: Sequim, WA
Posts: 539




Ignore
« Reply #11 on: December 27, 2015, 05:40:56 PM »

English units are fine for certain things like miles per hour and such but it can also lead to very weird things like slugs fathom per fortnight.
Shouldn't that be fathom slugs per fortnight?

Around 3.4 million fathom slugs/fortnight = 1 horsepower.
« Last Edit: December 27, 2015, 06:25:53 PM by tortoise » Logged
joea
Hero Member
*****
Offline Offline

Location: Rapid City SD
Posts: 1547




Ignore
« Reply #12 on: December 27, 2015, 08:21:03 PM »

One of the things I love about this site...2+2 can be discussed for 10 plus pages and 2 + years
Logged
Pages: [1]   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.21 | SMF © 2015, Simple Machines
Simple Audio Video Embedder
Valid XHTML 1.0! Valid CSS!


Google visited last this page October 13, 2017, 09:27:38 AM