Energy required to start motion

[ctopher63]

I'm out of my area here, but I'm trying to calculate the energy required to start a car moving. How do you calculate the energy required to overcome static friction and the cars inertia. Thanks in advance, Chris

 

[MintJulep]

First, energy doesn't move things. Force moves things.

Second, F=ma

 

[patprimmer]

I would place the car on the appropriate level surface then attach a load cell to the front of the car then pull against the load cell until the car moved.

There will be a lot of variables, like tyre pressure, temprature of lubricants, brake drag, nature of surface, wind, inclination of surface bearing preloads, time since last moved etc. This will give static friction.

The inertia is easier. As MintJulep says F=MA, however there is the linear acceleration of the entire mass, and the moment of inertia of the rotating parts.

Regards

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[ctopher63]

My question is how would I calculate the work required to apply a sufficient force to overcome an automobile's static friction? The standard Work = Force * Distance doesn't seem to apply.

 

[MintJulep]

This Wikipedia doesn't seem entirely wrong.

 

[patprimmer]

The static friction will vary greatly according to environment and other parameters. I think calculations will be useless unless you have lots of set criteria, lots of data and lots of computer resources.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[MintJulep]

the missing link:

http://en.wikipedia.org/wiki/Rolling_resistance

 

[GregLocock]

ctopher - you need a force, not a work calculation, to make a car move.

F=m*a+m*g*tan(theta)+m*Crr*g+a bit more for driveline drag etc+1/2*Cd*A*rho*v^2

at v=0, and a~0 the first and last terms are irrelevant

Problem is, that is for rolling friction, not stiction.

When as tire is cold and stationary the plies move around and then stick together, deforming the tire. That's why they can feel bumpy first thing in the morning.





Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[BobM3]

If you don't have a load cell maybe build a platform you can tilt with a jack or two. Jack it up until the car starts to roll then measure the inclination and use

F = weight x sine (theta)

 

[tygerdawg]

Instead of load cell, place a bathroom scale on the rear of the vehicle and push until motion starts. If you maintain the force, you could record a wheel's index mark on the pavement versus the time. Then you could chart it, estimate your acceleration and work/energy through integration.

Like several people stated, you are looking for Force as in F=ma.

Work/Energy will be the amount of Force through a distance.

Power is Work over time. You need Power to accelerate the mass up to a specified speed in a specified time.

So.....what do you want to do? Just move a vehicle off of its stiction point to its first motion, or move it up to a specific speed? Those are two different things with different amounts of force/power/work/energy.

TygerDawg
Blue Technik LLC
Advanced Robotics & Automation Engineering

http://www.bluetechnik.com

 

[RossABQ]

I would imagine the tires are the primary issue on a car, and extremely variable. For trains they have the bearing numbers down to a science, because it determines the coupler forces needed. You might check on a rail engineering site for bearing forces.

 

[MikeHalloran]

Chris, if you could reveal just a bit more about what problem you are actually trying to solve, you might get more useful answers.



Mike Halloran
Pembroke Pines, FL, USA