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Determine actual car mass

  1. Dec 3, 2011 #1
    So my physics teacher gave us a lab this weekend where we have to find the mass(kg) of an ACTUAL car. we need to do an experiment for this and i have absolutely no idea how to do it. does anyone know any ways to do an experiment that will determine the mass of the car?? (as in for eample::: driving it down a ramp for example to determine this or that, and then using that data for this or that..i dont know something like that) i dont have any fancy stuff. just "normal" home stuff, so it has to be SIMPLE. im in highschool physics so i dont think he wanted the experiment to be too hard to figure out. THANKS SO MUCH PLEASE HELP!!! ANY SUGGESTIONS ARE WELCOME!

    we have the basic formulas to use( F=MA, displacement equation, etc.)

  2. jcsd
  3. Dec 3, 2011 #2
    Re: <><><>EASY EASY QUESTION<>DETERMINE ACTUAL CAR MASS---help meh!!<><><><>

    Place the car on a material where you know the static coefficient of friction between the rubber tires and that surface. Then connect a wire to an electric motor and find the greatest force you can apply without motion. This force is equal to the force of friction which is the coefficient between the rubber and your surface multiplied by the normal force which is proportional to the car's mass :)
  4. Dec 3, 2011 #3
    Re: <><><>EASY EASY QUESTION<>DETERMINE ACTUAL CAR MASS---help meh!!<><><><>


    The basic idea is fine, but either you did not get it quite right or are missing something...

    ...when you say "greatest force you can apply without motion"...did you mean motion of the car with the emergency break on? in other words, slippage of all 4 tires at the same time?...

    ...that would make sense, but that is going to be a very very large force if you expect all four rubber tires to sliip...

    ...and if you were thinking of letting the car get into motion by letting the tires rotate..then, this experiment is no good...you would need to know the friction in the tires bearings, instead.


    JHamm's idea is great, but you need to reduce the friction coefficient for an easily do-able exercise

    Just put the car's 4 tires on top of something that has a small and knwon friction coefficient...

    for example, get 8 pieces of 2x4's or 2x6's, say, a couple of feet long...

    park the car in a place where you can work. Put the emergency brake on

    raise one tire at a time and put two pieces of wood under it, one on top of the other one (not side by side)

    to ensure that the friction of coefficient is large between the ground (garage floor?) and the first (bottom) piece of wood, put anti-slipery mat or 2-sided tape or something

    assuming firction coefficient between tire and top piece of wood is larger than in between the two pieces of wood ( should be ) ...the interface between the pieces of wood would be the weakest link...

    get a spring of known constant coefficient or a strong enough dynanometer and a few friends plus one...all but one pull on the car through the dynometer and one of you keep an eye on the reading...the maximum number before the car move is your starting number for your calculations

    ...if friction is too large...you can use other materials instead of bare wood...

    polished metal plates
    laminate flooring
    thick glass

    ...anything with known friction coefficient...or if it is not know...you can always conduct a preliminary experiment to determine the friction coefficient of whatever you are using.

    it firction force with the car is too large and need to use a motor ...I guess you need to make a current (amperes) reading and calculate torque and then force depending on pulley's radius, etc.

    does all this sound reasonable?
  5. Dec 3, 2011 #4
  6. Dec 3, 2011 #5
    well, yeah, you can also place a scale under every tire and add up the weight...I just don't know the range if a bathroom scale is large enough...start by looking up the weight of the car in its manufacture's site and look at the specs..that should give you an idea.
  7. Dec 3, 2011 #6


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    With a 1 foot long 2x12, you can use two bathroom scales. Put a piece of foam on each scale though to ensure you dont concentrate the weight.
  8. Dec 3, 2011 #7


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    How long is that 2x12? How does a 1 foot section of 2x12 allow you to use only 2 scales?

    Also, I am fairly certain that a car is going to blow the max tolerance of a bathroom scale - pretty sure 4 of them will - let alone 2 of them. Most won't do more than 300-350 lbs.

    And finally to all: this is a physics assignment. I have a feeling the teacher will be disappointed with the lack of imagination of simply weighing the car.
  9. Dec 3, 2011 #8
    Well, I'm new here, but since I am asking for help (different post) I should offer help if I can. First, I'm an engineer - not a physicist. So I tend toward: Understand the math model. Apply model to problem. Get-er-done.

    I think the bathroom scale concept is workable. You just need to multiple the range. Using two 2x6 - 5 foot long, jack up one tire, position the 2x6s on edge under the tire with 1' back and 4' forward. Put a fulcurm under the close end and the bath scale under the far end. That wil scale the bathscale reading up by a calculable factor. I'll leave the calcs to you. It's a pretty standard free body problem involving levers.

    For the fulcrum, use something about as high as the bath scale, 2" pipe, mom's wooden rolling pin. Nail the 2x6s together so they don't flip over easy. And KEEP YOUR HANDS CLEAR. Never, never, never reach under anything under load. Measure your distances as close as you can. That will help your accuracy.

    Weigh one front tire and one back tire. Maybe jack up the other side and put the tires on blocks to keep it level. Might help your accuracy a bit - might not

    Good luck. Let us know how it goes.

    Last edited: Dec 3, 2011
  10. Dec 3, 2011 #9


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    Ehh, 1 ft is probably too short, yeah. 18" should be plenty if you put them next to each other.

    For the weight issue, sure, the OP needs needs to check the specs of the car and scales. But two 300 lb scales will do a compact sedan if it is reasonably well balanced. If not, some extra wood and a saw will allow you to use 4 scales.
  11. Dec 3, 2011 #10
    well one possibility is pull on the car and see how much force it takes to get it to start rolling while in neutral.

    To do this you'll need to account for the resistance the car has to rolling plus the moment of inertia of the wheels - you could measure this roughly by letting the car roll down a hill from rest and see how fast it goes at the bottom, the acceleration due to gravity is independent of its mass, so the difference from this velocity from the velocity you'd calculate assuming no friction will give you some measure of how much the car resists rolling.

    Then pull on the car with a rope and a force gauge and see how much force it takes, and account for what you measured above.
  12. Dec 3, 2011 #11


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    OK, now I'm more confused. I wasn't just being nitpicky. I thought you were suggesting a 2x12 that goes from wheel to wheel.

    Oh, I get it now. You're putting two scales under one wheel.

    Even a compact weighs well over 1000lbs.

    Oh I see, you mean because it's only supporting half the weight of the car. Got it.

    Still, I say actually weighing it is not in the spirit of a physics lesson.
  13. Dec 3, 2011 #12
    Park the car on a slightly inclined surface facing either up or down hill. Park a second car 2 or 3 inches down-hill from the 1'st car. Have someone hold a bathroom scale between the 2 cars. Slowly release the brake on the first car and let it try to roll downhill, pinching the scale between the 2 cars. Now use a level and a ruler to measure the incline of the surface. Now you have the classic "rolling down an incline" problem that's in every basic physics text.

    You will have to compensate for the rolling resistance of the car. this can be measured by pushing the car at constant speed on a level surface with the scale between your hands and the car.
  14. Dec 3, 2011 #13
    nice, I like this last one...

    ...do you really have to account for the rolling resistance?...even if the car is not moving anymore?
  15. Dec 4, 2011 #14


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    Your teacher would want you to use the equations involving forces and motion, and you should do your 'experiment' to show a basic understanding of the priciples involved.

    1. Momentum
    2. F=ma
    3. equations of motion

    Find a flat fairly level long track of ground or rarely used piece of paved surface.
    Mark of a length of 500ft or so.
    Achieving a speed of 20 to 30mph, at the start of the length, take your foot off the accelerator and time how long it takes the car to coast from one end of the track to the other. Do this 3 or 4 times.
    Also record the speed of the car as it crosses to end of the track.
    Do this from the opposite direction also 3 or 4 times ( from both directions to even out the level-ness factor )
    Average all the times to obtain a time for the deceleration of the car on a level section.
    From an equation of motion you can find out the acceleration.

    Momentum: F x(t1 - t0 ) = = mv1 - mv0
    determine from an equation of motion, the time to travel the track, if your can was travelling at a constant speed along the track.
    From this you, can calculate the friction on the car.

    Mass of car:
    Solve for the mass of the car m in the equation F = m a , from the above values from you experiment.
  16. Dec 4, 2011 #15
    One at a time jack each corner of the car up, smear paint or grease on the bottom 1/3 of the tyre put a piece of paper down and lower the tyre onto it lift the car remove the paper and let the car down again, measure the pressure in the tyre, repeat for all four tyres (hint if you can get a Reliant Robin you cut your work load). Take the grease stained paper and measure the area of contact, from the pressure work out how much each tyre supports.
  17. Dec 4, 2011 #16
    I would think so for the following reason...

    There are 2 forces opposing the motion of the car, their sum must equal the force applied by gravity. They are the force applied by the 2'nd car and frictional force. The force applied by the second car is measured by the scale. The 1'st car is rolling ever so slightly as it applies the force to the scale so I believe it should be the rolling friction that is added to the scales measurement.
  18. Dec 5, 2011 #17


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    Two 300lb scales, one wheel at a time = 1/8 of a 2400lb car.

    I'd rather not have any span because I don't know what a 2x12 can support, but I do know we'd be using it wrong (in its weakest orientation).
    Last edited: Dec 5, 2011
  19. Dec 5, 2011 #18


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    Why all this attacment to bathroom scales and 2x4's to weight the car. The OP never said anything about a weekend experiment using fulcrums. he stated:

    So, momentum, equations of velocity, and acceleration are the key.

    If the car needs to be weighed, just take it to a roadside trucker scale
    Last edited: Dec 5, 2011
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