# Pushing a block - Car project

Pushing a block -- Car project

To begin w/ i would like to say that my knowledge on the subject of physics is fairly minute. I have a project assigned to me in my Intro to engineering class where my group has to build a car out of legos and later race it competitively. We have to make preliminary design reports were my group will discuss the various design aspects of our car. We split the work between team members and I have to find the ideal weight for our car. The most obvious choice is as light as possible, but that might not be exactly true in my case. As part of our obstacle course, our car will be required to push a 1kg block some distance. Our car is going to be powered by an electrical lego motor provided to us as part of the project. That's about all i know about the motor. We did, however, run a few tests on it one day in lab where we attached a weight to the motor and timed how long it took for the motor to move that weight 1.143m. From which we determined the rpm of the motor during that time period. I thought that being as we lifted the weight straight from the floor (there was a string attached to a fixed point on the wheel that picked the weight up as it turned) i could solve this problem with the work-energy theorem by finding the $$\Delta$$W then dividing that by the distance to find force (w=F*D). well i found force, but that evidently is not the number i need because between the different trials (with different weight) the force will vary according to the mass of the object lifted (I checked these force measurements by finding tension T was indeed equal to F). Now being as the F varies between the different cases its obviously the information I'm looking for here (being as im trying to figure out the capabilities of this single motor), so any well you all could give to point me in the right direction would be great.... I added a zip document showing the what we found in lab (just to add any other relevant information due to the properties of the motor as V$$\rightarrow\infty$$ A$$\rightarrow$$0, and the D of the wheel was 0.01905m)

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berkeman
Mentor

To begin w/ i would like to say that my knowledge on the subject of physics is fairly minute. I have a project assigned to me in my Intro to engineering class where my group has to build a car out of legos and later race it competitively. We have to make preliminary design reports were my group will discuss the various design aspects of our car. We split the work between team members and I have to find the ideal weight for our car. The most obvious choice is as light as possible, but that might not be exactly true in my case. As part of our obstacle course, our car will be required to push a 1kg block some distance. Our car is going to be powered by an electrical lego motor provided to us as part of the project. That's about all i know about the motor. We did, however, run a few tests on it one day in lab where we attached a weight to the motor and timed how long it took for the motor to move that weight 1.143m. From which we determined the rpm of the motor during that time period. I thought that being as we lifted the weight straight from the floor (there was a string attached to a fixed point on the wheel that picked the weight up as it turned) i could solve this problem with the work-energy theorem by finding the $$\Delta$$W then dividing that by the distance to find force (w=F*D). well i found force, but that evidently is not the number i need because between the different trials (with different weight) the force will vary according to the mass of the object lifted (I checked these force measurements by finding tension T was indeed equal to F). Now being as the F varies between the different cases its obviously the information I'm looking for here (being as im trying to figure out the capabilities of this single motor), so any well you all could give to point me in the right direction would be great.... I added a zip document showing the what we found in lab (just to add any other relevant information due to the properties of the motor as V$$\rightarrow\infty$$ A$$\rightarrow$$0, and the D of the wheel was 0.01905m)
Welcome to the PF, Tam. I deleted your attachment. Could you please re-post it in PDF format? Fewer issues with potential macros and things. If you need a PDF writer, you can get a free download of PrimoPDF.

I'm also moving your thread to Homework Help, Intro Physics for now.

EDIT -- I also changed the thread title some to make it more descriptive.

Ok so running over this data again I have found some quantity that is constant in all both cases we did in lab- Average Angular Acceleration. Now what this tells me I'm not to sure because I've just now started dealing with problems involving torque in physics. I restate my assignment I must find the ideal weight for our car so that its top speed will be as high as possible while still being massy enough to push a 1kg block. The average $$\alpha$$ of all cases was=0.328987$$\Theta$$/s^2. I think that since the motor in this case was doing work against gravity i might need to add gravity to that acceleration some how... Not sure how to go about doing that... And even less of an idea about how to go about figuring Ideal weight

berkeman
Mentor

Ok so running over this data again I have found some quantity that is constant in all both cases we did in lab- Average Angular Acceleration. Now what this tells me I'm not to sure because I've just now started dealing with problems involving torque in physics. I restate my assignment I must find the ideal weight for our car so that its top speed will be as high as possible while still being massy enough to push a 1kg block. The average $$\alpha$$ of all cases was=0.328987$$\Theta$$/s^2. I think that since the motor in this case was doing work against gravity i might need to add gravity to that acceleration some how... Not sure how to go about doing that... And even less of an idea about how to go about figuring Ideal weight
I don't see how there's an advantage to a heavy car, unless you intend to bash it into the 1kg weight to move it. If the power to move the block comes from the engine, then the lightest car would seem to be best for quickness.

What other variables are you considering? Gear ratio from the motor to the wheels? Diameter of the wheels?

To answer ur question the diameter of the wheels is still a variable ,but the way we divided the work I have no control over that variable.
Truthfully I cant really see an advantage to having a heavier car either, only advantage that i see from it would to make sure the car has enough high enough friction so it wont just hit the block and start spinning tires but that brings in a lot of things I don't know. However, I have to present some sort of data that represents our decision on what the car should weigh aka prove what ranges are acceptable... So like any an answer like the car should weight 0><100lbs would be applicable assuming that I found that the motor could no longer push the car if it weighed that much, but I would need some sort of proof that the car is no matter what better off as light as possible. Proof being mathematical representation of that idea.
What would be ideal i think is to say if the car weighs xlbs it would no longer be able to push the block and itself and have some sort of way to prove that no matter what its better off as light as possible.

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berkeman
Mentor

To answer ur question the diameter of the wheels is still a variable ,but the way we divided the work I have no control over that variable.
Truthfully I cant really see an advantage to having a heavier car either, only advantage that i see from it would to make sure the car has enough high enough friction so it wont just hit the block and start spinning tires but that brings in a lot of things I don't know. However, I have to present some sort of data that represents our decision on what the car should weigh aka prove what ranges are acceptable... So like any an answer like the car should weight 0><100lbs would be applicable assuming that I found that the motor could no longer push the car if it weighed that much, but I would need some sort of proof that the car is no matter what better off as light as possible. Proof being mathematical representation of that idea.
Ah, good point about having enough traction to be able to push the block! I think that may be a good key in your calculation. I don't see any other reason for increasing the weight above the minimum possible, so use that one. You will need the mu (static) value for the block and the ground surface, and you will also need the mu (static) value for your tires on the surface. You should be able to calculate the minimum car weight needed to be able to break the block loose and start it sliding. See, you answered your own question!

the problem is that I have no idea what the coefficients of frictions are here and I really don't have the ability to test that. Being as i have no clue as to find the force of frictions without those coefficients being given to me I'm kinda screwed. Now i could maybe find some sort of estimate for these coefficients on the internet but i have no idea where to start looking for that

berkeman
Mentor

the problem is that I have no idea what the coefficients of frictions are here and I really don't have the ability to test that. Being as i have no clue as to find the force of frictions without those coefficients being given to me I'm kinda screwed. Now i could maybe find some sort of estimate for these coefficients on the internet but i have no idea where to start looking for that
Well, you've clearly identified a need for knowing those coefficients in the project. So you either have to figure out how to come up with them, or request that they be added into the project information by the instructor.

What are the materials of the block, floor and your wheels/tires? You know the trick of inclining a plane until the block slips to find the mu, right?

Well, you've clearly identified a need for knowing those coefficients in the project. So you either have to figure out how to come up with them, or request that they be added into the project information by the instructor.

What are the materials of the block, floor and your wheels/tires? You know the trick of inclining a plane until the block slips to find the mu, right?
Yeah i know the incline plane trick I've had physics homework on that. The problem is how do i do that i mean the floor is flat and i don't think the school would approve of me pulling up a piece of tile to perform an experiment.

I have a new idea though I don't know sure it's real though. If I could find transitional velocity by using torque somehow. I could make a spreadsheet and put in a bunch of numbers for mass and find my torque in every case then use that to find transitional velocity and then I apply those to the idea of inelastic collisions and set my v final is=0 giving me the minimum value for mass then figure out how much the car would have to weigh before the motor could no longer propel it which would give me the maximum. Now obviously these numbers wont be exactly correct the minimum would be to small the maximum to big because friction is presented but for the purpose of this report I think they would suffice

of course i just realize now that the only number for mass that will make my v final zero is zero... suck

is there a way I could do the slope trick on flat ground

How about this how could I find the the maximum mass the car could have and still be able to move itself plus the mass of the block

berkeman
Mentor

You don't want to maximize the car's mass, you want to minimize it.

If you have a sensitive fish scale or similar, you can find the mu on flat ground. By fish scale, I mean one of those hand-held spring scales use to hang the fish to weigh it. You would get a piece of the material (block, wheel) big enough to register on the scale as you pull the object horizontally.

Or, if you have access to a small platform scale, say used to weigh small amounts of, er, stuff. If it works when held sideways, you could push the two blocks of material (block material and wheel material) to see how much force it takes versus their mass.

What material are the wheels or tires made out of. They need to have a high mu value with the floor, or you're not going to be pushing that block much...

the tires are rubber as of now, but really they can be anything i want them to be assuming that I can afford to buy it being as this is an engineering class

ok i experimented today $$\mu$$_block=0.42857 $$\mu$$_tire=0.75.... Now what?

berkeman
Mentor

ok i experimented today $$\mu$$_block=0.42857 $$\mu$$_tire=0.75.... Now what?
How much force will it take to break the block loose and get it sliding? What force can the car generate before its wheels spin, as a function of the car mass? Off you go!

ok let me say that was a terrible post... sorry I was kinda rushed. Rly I know what to do right there. What I really need to know now is how do I show mathematically that if mass of my car higher than that number for mass ill get it will produce a negative effect in terms of the cars motion

berkeman
Mentor

ok let me say that was a terrible post... sorry I was kinda rushed. Rly I know what to do right there. What I really need to know now is how do I show mathematically that if mass of my car higher than that number for mass ill get it will produce a negative effect in terms of the cars motion
F=ma

Higher mass does what to your acceleration capabilites, given a constant engine size (F available)? You are competing on time, right?

Still having trouble sorry guys...lol. I think I solve this problem by f_(staic block)=$$\Sigma$$t=t_motor-f_(static car)*r. However that gives me a negative number for mass. I tried it the other way $$\Sigma$$t=f_(static car)*r-t_motor that told me my mass should be 571.5485g, which seemed somewhat reasonable to me but backwards in concept. So I was just trying to check my work so to speak

berkeman
Mentor

Still having trouble sorry guys...lol. I think I solve this problem by f_(staic block)=$$\Sigma$$t=t_motor-f_(static car)*r. However that gives me a negative number for mass. I tried it the other way $$\Sigma$$t=f_(static car)*r-t_motor that told me my mass should be 571.5485g, which seemed somewhat reasonable to me but backwards in concept. So I was just trying to check my work so to speak
What are the "t" variables? Torques? I'm looking for equations of the form F = mu * N.

...so F_static block=F_static wheels?

berkeman
Mentor

...so F_static block=F_static wheels?
Yeah, basically. You will want some margin built in, so the vehicle can for sure break the block loose, but the smaller you make that margin, the quicker the car can be overall. Make sense?

But, is your vehicle 4-wheel drive? Or are only two wheels driven? That will change your calculation, right? What would you recommend to the rest of the team about 2/4 wheel drive, and why?

Our car is gonna be 2 wheel drive that decision is being based not on mechanical advantage but simply the simplicity of design. I feel its a fairly wise choice. I know as far as traction goes we'd be better off with a 4wd but It would be pretty complicated to make it work. I guess though its better off because two more wheels get to push against that friction so I guess i would get to cut the mass of the car in half

berkeman
Mentor

Our car is gonna be 2 wheel drive that decision is being based not on mechanical advantage but simply the simplicity of design. I feel its a fairly wise choice. I know as far as traction goes we'd be better off with a 4wd but It would be pretty complicated to make it work. I guess though its better off because two more wheels get to push against that friction so I guess i would get to cut the mass of the car in half
Yep, you've shown that there is an advantage to 4WD for this project, as long as the drive train for 4WD doesn't detract too much from the overall drivetrain efficiency (it shouldn't). Are you sure you can't feed this conclusion back to the overall design team? It's not good team dynamics to avoid/overlook an optimization just because a previous decision was made for other reasons. It depends on the phase of the project, of course, but if there is still time, it could give your team a competitive advantage.

And if another team has figured this out, and decided to go with 4WD, then, well, you know...

Good point but of course this is just an intro to engineering class and believe it or not its supposed to be intro to electrical engineering but it hardly seems like it, but i mean I can definitely look into it and try to think of something

First off let me say the help so far I've really been racking my brain on this problem for the last few days, but I have one final question How do I find the torque of my motor? I attached a pdf here to show a lab we did it has a little diagram I made to show the experiment. Now I've been over these numbers a few times, but am yet to find any convincing results, but I do believe that: (T=tension t=torque) T*R= t_motor-m*g*R is this right?

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