Need a realistic Statics Problem

[sanddrag]

I need to take a realistic situation, simplify it, and analyze it as a statics problem. It doesn't have to be super complicated, it just needs to be a good problem to illustrate the principles of vector statics. I was kind of thinking of doing some sort of 3D problem, but I was also thinking it would be cool to analyze something like the arm of a backhoe tractor (in a simplified 2D way).

Areas of study include trusses, friction, force and moment equilibrium, and frames and machines.

I'd kind of prefer a force and moment equilibrium problem but I'm open to any suggestions.

I need a problem that is not too difficult, but not too easy.

Thanks.

[Greg Needel]

i think you will like this and i had to do a similar thing when i took statics

Take a picture of a robot from last year (profile view) holding a tetra in the air and solve for the reactions at each one of the tires, or prove that it wont fall over. all you will need is the dimensions of the robot, the weight of the robot, weight of a tetra, and the angle of the arm (which can be done by trig). Take the photo into photoshop (or equivolent software) draw dimention lines on the picture and put that at the top of the page above the solution. Your professor will be impressed.

[Gasperini]

http://dept.physics.upenn.edu/course...tion4_1_5.html

I hope I'm not doing your homework for you.

[KenWittlief]

our team discovered this a few years back. In the drawing, the red arms pivot at all four points on the black frame

the L shaped platform is moved up and down by the torque on the arms.

Now heres the weird part - moving the load (blue box) left and right has no effect on the amount of torque required to raise the arms

as if leverage no longer exists.

[Chris Hibner]

Originally Posted by KenWittlief

our team discovered this a few years back. In the drawing, the red arms pivot at all four points on the black frame

the L shaped platform is moved up and down by the torque on the arms.

Now heres the weird part - moving the load (blue box) left and right has no effect on the amount of torque required to raise the arms

as if leverage no longer exists.

This is what I figured out the day after the 2002 kickoff. Our robot lifted the goals completely off the ground that year. My calculations showed that with a 4 bar linkage, the distance from the pivot became irrelevant as to how much torque was needed to lift the goals (the distance only changed the amount of compression and tension in the upper and lower arms). These calculations convinced our team to make the lifting robot - and it turned out to be not that difficult to do.

See, this is what an engineering education does for you. Without knowing statics, we probably wouldn't have attempted the lifting robot. That or we might have attempted and failed. It turned out pretty good for us that year. It was also a great message to our students to show how an education can help you solve difficult problems.

[KenWittlief]

the Fairport team used the concept for the 1999 game, lifting the 'floppies' 8 feet in the air. We had a basket attached with four arms, like the drawing, and a geared down van door motor supplying the torque.

Lots of teams had a difficult time lifting their floppies in the air that year. Our bot (Floppy Joe) could raise and lower a full basket in about 2 seconds, all day long.

[sanddrag]

Originally Posted by Greg Needel

i think you will like this and i had to do a similar thing when i took statics

Take a picture of a robot from last year (profile view) holding a tetra in the air and solve for the reactions at each one of the tires, or prove that it wont fall over. all you will need is the dimensions of the robot, the weight of the robot, weight of a tetra, and the angle of the arm (which can be done by trig). Take the photo into photoshop (or equivolent software) draw dimention lines on the picture and put that at the top of the page above the solution. Your professor will be impressed.

I've selected a problem very similar to this. Thanks for the idea. I had thought of it before, but to hear someone else mention it makes me think it is indeed a good selection.

For Ken, would you mind explaining why that is true? I don't intend to use that example in my report, but it seems interesting nonetheless.

[KenWittlief]

I could, but it would be better if you figured it out yourself :^)

moving the blue box left and right does have an effect on the system, but not on the force required to raise and lower the arms.

[sanddrag]

I have a feeling it has something to do with this: when you move the blue box out (away), you are only increasing the reaction forces where the red bars meet the black piece.

Is this correct or no?

[sciguy125]

As to the problem Ken presented:
I haven't taken statics, so I don't know nothin' 'bout these "reaction forces" sanddrag talks of. But, I think I have an idea. The only torque that the red bars see is at the pivot between it and the black piece. Moving the blue box doesn't change this torque. I think that the ratio of the forces between each individual red bar and the L will change, but I have a feeling that the net torque that both bars see will remain constant.

[KenWittlief]

yes, you are both correct.

when you move the box to the right the twist on the frame increases - the frame wants to deform on both sides of the red arms, but the torque required to lift the L shape platform up and down remains constant

the reason its counter-intuitive is this: if you used a straight arm, then the further to the right you moved the box, the further it would move up and down (in feet) per degree of arm rotation, and therefore the torque required to move it changes

but with the double arms the box moves the same vertical distance no matter where you place it on the L shape, so the torque is the same.