Engineers???

[Court Jester]

Maybe an odd question to bring to a motorcycle forum but there's some very smart people here and a hand full of engineers as well that might be able to offer up some insight and save me some money. So here it is.

I'm working with a company to help reduce some ergonomic issues they are having. One of the things I am going to have them do is put together a sheet that will list the weight of their carts and the weight of their products on the cart. I also have a scale that they will use to measure the amount of weight pulled on a loaded cart. Each time reducing the cart by one part then checking weights again.

Now I would assume weight is weight. Things may at times feel heavier depending on how you are handling a cart/part and where it is in relation to the body. So just double-checking before I go out and spend  $800 on another scale.

If 100 pounds of force is required to pull a cart, given the exact same cart, parts, and setting, would it also require only 100 pounds of force to push that same cart???

Thanks for the advice.

[Garywc]

wheres zeroice when you need him?

[Court Jester]

I think he's driving back from Blackhawk now.

[superspud]

I think he's driving back from Blackhawk now.

speaking of... where were you last weekend CJ?  Thanks for letting me borrow the crutches; I brought them to the track but didn't see you. 

[SV88]

Jester:  Basic mechanics F=ma where m is the mass of the object and a is the acceleration.  As soon as the cart is moving, you have overcome static friction so the force to keep the cart moving is reduced.  But yes the force to get the cart in motion is directly related to the mass on the cart.  The big problem is keeping the accelaration a constant.  If you can capture the force required as soon as the cart is in motion, then you can correlate it to cart weight (mass).  Another approach would be to use the momentum equation F=mv2 (velocity squared) but this would only apply as long as the velocity was constant.  Ie no accelaration.

It would be far easier to use either strain gages on the cart or a weigh cell.

All this from a licenced Chemical Engineer (as I like to remind critics of my mechanical skills at the track)...

steve.

[Court Jester]

speaking of... where were you last weekend CJ?  Thanks for letting me borrow the crutches; I brought them to the track but didn't see you. 

I was in Tennessee this week. I had work stuff and a death in the family, both in tennessee.

[Court Jester]

Jester:  Basic mechanics F=ma where m is the mass of the object and a is the acceleration.  As soon as the cart is moving, you have overcome static friction so the force to keep the cart moving is reduced.  But yes the force to get the cart in motion is directly related to the mass on the cart.  The big problem is keeping the accelaration a constant.  If you can capture the force required as soon as the cart is in motion, then you can correlate it to cart weight (mass).  Another approach would be to use the momentum equation F=mv2 (velocity squared) but this would only apply as long as the velocity was constant.  Ie no accelaration.

It would be far easier to use either strain gages on the cart or a weigh cell.

All this from a licenced Chemical Engineer (as I like to remind critics of my mechanical skills at the track)...

steve.

I have a bachelors in engineering so I should know the answer myself. I must have been drunk and/or stoned that day. I dunno. But I'm working on a psychology degree at the moment too, so I'm gradually forgetting the engineering jazz anyway.

I understand all that you posted, but what we are looking at is the force required to get the cart up to, say 3 mph as when a team member is pushing/pulling the cart from one work center to another. The company wants a solid documented reasoning to justify a change in policy or a new policy. With several back injuries over the past year coming from pulling carts, this is what I'm looking at so that we might get a safe standard for number of parts on a cart and a maximum weight to be loaded on each cart.

Obviously, pushing is better ergonomically than pulling. As the greatest burden on the team member will be getting the cart up to speed, the force required to keep it at speed isn't a concern. What is the concern is the amount of force required to get the cart up to say 3 mph.

So you have two carts side by side, both exactly the same weight, loaded with the same parts distributed the same across the cart, and you try to get them to 3 mph in five seconds. Is there a difference in the force (in pounds) required to do this if you push one and you pull the other? Or would the force be the same as both carts are exactly the same? I'm almost positive that each would require the same force, I just don't want to make an assumption and be wrong and I don't want to buy a scale if I'm correct. 

God I need a drink. All this thinking's giving me a headache.   

[tzracer]

So you have two carts side by side, both exactly the same weight, loaded with the same parts distributed the same across the cart, and you try to get them to 3 mph in five seconds. Is there a difference in the force (in pounds) required to do this if you push one and you pull the other? Or would the force be the same as both carts are exactly the same? I'm almost positive that each would require the same force, I just don't want to make an assumption and be wrong and I don't want to buy a scale if I'm correct. 

God I need a drink. All this thinking's giving me a headache.   

How the force is applied also affects the force applied. How are you pulling? Is the force horizontal or up or down at some angle? If the force is applied at an angle it will affect the normal force which will affect the friction. Again the best way would be some sort of force transducer. Chances are you will not get any sort of smooth acceleration curve, it will probably have quite a few large spikes.

BTW when you really need to know what is happening, skip the engineers and go directly to your local physicist.

[superspud]

I was in Tennessee this week. I had work stuff and a death in the family, both in tennessee.

I'm sorry to here about your loss.  Just let me know whenever you'd like them back, no rush.
Thanks for letting me borrow them.