Moving without the application of an external force

[monilmodi]

let this be clear the max movement possible for you is when u remain in the cart and at the same time walk on the cart floor. this will happen only because position of centre of mass is to be maintained in absence of external forces as per the law of conservation of momentum. so it is like this:you may move but nothing beyond a distance that will be lesser than the length of cart.

m.x = M.X

(where

m=your mass
M=cart mass
x = your moved distance
X= dist moved by cart

AND x,X < L

L=length of the cart
)

so you r right, you can move but not as much as you want to.

and pushing the insides of the cart will not help at all 'cause there are two opposite sides to balance any force you may apply.

i hope i cleared this matter.

 

[smallphi]

The car can move more than that because it is NOT a closed system - it interacts with the floor and that interaction can have horizontal force component.

Cosider this more obvious example. Imagine your are sitting in a light chair, legs not touching floor. Can you and the chair move forward without touching the floor by 'jumping'. Almost everybody has done this at some poin in his/her life and we all know that yes you can move. Why the center of mass moves? Because the system of you+chair is not closed, it interracts with the floor and your are using the forces from the floor which respond to your jerks in your advantage.
Note: I tried it with my kitchen chair. I was unable to move it while sitting on it but then I stood up on it and pulled the chair from above I was able to move the whole system quite a lot.

Your derivation based on the center of mass not moving applies only to a system for which the total horizontal force from the environment is zero.

Another proof that the center of mass can move is that if you walk in the car carefully enough in one direction the cart won't move so the center of mass will move in your direction. That's because the forces holding the cart in place (ellastic forces and friction forces with floor) have a threshold and as long as the force from your feet remains below that threshold the car won't move. When you are at the store, in order to make the cart start moving, you have to push ABOVE that threshould.

 

[apope]

whether there is friction or not isn't important.. you want to know if you can apply a force forward moving the shopping cart, the magnitude of the force to overcome friction isn't important.. unless you tell us the strength of the person in the cart, the coefficient of friction and the weight of the person in the cart.

 

[rewebster]

whether there is friction or not isn't important.. you want to know if you can apply a force forward moving the shopping cart, the magnitude of the force to overcome friction isn't important..
unless you tell us the strength of the person in the cart, the coefficient of friction and the weight of the person in the cart.

hmmmmm...

well, yeah--and maybe the cart is 40 feet long, and maybe the cart has no wheels (or just three wheels), and maybe its a baby inside, and maybe your arms and legs are tied up, and maybe the cart is going uphill, and maybe there's a black hole up ahead, and maybe the cart is on the moon, and maybe...

 

[Saladsamurai]

hmmmmm...

well, yeah--and maybe the cart is 40 feet long, and maybe the cart has no wheels (or just three wheels), and maybe its a baby inside, and maybe your arms and legs are tied up, and maybe the cart is going uphill, and maybe there's a black hole up ahead, and maybe the cart is on the moon, and maybe...

..what about that cart with one jammed wheel?...the one that constantly tries to make a left while simultaneously making a loud farting noise...

 

[rewebster]

..what about that cart with one jammed wheel?...the one that constantly tries to make a left while simultaneously making a loud farting noise...

yeah!----


Actually, there's a 'term' (which doesn't come to mind right now) NOT to assign/bring in more variables to a situation than what one can assume from the amount of information 'given' to try to resolve a situation. The term is similar to being 'over speculative' both/either in science and/or philosophy, but more specific to assumptions.

 

[rewebster]

yeah!----


Actually, there's a 'term' (which doesn't come to mind right now) NOT to assign/bring in more variables to a situation than what one can assume from the amount of information 'given' to try to resolve a situation. The term is similar to being 'over speculative' both/either in science and/or philosophy, but more specific to assumptions.

-------------------------------

The term (that I still can't place) is something similar to 'significant figures' in math, but relates to not bringing in extra parameters that weren't 'suggested/implied/inferred/??' to ANY more parameters that in the original problem. what is THAT term?



Any help from the 'parameter' gallery?