Help cant figure this question out

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Homework Help Overview

The problem involves a 20kg block on a horizontal surface connected to a 3.5kg mass via a frictionless pulley. The task is to prove that the 3.5kg mass is insufficient to start the block moving, given a coefficient of static friction of 0.2, and to determine the additional mass required to initiate motion.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the forces acting on the blocks, including friction and weight. Questions arise about the conditions necessary for motion and the relationship between force and mass. There is also a focus on understanding the calculations related to frictional force and required additional mass.

Discussion Status

Some participants have provided insights into the calculations and the forces involved. There is an ongoing exploration of the necessary conditions for the blocks to move, with some suggesting that additional force is needed. However, there is no explicit consensus on the final answer or approach.

Contextual Notes

Participants are navigating through calculations and assumptions about forces, with some confusion about the acceleration and its relevance to the problem. There is mention of needing to apply an additional force to initiate motion, but the exact requirements remain under discussion.

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Homework Statement


a 20kg block rests on a horizontal surface and is attatched to a mass of 3.5kg by a cable,the pulley is frictionless ans the coefficient of static frition is 0.2

prove that the mass is not suffcient to start the block moving,what is the additional mass that would be requaired to start motion


Homework Equations


http://img818.imageshack.us/img818/9962/37642403.png

The Attempt at a Solution


20kg block Ff=39.24 Fn=196.2
3.5kg block Fw=34.33
i can work out the acceleration which is 6.68 m/s

but i just don't know hwo togo about working out what it is asking to do,as in prove the mass is not sufficient to start the block moving,what additonal forceis needed

any help would be great Thanks
 
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What makes a body move from rest? Can you relate to any laws?
 
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Hi camo2412! Welcome to Physics Forums.

I'm puzzled about the acceleration you say you have worked out. The masses won't move, because the smaller one exerts a horizontal force of (according to your figures) 34.33N, while your calculations determined that a force of 39.2N is needed to overcome the bigger block's friction.
 
thanks nas
the acceleration calculation i did shouldn't have been in the that was another question is was working out sorry.
but yes from what i undertstand it won't move until another extra 5n is applied,
therefore i think is its an extra .5kgs that is needed
.5x9.81=4.9 which rounded up let's say 5n

just unsure if this is correct answer to the question?
 
camo2412 said:
it won't move until another extra 5n is applied,
therefore i think is its an extra .5kgs that is needed
.5x9.81=4.9 which rounded up let's say 5n

just unsure if this is correct answer to the question?
Sounds about right to me. :smile: