Help cant figure this question out

[camo2412]

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

 

[azizlwl]

What makes a body move from rest? Can you relate to any laws?

 

[NascentOxygen]

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.

 

[camo2412]

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?

 

[NascentOxygen]

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.