Number of Workers Needed to Hold 2600kg Stone Block at Rest

[TyFelmingham]

Suppose there is a 2600kg stone block on a ramp inclinded at 12 degrees to the horizontal.Assume that each worker can exert a pulling force of magnitude at most 360 Newtons.

a) Determine the number of workers it would take to hold the stone block at rest

b)Although lubrication can reduce friction, it cannot eliminate it. Assume that
the magnitude of the static friction exerted by the ramp on the stone block
is 2560 N. Taking into account the friction acting upwards along the ramp
(reducing the tendency of the block to slide down the ramp), how many men
are required to hold the block at rest?

Ok, so for a,

mg* cos 12
=24923 N

Divide it by the amount a worker can pull

= 70

Is that right?

For b, I am stuck...I have no idea where to start

 

[dextercioby]

It should be a sinus there...Make the FBD again & pay attention to those triangles.

Daniel.

 

[TyFelmingham]

For b), would the force just be the F calculated in question a minus 2560 N?

Cheers

 

[ShawnD]

Yes that's how you do part B.
I also agree with dex when he says to use sine instead of cosine.