Calculating force required for lifting

[sozener1]

Homework Statement

say you have a 240kg rectangular mass

it is pulled upwards by two cables symetrically

since force due to gravity is 240g so each cables would have to produce 120g each to lift the mass.

but the resultant force sum to zero

so if we were just given a FBD of the 240kg mass with two cables attached how would we know it is being lifted up??

Homework Equations

The Attempt at a Solution

 

[tiny-tim]

hi sozener1!

so if we were just given a FBD of the 240kg mass with two cables attached how would we know it is being lifted up??

the forces (and the FBD) will be the same no matter whether the mass is stationary, is being lifted up at constant speed, or is being lowered at constant speed

 

[sozener1]

hi sozener1!


the forces (and the FBD) will be the same no matter whether the mass is stationary, is being lifted up at constant speed, or is being lowered at constant speed

so if you want to accelerate the mass upwards from 0m/s to 2m/s in 3seconds

the accelerations would be 2/3=0.667m/s2

and F=ma so 240 times 0.667 gives something like 160N

Does this force would have to be an additional force applied to the cables??

so that the total force upwards is 240g+160N ??

 

[Fewmet]

so if you want to accelerate the mass upwards from 0m/s to 2m/s in 3seconds

the accelerations would be 2/3=0.667m/s2

and F=ma so 240 times 0.667 gives something like 160N

Does this force would have to be an additional force applied to the cables??

so that the total force upwards is 240g+160N ??

It really pays to keep track of the laws of motion. What tinytim replied is true because of the first law: when the net force on a body is zero, its motion will not change (i.e., it will not accelerate).

Your last question is really a matter of the second law: when there is a net force, the body accelerates in the direction of the net force. You showed that the net force is 160 N, which means
downward force + upward force = 160 N

(Remember to choose one direction as positive and one as negative).