Newton's Laws Of motion Please Reply

[chazgurl4life]

This is my problem:

A window washer pulls herself upward using the bucket-pulley apparatus The mass of the person plus the bucket is 70 kg.

A) How hard must she pull downward to raise herself slowly at constant speed?...N
(b) If she increases this force by 9%, what will her acceleration be?
.....m/s2

I figured out part A) by taking 70 kg and multiplying it by 1/2 A which equals 4.9m/s2 and so the answer is 343 Newtons. But for part b i am confused as to how to use the the 9% and how to work it out..

 

[Hootenanny]

Well what is 9% of 343? And what does an unbalanced force create?

 

[chazgurl4life]

9 percent of 343 is 30.47 N ..so if her new force net equal 374 N where should i go from there?.. I am sorry i just been working on this problem for a while and I am still stuck

 

[Hootenanny]

Ok, so what is the net force acting on her and the bucket?

 

[chazgurl4life]

isnt the net force =343+/-30.97?

 

[Hootenanny]

No. I mean the resultant force.

 

[Hootenanny]

Think about what force is pulling the bucket and the girl down and what force is pulling them up.

 

[chazgurl4life]

the force pulling on the bucket and the girl is mg= mass x gravity (9.8 m/s2)...the normal force-the force she is pulling the pulley down is the only other force i can think of

 

[Hootenanny]

Hang on a minute, why did you half the acceleration due to gravity?
Downwards force = 70 \times -9.8 = -686N.
Therefore, the girl must be pulling 'up' with 686N to travel at a constant speed. However, if she pulls with an addition 9% of force then the upward force will be 70 \times 9.8 \times 1.09 = 747.74N.
This means there is an unbalanced force of 747.74-686 = 51.74N. Using F = ma \Rightarrow a = \frac{F}{m} = \frac{51.74}{70} = 0.739 m\cdot s^{-2}

 

[helppleaseee]

i was confused on this same problem but i don't understand where the 1.09 comes from...?

 

[mybsaccownt]

1.09*x will give you the sum of the value of x plus 9% (0.09) of x

(instead of doing 9% of x, then adding to x)

 

[helppleaseee]

ok i got it thanks so much!