
#1
Oct1907, 05:44 PM

P: 56

A 61.0kg box hangs from a rope. What is the tension in the rope if:
(a) The box is at rest? (b) The box moves up a steady 5.10m/s? (c) The box has v_y = 5.10m/s and is speeding up at 5.10m/s^2? The y axis points upward. (d) The box has v_y = 5.10m/s and is slowing down at 5.10m/s^2? This problem seems easy, but because I cannot even get part (a) I am confused and need help. To me it seems like part (a) should be either 0N or 61N. F=ma F=61kg(0m/s^2) > box at rest means acceleration is zero right? F=0N 



#2
Oct1907, 05:47 PM

Mentor
P: 40,905

Analyze the forces acting on the box (there are two) and apply Newton's 2nd law (Fnet=ma). You're right that "ma" = 0, but that means that the net force is zero.




#3
Oct1907, 06:12 PM

P: 56

I am still a little confused.
The two forces are Tension and Weight, which if the Acceleration is zero, doesnt this mean that the force of the tension and weight are the same. 



#4
Oct1907, 06:17 PM

P: 56

Box Hangs From Rope
Gravity....
So it would be 61kg times 9.8m/s^2 = 597.8N for part (a) 



#5
Oct1907, 06:21 PM

P: 56

How do I tackle part (b)?




#6
Oct1907, 06:31 PM

Mentor
P: 40,905





#7
Oct1907, 09:35 PM

P: 56

5.10 ???




#8
Oct2007, 06:48 AM

Mentor
P: 40,905

So, what's the acceleration of something moving upward at a steady speed? 



#9
Oct2007, 11:07 AM

P: 56

Zero




#11
Oct2007, 11:26 AM

P: 56

Ok so it 598N again.
Now part (c)... It is accelerating at 5.10m/s^2 Tmg=ma ma= 61(5.1) = 311N mg= 598N T= ma+mg= 311N+598N = 909N 



#12
Oct2007, 11:28 AM

P: 56

Part (d) would then be 598N  311N = 287N
Thanks for your help!!! 


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