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Mass of block given μK and Force? 
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#1
Nov1512, 05:02 PM

P: 52

1. The problem statement, all variables and given/known data
A student pushes a block with 240 N of force across a horizontal surface with a coefficient of kinetic friction of 0.4. The block accelerates at a rate of 0.88 m/s^{2}. Find the mass of the block. 2. Relevant equations μ_{K} = F_{K} / F_{N} F=ma 3. The attempt at a solution My first attempt was to figure out mass using F=ma m = f/a = 240 N / 0.88 m/s^{2} = 272.7 kgbut this is not taking into account the kinetic friction... So what I did was F_{K} = μ_{K} x F_{N} = 0.4 x (272.7 kg x 9.8 m/s^{2} = 1069.1 NI know I'm doing this wrong and I need to know how to derive at net force to find acceleration 


#2
Nov1512, 05:14 PM

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P: 41,321

Instead, just call the mass "m" and set up your equation using the net force. Then you can solve for the mass. What's the friction force in terms of "m"? 


#3
Nov1512, 05:45 PM

P: 52

F_{N} = F_{K} / μ_{K} mg = F_{K} / μ_{K} m = (F_{K} / μ_{K}) / g Like this? 


#4
Nov1512, 05:58 PM

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P: 41,321

Mass of block given μK and Force?
Use ƩF = ma. First work on the forces. There are two forces acting. What are they? 


#5
Nov1512, 06:40 PM

P: 52

force applied of 240 N and kinetic friction
I can't find magnitude of kinetic friction because I cannot find Force normal. 


#7
Nov1512, 07:07 PM

P: 52




#9
Nov1512, 07:49 PM

P: 52

μ_{K} = F_{K} / F_{N}
F_{K} = μ_{K} × F_{N} F_{K}= 0.4 x 9.8m I have 2 unknowns? 


#10
Nov1612, 04:01 AM

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P: 41,321

Keep going. What is ƩF? 


#11
Nov1612, 05:04 AM

P: 52

F_{applied} = 0.88m
just guessing here: Fnet = F_{A} [fwd] + F_{f} [bwd] Fnet = F_{A} [fwd]  F_{f} [fwd] Fnet = 0.88m  (0.4 x 9.8m) 


#12
Nov1612, 05:08 AM

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#13
Nov1612, 05:17 AM

P: 52

oh wow my bad...
Fnet = 240  (0.4 x 9.8m) = 240  2.92mStill don't get what I need to do next? 


#15
Nov1612, 05:50 AM

P: 52

240  2.92m = 0.88m
240 = 0.88m + 2.92m 240 = 3.8m m = 63 kg 


#16
Nov1612, 05:58 AM

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#17
Nov1612, 06:05 AM

P: 52

Fnet = 240  (0.4 x 9.8m)
= 240  3.92m 240  3.92m = 0.88m 240 = 0.88m + 3.92m 240 = 4.8m m = 50 kg wohooooooo thanks a lot! 


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