
#1
Jul2009, 08:00 AM

P: 5

Hi,
Why is F = ma equals to F =mg? Also what is F = Fa  Ff? It came from this question. The question is c) If the box moves at a constant speed of 2m/s1 when a 50N force is applied, what is the frictional force? (Box is 10kg) Answer: Constant speed => a = 0 F = ma = 0 F = Fa  Ff (what's this suppose to mean?) Ff = Fa = 50N 



#2
Jul2009, 08:06 AM

Mentor
P: 40,871





#3
Jul2009, 08:11 AM

P: 5

Thank you so much for your answers, you just saved me.
@ Doc Al, since the equation is incorrect, is it possible for you to work out the sum to find the actual answers or you need more details on the question? EDIT: I'm sorry, mistook the equal for a minus. So is the equation correct now? 



#4
Jul2009, 08:17 AM

Mentor
P: 40,871

F = ma also equals to F = mg? F = Fa  Ff? 



#5
Jul2009, 08:20 AM

P: 5

Hmm, so to find out Ff is this below way done correctly?
Since F = ma = 0, and F = Fa  Ff, I'm suppose to find out what's Ff. So Ff = Fa  F? And since F = 0, it'll be Ff = Fa(50)  0? So Ff = 50  0 = 50 thus Ff = 50? 



#6
Jul2009, 08:29 AM

Math
Emeritus
Sci Advisor
Thanks
PF Gold
P: 38,877

The friction force always opposes the velocity, and so the applied force. The "net force" on the object, F, is the difference between the applied force and the friction force, F_{a} F_{f}. Since there is no acceleration F= ma= m(0)= 0, so F_{a} F_{f}= 0 and, from that, F_{a}= F_{f}. 



#7
Jul2009, 08:45 AM

Mentor
P: 40,871

ΣF = ma = 0 Fa  Ff = 0 so: Ff = Fa thus: Ff = 50N (since you know that Fa = 50N) 



#8
Jul2009, 08:49 AM

P: 5

Another question, Ff = Fa does not apply on every question EXCEPT when Fa  Ff = 0 am I correct to say that?




#9
Jul2009, 01:35 PM

P: 767

yes, that's right



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