Kinetic Friction Lab Question

  • #1

Homework Statement


A 4.5 kg block is on an incline with a coefficient of static friction of 0.50. The angle that the ramp makes with the horizontal is increased gradually, until the block begins to slide down the ramp. If you know that the coefficient of kinetic friction between the block and the plane is 0.10, find the time it will take the block to slide down the ramp a distance of 0.65 m starting from rest.


Homework Equations



Kf = (coef of frict)*m*g*cos(theta)



The Attempt at a Solution



tan^-1 (.5) = 26.6
theta = 26.6 degrees
F_g = mg
F_g = (4.5)*(9.81) = 44.1 N
F_gx = 44.1*sin(26.6) = 19.7
a_x = 19.7 N / 4.5 kg = 4.39 m/s^2
4.39 - (coef of frict)*m*g*cos(theta)
4.39 - (.1)(4.5)cos(26.6)
4.39 - .402 = 3.99 m/s^2
d = .65
d = 1/2at^s
.65 = 1/2 (3.99) t^2
.65 = 1.995t^2
t^2 = .326
t = .571 seconds.


I'm really confused, as we haven't covered this stuff in lecture, yet we have labs that deal with it. I'm sorry if I completely fudged up something because I don't know what I'm doing...
 

Answers and Replies

  • #2
PhanthomJay
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Homework Statement


A 4.5 kg block is on an incline with a coefficient of static friction of 0.50. The angle that the ramp makes with the horizontal is increased gradually, until the block begins to slide down the ramp. If you know that the coefficient of kinetic friction between the block and the plane is 0.10, find the time it will take the block to slide down the ramp a distance of 0.65 m starting from rest.


Homework Equations



Kf = (coef of frict)*m*g*cos(theta)



The Attempt at a Solution



tan^-1 (.5) = 26.6
theta = 26.6 degrees
F_g = mg
F_g = (4.5)*(9.81) = 44.1 N
F_gx = 44.1*sin(26.6) = 19.7
ok so far
a_x = 19.7 N / 4.5 kg = 4.39 m/s^2
you went astray here. Do not calculate acceleration in x direction until you have first determined all forces acting on the block in the x direction. You should note your units become inconsistent when subtracting force units from acceleration units.
 
  • #3
ok so faryou went astray here. Do not calculate acceleration in x direction until you have first determined all forces acting on the block in the x direction. You should note your units become inconsistent when subtracting force units from acceleration units.

ok, so I should subtract Kinetic force from my F_gx first before calculating acceleration?
 
  • #4
PhanthomJay
Science Advisor
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509
Yes.


I'm really confused, as we haven't covered this stuff in lecture, yet we have labs that deal with it
I see things haven't changed much in the 40 years since my college days!
 
  • #5
656
2
ok, so I should subtract Kinetic force from my F_gx first before calculating acceleration?

Yep, force due to kinetic friction.

Then, kinematics.
 
  • #6
Yes.


I see things haven't changed much in the 40 years since my college days!

Haha I guess not! Thank you all for your help, .61 s is the answer. I will definitely come back to this forum for help (if I need it) again!
 

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