- #1
FaraDazed
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Homework Statement
A block of mass 100g is at rest on a rough horizontal plane. It has a massless rope attached to one end inclined at 20° above the horizontal. When the tension in the string is 0.5N the object is found to be in limiting equilibrium.
Part A: Find the coeffecient of static friction between the object and the plane.
Part B: What would the tension in the string have to be to make the object accelerate at 1.5m/s^2.
Homework Equations
Fnet=ma
R=mg (R is the normal reaction force)
The Attempt at a Solution
For clarification, F_f is the frictional force. and P is the unkown foce in part B.
Part A:
[itex]
0.5cos20=0.47\\
0.5sin20=0.17\\
mg=R+0.17\\
0.98=R+0.17\\
∴ R=0.98-0.17=0.81\\
F_f=0.5cos20 (because \space the \space force \space applied\space by\space the\space string\space to\space the\space horizontal \space must \space equal \space the \space frictional \space force)\\
∴F_f=0.47\\
μR=0.47\\
μ0.81=0.47\\
∴ μ \frac{0.47}{0.81}=0.58 \\
[/itex]
Part B:
[itex]
R+Psin20=mg \\
R+Psin20=0.98 \\
∴ R=0.98-0.34P \\
\\
Pcos20-F_f=ma\\
Pcos20-μR=0.15\\
∴0.94P=0.15+0.58R\\
0.94P=0.15+0.58(0.98-0.34P)\\
0.94P=0.15+0.57-0.2P\\
1.14P=0.15+0.57\\
1.14P=0.72\\
∴ P=\frac{0.72}{1.14}=0.63N \\
[/itex]
My main issue is if I have done it right because the answer to Part B seems rather low compared to the force when it is in limiting equilibrium i.e. only 0.13N more.
Any help/pointers is appreciated.
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