Homework Help: Static/Kinetic friction problem help! (several problems - Diagrams included)

1. Oct 7, 2012

Xelb

PROBLEM 1
1. The problem statement, all variables and given/known data

A block of mass m = 12.5 kg rests on an inclined plane with a coefficient of static friction of μs = 0.08 between the block and the plane. The inclined plane is L = 4.5 m long and it has a height of h = 3.25 m at its tallest point.

m = 12.5 kg
μs = 0.08
L = 4.5 m
h = 3.25 m

2. Relevant equations

Fs = μsN
W=mg
sin(theta) = opposite/hypotenuse

3. The attempt at a solution

There are four parts to this problem:

A) What angle, in degrees, does the plane make with respect to the horizontal?

-Should be simple, right? According to the expert TA system...apparently not. This is what I did:

sin(θ) = opposite/ hypotenuse
= 3.25m / 4.5 m
= .0126°

This has to be the right answer, but whenever I input it, it comes off as wrong. I click the "hint" button, and it basically tells me to do exactly what I just did.

B) What is the magnitude of the normal force, FN in Newtons, that acts on the block?

-Since the weight of the Earth on the box is equal and opposite (according to Newton's third law), I used W = mg and plugged in 12.5kg and 9.8 m/s^2 respectively. I produced an answer of 122.5N. Shouldn't this mean that the normal force that acts on the box is the same number?

C) What is the force of gravity along the plane, Fgx in Newtons?

Isn't this asking the same thing as B? UGGGGH.

D) Write an expression for the magnitude of the force due to static friction, Fs, just before the block begins to slide.

Isn't that just basically Fs≤μsN?

PROBLEM 2
1. The problem statement, all variables and given/known data

2) A block that has a mass of m = 4 kg rests on a horizontal plane. The coefficient of static friction, μs, is 0.15. A horizontal force, F, is applied to the block, and it is just large enough to get the block to begin moving.

a) Write an expression for the sum of the forces in the x-direction using the variables from the above Free Body Diagram.
b) Given the coordinate system specified in the problem statement, write an expression for the sum of the forces in the y-direction.
c) Write an expression to show the relationship between the maximum friction force, Ff, and the normal force, FN.
d) Calculate the magnitude of F, in Newtons, if Ff is at its maximum.

m = 4 kg
μs = 0.15

2. Relevant equations

Fnet=ma?

3. The attempt at a solution

A) First I used F=(4kg)(9.8m/s^2) and came up with 39.2N. But since it's just an expression, I'm kind of stuck as to what I should do next. v_v

B) Same thing as A...not really sure.

C) Same thing as B and A. Ughh.

D) Wouldn't this just be the number I calculated in part A (39.2N)?

PROBLEM 3
1. The problem statement, all variables and given/known data

3) A block with mass m1 = 7.2 kg rests on the surface of a horizontal table which has a coefficient of kinetic friction of μk = 0.52. A second block with a mass m2 = 8.3 kg is connected to the first by an ideal pulley system such that the second block is hanging vertically under the force of gravity. The second block is released and motion occurs.

a) Using the variable T to represent tension, write an expression for the sum of the forces in the x direction, ΣFx for block 1.
b) Write an expression for the magnitude of the acceleration of block 2, a2, in terms of the acceleration of block 1, a1. (Assume the cable connecting the masses is ideal.)
c) Write an expression using the variables provided for the magnitude of the tension force, T.
d) What is the tension, T in Newtons?

m1 = 7.2 kg
m2 = 8.3 kg
μk = 0.52

2. Relevant equations

Friction equations
F=ma
W=mg?

3. The attempt at a solution

...we didn't even go over tension in class. I don't even know how to do this problem...yet the HW is due by midnight. I'm not sure what my professor even wants with this one. T_T

EDIT: Each diagram corresponds to the order of the problems. For example, the first diagram on the very left corresponds to problem one, the middle for problem two, and the one on the right for problem 3.

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2. Oct 7, 2012

Xelb

I've been approaching this from every possible angle. If I submit wrong answers, I lose points. Even when I submit an apparently "correct" answer, I'm still being marked wrong. I really desperately need help. T_T

3. Oct 7, 2012

Xelb

I attempted the assignment for real. I failed it. No need to respond to this thread anymore. Thank you, Physics forums! T_T