How Does Friction Impact Crate Movement on a Rough Surface?

[fubag]

During an experiment, a crate is pulled along a rough horizontal surface by a constant force F_vec and the magnitude of the acceleration along the x direction, a_x, is measured. View Figure The vector F_vec has a component along the x direction of magnitude F_x. The experiment is repeated several times, with different values of F_x each time.

http://img229.imageshack.us/img229/3708/1011642asf3.th.jpg


Create a plot of the force of static friction, f_s, versus the x component of the pulling force, F_x, for the experiment. Let the point F_min, along the horizontal axis, represent the minimum force required to accelerate the crate. Choose the graph that most accurately depicts the relationship among f_s, F_x, and F_min.

http://img398.imageshack.us/img398/3683/1011642csr5.th.jpg

Create a plot of the force of kinetic friction, f_k, versus the x component of the pulling force, F_x, for the experiment. Let the point F_min, along the horizontal axis, represent the minimum force required to accelerate the crate. Choose the graph that most accurately depicts the relationship among f_k, F_x, and F_min.

http://img453.imageshack.us/img453/3871/1011642dtb9.th.jpg

After all the trials are completed, a graph of acceleration a_x as a function of force F_x is plotted. Assuming the presence of both static and kinetic friction, which of the following graphs View Figure is most nearly correct?

http://img230.imageshack.us/img230/3650/1011642eia5.th.jpg


I am too sure about these questions, but so far I have guessed.

For the first one, I am thinking it is D, because the static friction to overcome is needed only for few seconds.

For the second one, I am thinking it is D because force of kinetic is required after a few seconds.

For the third one, I think it is B because the force applied must be equal to or greater than the force of friction for the block to move.

Please comment on my answers, I am not too sure about them.

 

[Mentz114]

The force of kinetic friction remains constant, so any graph with it rising can be rejected.
Also, a = F/m which is a straight line with slope 1/m, which should help with the third one.

 

[fubag]

ok

but still

is my logic here ok?

I mean for Parts A and B...

for C I am assuming it is Graph A now.

 

[Mentz114]

For, C it is C. There is no acceleration until the static friction is overcome.
I don't know about the others.

 

[fubag]

can anyone help me with the first two parts?

im thinking part b is D

 

[fubag]

and for part A i think it is D but i need more clarification

 

[Doc Al]

For the first one, I am thinking it is D, because the static friction to overcome is needed only for few seconds.

Realize that time is not being plotted in these diagrams, just forces. But D is correct because until the crate starts to move, the static friction equals F_x. (Note that the slope equals 1.)

For the second one, I am thinking it is D because force of kinetic is required after a few seconds.

Again, only forces are being plotted, not time. But yes, D is correct. Once the crate starts moving, kinetic friction acts--and the kinetic friction is constant.

For the third one, I think it is B because the force applied must be equal to or greater than the force of friction for the block to move.

Plot B implies that the acceleration is constant. That can't be true: Until F_x exceeds some value, you won't even be able to move the crate (acceleration = 0); Once it starts moving, as F_x increases the net force on the crate increases (thus acceleration increases).