Physics! (pulley, friction)

[Cerb]

A 10kg mass is on a 50º incline. The incline has a standing friction of 0.74u, and sliding friction of 0.57u.
At the top of this incline is a magical frictionless pully, and a 6kg mass hanging freely from it.
Will either mass actually move, and what equations exactly are used for which forces?

I thought I'd have a lot more, but the rest seem to be coming to me. problems like this, though, I just can't put it all together right to get an answer matching the book (studying for a test).

 

[zimu]

could've answered this 5 years ago, not any more!
try working out how much force is required to make a 10kg object to start moving on a 50 degree incline with a given standing friction. if that exceeds the force exerted by a 6kg mass, then it moves.

can't remember any equations, but likely someone in this forum knows the answer in a sec.

 

[Cerb]

Logically I can get it, but it is figuring out which trig bit goes to which force that gets me when I try to do it unassisted. I figure maybe an explanation from someone here would be better than the prof--not that that'd be hard.

 

[Triumph]

Where are your free body diagrams? I'll tell you that the tension in the rope is the same everywhere....

 

[edro]

I don't think it will move.
This probably contains your answer.

 

[DingDingDao]

Originally posted by: edro13
I don't think it will move.
This probably contains your answer.

Wow that website definitely gives you the answer in a hurry. Not very helpful, though, in the sense that it will give the correct answer, but in the end, the OP won't have this website to access during his midterms/finals. So he probably ought to learn the concepts before using this site.

 

[Triumph]

Originally posted by: DingDingDao

Originally posted by: edro13
I don't think it will move.
This probably contains your answer.

Wow that website definitely gives you the answer in a hurry. Not very helpful, though, in the sense that it will give the correct answer, but in the end, the OP won't have this website to access during his midterms/finals. So he probably ought to learn the concepts before using this site.

Agreed. Most engineering teachers I had didn't even care if you got the right answer - if you had the process to the solution correct, you got maybe 8 out of 10 on a homework problem. Getting the right answer only shows that you put the right numbers into the right equation... the real key is getting the right equation.

 

[edro]

Originally posted by: Triumph

Originally posted by: DingDingDao

Originally posted by: edro13
I don't think it will move.
This probably contains your answer.

Wow that website definitely gives you the answer in a hurry. Not very helpful, though, in the sense that it will give the correct answer, but in the end, the OP won't have this website to access during his midterms/finals. So he probably ought to learn the concepts before using this site.

Agreed. Most engineering teachers I had didn't even care if you got the right answer - if you had the process to the solution correct, you got maybe 8 out of 10 on a homework problem. Getting the right answer only shows that you put the right numbers into the right equation... the real key is getting the right equation.

We all know that... I wasn't just giving him the answer. That page also contains the equation: (m1-m2)sin50/(m1+m2)

I don't know where to put friction into that equation though.

 

[DrPizza]

microsoft painting a picture right now to help.
be back to this thread in a bit (if I finish it before I go to lunch)

 

[chickchick]

my brain is on a vacation. sorry

 

[Cerb]

Originally posted by: chickchick
my brain is on a vacation. sorry

Mine wants to be. Badly . All I can do is be glad I'm not taking Chemistry for my science hours.

 

[maziwanka]

that website is freaking awesome. it shows all the necessary diagrams in order to work out a proper solution. the relevant coeff of friction is the standing one (the block has to overcome that one first in order to begin moving).

study the diagrams on the site. if you still have trouble understanding ask more questions and we can surely help.

 

[DrPizza]

Sorry... didn't beat the lunch break and had students in for extra help. THen calc class... now I'm back with a general explanation....

 

[DrPizza]

Okay. Look at the picture
inclinedplaneproblems.jpg

Use the acceleration of gravity to find the gravitational force (always straight down). F=ma
Now, use simple trig to find the other two sides of the red triangle. Generally, on the inclined plane problems you will need these two: parallel force and normal force.

Basically, you just broke the gravitational force into 2 component forces that are perpendicular to each other... if you add those two force vectors together, the equivalent force will be equal to the gravitational force. (a check)

Now, gravity is exerting a force down the plane equal to that parallel force you found. It is also exerting the normal force perpendicular to the surface. (well, actually the normal force is the force the surface is exerting back up - the two are equal (Newton)

Use the normal force and your coefficient of static friction to determine the force necessary to get the object to move.

You have 2 forces opposing each other, both parallel to the slope. One is the parallel force that you calculated. The other is the force caused by the rope/pulley/mass system. Find the net force parallel to the surface.

Is the net force larger than the force necessary to move the object?




Note: on a future problem if your teacher wants to make it really tricky, he may ask for the acceleration of the object. If you do a F=ma, don't forget: there are 2 masses accelerating - the one on the inclined plane and the one hanging. (plus, you'll have to switch to the coefficient of kinetic friction)