Given: coefficient kinetic and coefficient static frictions

In summary: A = 0.30F = (244lb)(sin(25.8))F = 105lbF = (244lb)(cos(25.8))F = 214lbFriction at A = 105lb <--- using the angle of the static frictionNormal force at A = 214lb <--- using the angle of the static frictionFriction at B = 214lbNormal force at B = 105lbIn summary, the blocks shown in the figure are connected by cords passing over a frictionless pulley. The coefficients of friction differ at each block, with values of Ms = 0.30 and Mk = 0.20 at Block
  • #1
Edwardo_Elric
101
0

Homework Statement


A problem in engineering mechanics:
The blocks shown in the figure are connected by flexible inextensible cords passing over frictionless pulley. At Block A the coefficients of friction are Ms = 0.30 and Mk = 0.20 while at Block B they are Ms = 0.40 and Mk = 0.30. Compute the magnitude and direction of the friction force acting on each block.

coefoffriction.jpg


the answer is Friction at A = 48lb
Friction at B = 36lb

Homework Equations


R is along the angle of friction w/c has two components:
Normal force at the vertical
Friction at the horizontal



The Attempt at a Solution


I do not know how to approach the problem since there are two given coefficient of friction and it seems very confusing...

however i tried solving at block A using the coefficient of kinetic friction at A = 0.20:

F = friction
R = the force along the angle of friction
frics.jpg

the angle of friction here lies along with R which is the coefficient of friction
tan(theta) = 0.20
theta = 11.3 degrees

so i used sine law:
[tex]\frac{\sin{79}}{300} = \frac{\sin{53}}{R}[/tex]
R = 244lb

R has two components: the normal force N and the friction F
F = (244lb)(sin(11.3))
F = 48lb <<< at A

i am not so sure about this..
can u give some hints its very confusing
 
Last edited:
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  • #2
I don't quite understand what you're doing. Where did the angle of 11.3 come from?

The first thing I'd do is figure out if this thing moves or not. Does it?
 
  • #3
no?

so ill try to use static friction
 

1. What do the coefficients of kinetic and static frictions represent?

The coefficients of kinetic and static frictions are measures of the amount of resistance or friction between two surfaces in contact. The coefficient of kinetic friction represents the resistance between two surfaces in motion, while the coefficient of static friction represents the resistance between two surfaces at rest.

2. How do the coefficients of kinetic and static frictions affect the motion of an object?

The coefficients of kinetic and static frictions play a crucial role in determining the motion of an object. The coefficient of static friction must be overcome in order for an object to start moving, while the coefficient of kinetic friction affects the speed and acceleration of the object.

3. How are the coefficients of kinetic and static frictions determined?

The coefficients of kinetic and static frictions are determined experimentally by measuring the force needed to move an object or keep it in motion on a particular surface. The coefficient is then calculated by dividing the force by the weight of the object.

4. Can the coefficients of kinetic and static frictions change?

Yes, the coefficients of kinetic and static frictions can change depending on various factors such as the type of surfaces in contact, the roughness of the surfaces, and the presence of any lubricants. They can also vary with temperature and pressure.

5. How do the coefficients of kinetic and static frictions relate to each other?

The coefficient of static friction is typically higher than the coefficient of kinetic friction because it takes more force to overcome the initial resistance and set an object in motion. However, the coefficients can be equal if there is no difference in the amount of force needed to start and maintain the motion of an object.

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