Equilibrium Forces: Force to Move a Mass

[JoeyBob]

Homework Statement

see attached

Relevant Equations

0=Tsin(angle)+Ff

So I already know the normal force is 268.08 from a previous part of the question. I thought that the friction force must be less or equal to uFN for an object to stay in static equilibrium.

So Tcos(angle)=uFN

T=uFN/cos(angle)=116.49

But the answer is suppose to be 133.37.

 

[TSny]

So I already know the normal force is 268.08 from a previous part of the question.

But the normal force depends on the force $T$ which is an unknown.

 

[JoeyBob]

But the normal force depends on the force $T$ which is an unknown.

T isn't an unknown. T=34

 

[TSny]

T isn't an unknown. T=34

Isn't the value T = 34 N used for a previous part of the problem? In the part of the problem that you are now working on, you are looking for the maximum value that T can have before the block starts to slide. So, the value of T that you are looking for is unknown. You cannot assume that the normal force is the same in both parts of the problem. As T is increases, N increases.

 

[JoeyBob]

Isn't the value T = 34 N used for a previous part of the problem? In the part of the problem that you are now working on, you are looking for the maximum value that T can have before the block starts to slide. So, the value of T that you are looking for is unknown. You cannot assume that the normal force is the same in both parts of the problem. As T is increases, N increases.

So FN=mg-Tsin(angle).

Ff=umg-uTsin(angle)

0=Tcos(angle)-umg+uTsin(angle)

T=umg/(cos(angle)+usin(angle))

This gives me 94.65, which is still wrong.

 

[TSny]

So FN=mg-Tsin(angle).

Check the signs in this equation. Otherwise, things are looking good.

 

[JoeyBob]

Check the signs in this equation. Otherwise, things are looking good.

Thanks