EK 1001 physics #277

[divinelyawesome]

I'm geting confuse about the application of newton second law here:

The plane above is inclined at a 60 degrees angle. The coefficient of static friction is 0.2. The mass is 100 kg. What is the minimum force F necessary to start the object moving up the plane? Basically, the object is being pulled up the incline by a cord. So we have:
Sum of forces in x direction=T-f-W.sin(60degrees)=m.a
In y direction:N=W.cos(60)
a is zero in y direction, but I don't understand why it's zero in the x direction. If the object is moving upward, it has an acceleration. Should I assume it's moving at cst velocity?

---

Members don't see this ad.

 

[syoung]

If it's moving it doesn't mean it has acceleration. You need to look at all of your forces.

 

[DrknoSDN]

In this question it would be accelerating but that isn't relevant. They gave you coefficient of static friction and at a certain force you will overcome that static friction and have less friction because the coefficient of kinetic friction is smaller.
That's why the question asked at what minimum force will it break free of the kinetic friction and begin to move.

Basically once it starts sliding it will continue sliding.
This is true for just about every scenario with friction because kinetic is always smaller than static.

 

[divinelyawesome]

Okay, I get it. Thanks guys.