# Normal force and friction

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1. Feb 16, 2017

### Pysics86

• Member advised to use the homework template for posts in the homework sections of PF.
Hi all,
I have problem to solve this question:

Wb=150N
Wa=50N
T=150N
alph =?
µ =?
- There is friction between A and B only(Between A and the ramp there isn't).
- A and B not moving(first Newton's law).

3. I tried to solve as:

But the answer is wrong, can anyone help me please, and explain where is the problem?

Regards Jack.

2. Feb 16, 2017

### ehild

3. Feb 16, 2017

### Pysics86

The correct anser is Miu=0.25

4. Feb 16, 2017

### ehild

How did you get it?

5. Feb 16, 2017

### Pysics86

By writing the forces as i the diagram forces, did the diagram, i don't care about the final answer and don't care about solving and finding the Miu, what i want to know is if the forces are right as the diagram or not, if not what is the problem, in other world i want to understand the physics not from the question from who understand physics and can explain.

6. Feb 16, 2017

### TomHart

Hi Physics86. Welcome to Physics forums. It looks like your diagrams are correct. The N??? in diagram for A would be equal to Nb because those forces have to be equal and opposite. I worked through the problem and got μ = 0.25.

7. Feb 16, 2017

### ehild

You need to collect the forces acting on block A. You have some confusion here, as there is normal force from the ramp (Na) and there is normal force from B acting on A, that can be the blue vector N???.
The forces acting on B are correct, taking in mind that the normal force Nb is the force exerted by A, and according to Newton's Third Law, it is equal in magnitude to N???.
You need to use the correct magnitudes to get the angle alpha. Remember, the friction is static, so F≤μN, and N is the magnitude of the normal force between A and B.

8. Feb 16, 2017

### TomHart

F≤μN. That is an important point. I should have pointed out that, although it wasn't stated, I assumed that one of the conditions of the problem was that block A was just at the point where it was going to slip. In that case, F would be equal to μN (F=μN). Otherwise - if that was not a condition of the problem - you would not be able to calculate μ.