# Unsure about angle inside Mohr's circle?

1. Apr 5, 2015

### influx

Can someone explain why the angle inside the triangle inside Mohr's circle is θP2 rather than θP1 ? I understand everything else but just this small point is frustrating me.

Thanks

2. Apr 6, 2015

### paisiello2

I don't see θP1 defined anywhere but I assume that the subscript 2 is used with θP2 to correspond with σ2.

3. Apr 6, 2015

### influx

θP2 is the angle that corresponds to σ2 and θP1 is the angle that corresponds to σ1 but what I am confused about is why the angle inside the triangle is θP2 rather than θP1? How did they know which one it is?

4. Apr 6, 2015

### paisiello2

Point D corresponds with σ2 and point B corresponds with σ1. If σ2 is the minimum principle stress then by definition 2θP2 would be the angle that point A makes with the x-axis. And 2θP1 would make the angle with point E which would be 180° from point A in Figure 22.9.

Last edited: Apr 6, 2015