# What is the value of these angles ?

1. Apr 7, 2014

### yoxter

i know that question is a little basic but i have no one to ask.

for this image i think that the angle between the horizontal and the normal force that the semi-cylinder
exert on the bar is is α - 90°

Original Image

for this one i think that the angle between the horizontal and the normal forces that the cylinders exert on the bar is O - 90°

but my teacher says that is O and i can't figure why.

some one can explaimed why ? thanks.

Original Image

Last edited: Apr 7, 2014
2. Apr 7, 2014

### dauto

An angle is measured between two directions. You can't say "the angle of the normal force that the semi-cylinder
exert on the bar" and expect people to know what you're talking about because you only provided one direction - namely, the direction of the normal force. Are you measuring the angle between that force and the horizontal direction? If that's the case than the answer to the first picture that the angle is equal to α. Why can't you ask that question to your teacher?

3. Apr 7, 2014

### yoxter

thanks.

i've edited the post as you say, i can't ask to my teacher because my exam is the next class.

4. Apr 7, 2014

### PhanthomJay

There is no difference between the 2 figures in regard to the angle that the normal force makes with the horizontal. The angles of a triangle sum to 180 Degrees and angles on one side of a straight line also sum to 180 degrees. In your first figure, Try extending the horizontal line through D to your dashed line and examine the triangle so formed to see if that helps to determine the required angle of the normal force, using the 180 degree sum rules noted.

5. Apr 7, 2014

### yoxter

Thanks

I did as you said and the first figure the angle is α and in the the second the angle is o

it is a little more clear to me now.

6. Apr 7, 2014

### PhanthomJay

OK and a belated welcome to PF!