Section 2 Question 42 Berkeley Physics

[pencap]

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Hey, I have a question regarding 42. How does one know what the frictional force is. Basically how do I know when it should be cos or sin. This aspect confuses me. Thanks

 

[reising1]

Can you post the actual question?

 

[pencap]

Can you post the actual question?

The frictional force acting on a block when it begins to slide down an inclined surface is best described by:

answers are
uk mg cos theta
uk mg sin theta
us mg cos theta
us mg sin theta

Is frictional force always cos theta? And when is something mgsintheta. I'm lost at this concept of things.

 

[Deadlifts]

The frictional force acting on a block when it begins to slide down an inclined surface is best described by:

answers are
uk mg cos theta
uk mg sin theta
us mg cos theta
us mg sin theta

Is frictional force always cos theta? And when is something mgsintheta. I'm lost at this concept of things.

Draw a free body diagram. The geometric relationships of the block on ramp with forces should show you whether it's cos or sin theta. The theta they use in that question is from the ramp bottom angle.

Friction force is Mus or Muk * Normal Force.

On a ramp, normal force is perpendicular to the ramp so we can calculate our normal force from the magnitude of the component of gravitational force in that vector component.

Do geometry with the drawing. SinTheta = O/H and CosTheta = A/H

Therefore, Muk * mgCosTheta

http://www.askiitians.com/iit-jee-p.../free-body-of-the-block-on-inclined-plane.jpg