Force Relative to the vertical?

Scorry · Oct 13, 2015

Can you explain why cosine theta is vertical, and sin theta is horizontal on this example? I am confused.

berkeman · Oct 13, 2015

Scorry said:

Can you explain why cosine theta is vertical, and sin theta is horizontal on this example? I am confused.

Link?

Does it look like this?

https://www.cdli.ca/sampleResources/physics3204/unit01_org02_ilo03/u01-s02-ls03-lessonfig07.gif

Free-body diagram of a block on a 45 degree incline showing gravity mg, normal force Fn = mg cos θ, friction Ff, and components mg sin θ parallel and mg cos θ perpendicular to the slope.

Scorry · Oct 13, 2015

Thanks. I attached a photo, but it's not showing up. Your example is what we're covering next, I haven't gotten there yet.

Textbook page showing Example 4-15 Accelerometer with two photo-style diagrams of a car interior with a hanging pendulum, and a free-body diagram with forces F_T, F_T cos θ, F_T sin θ and mg; solution text gives tan θ = a/g = 1.20 m/s²/9.80 m/s² = 0.122, θ = 7.0°.

berkeman · Oct 13, 2015

It's because of which angle theta they are using (they show theta between the force vector and the vertical axis). If they defined/chose theta to be between the force vector and the horizontal axis, that would switch the sin() and cos() terms, right?

Scorry · Oct 13, 2015

berkeman said:

It's because of which angle theta they are using (they show theta between the force vector and the vertical axis). If they defined/chose theta to be between the force vector and the horizontal axis, that would switch the sin() and cos() terms, right?

Thanks man, it makes a little more sense. I haven't taken trig in a few years.

Force Relative to the vertical?

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