BR Physics Trig

[bluishgreen]

Just started on BR Physics today. It is so darn confusing with the Trigonometric functions. Theres something like cos^2, etc. Too many equations with sin, cos. Can someone narrow down to what we need to know? Because the material in the book is TMI IMO. I took algebra based physics, and we BARELY had any trig in it.


And how do I do these functions without using calculator?

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[epsilonprodigy]

Memorize sin and cos values for commonly-used angles such as 30, 60, 45, 90 and 180.

Know the ratios associated with a 30-60-90 triangle without having to derive them from the trig functions. Know how tangent relates (sin/cos) but I wouldn't bother memorizing tangent values.

sin cos
30 1/2 sqrt 3/2

45 sqrt 2/2 sqrt 2/2

60 sqrt 3/2 1/2

90 1 0

180 0 1




ugh, sorry, I can't get the formatting right. Hopefully that makes sense, if not, google images will pull you up a nicer table.

 

[muhali3]

You only really have to memorize 3 numbers.

cosine - 0 (1) , 30 (.86), 45 (.71), 60 (.5), 90 (0)

The values for 0 degrees and 90 degrees should be common sense.
The middle three numbers (.86, .71, .5) are really the only numbers you need to remember. The values for sin are the same but in the reverse order.

sin - 0 (0) , 30 (.5), 45 (.71), 60 (.86), 90 (1)

 

[bluishgreen]

You only really have to memorize 3 numbers.

cosine - 0 (1) , 30 (.86), 45 (.71), 60 (.5), 90 (0)

The values for 0 degrees and 90 degrees should be common sense.
The middle three numbers (.86, .71, .5) are really the only numbers you need to remember. The values for sin are the same but in the reverse order.

sin - 0 (0) , 30 (.5), 45 (.71), 60 (.86), 90 (1)

So do I need to knw how to find Sin, Cos, and Tan? Or will I be ok memorizing these 3?

 

[BerkReviewTeach]

So do I need to knw how to find Sin, Cos, and Tan? Or will I be ok memorizing these 3?

You need to know those values mentioned by epsilon and muhali above (sin 30 degrees = cos 60 degrees = 0.50, sin 45 degrees = cos 45 degrees = 0.71, and sin 60 degrees = cos 30 degrees = 0.86) and SohCahToa. You can figure anything out from those numbers and good visualization.

If you are talking about cos^2 in Figure 1.9, that's not a solution, that's a summary. If it's not obvious at first glance that it's the Pythagorean theorem applied to vectors, then don't worry about it. As long as you know that the x-component of a vector depends on cosine and that the y-component of a vector depends on sine, you'll be okay.

So here are some examples of how to apply the numbers mentioned by epsilon and muhali:

Straight-forward application question​

1. What is the initial x-direction speed of a projectile launched at a 60 degree angle at 40 m/s?

• a) 12 m/s

• b) 20 m/s

• c) 34 m/s

• d) 80 m/s

Harder calculation question​

2. What is the initial y-direction speed of a projectile launched at a 30 degree angle if the initial x-direction speed of a projectile is 50 m/s?

• a) 50 m/s

• b) 29 m/s

• c) 86 m/s

• d) 11 m/s

Conceptual twist incorporated with a math question​

3. What is the y-direction speed at impact for a projectile that was launched at a 48 degree angle if the initial speed of a projectile was 31.7 m/s?

• a) 22.5 m/s

• b) 23.4 m/s

• c) 21.6 m/s

• d) 27.0 m/s