Physics help please?

[fifa518]

2. Vector A is 3 units in length and lies along the positive x-axis. vector B is 6 units in length and points along a direction 120° from the positive x-axis. What is the magnitude of the resultant when vectors A and B are added?
(cos 60° = 0.5, sin 60° = 0.87)

Answer 5.2




3.What is the magnitude of force necessary to stretch a 1 mm radius steel wire by 1 %? (Y = 2 x 1011 N/m2)

Answer: 6,280


4. The inductive reactance in an ac series circuit is 63 ohms and the frequency is 50 Hz. What is the inductance in the circuit?

Answer: 200 mH

….. don't think i ever learned how to solve for the inductance
Thanks!

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[Mark2K8]

3. Basically using Young's modulus equation to solve for Force.
F = (change in length)(Cross sectional area)(Young's modulus)
Change in length = is given, 1%, which transformed to decimals is 0.01.
Area = First, convert 1mm to meter, so 1x10^-3 meters. It is a wire, so area is going to be of a circle (pi*r^2), thus area is: (pi)(1x10^-3 m)^2 = (pi)(1x10^-6 m)
Young's modulus = given, 2 x 10 ^11 N/m^2

Plug and chug: (0.01)(pi)(1x10^-6)(2x10^11)= 6280
If you get a problem like this on the exam, it will most likely be easy numbers to work with. But definately know the equation and how to use it/what each variable means.

4. Another plug and chug one:
Inductance in A/C Circuit = XL= (2pi)(f)(L)
f = frequency ; L = inductance of coil (in henries, and what you are trying to solve) ; XL = inductive resistance (in ohms)
L = (XL)/(2pi)(f) ; where XL=63, f=50
L = (63)/(2pi)(50) = 0.200 Henries
Convert to mH = 200mH

 

[mathcod]

#2. The math can be tricky and time consuming. I just did graphical analysis of vectors (no math, just drawing a picture TO THE BEST SCALE I COULD) and approximated ~5 as my answer in 6 seconds; simply draw a picture to scale. Since this is multiple choice math, you need not be as accurate.

 

[byhc]

Thought I'd mention that AC circuits are not part of the OAT. Make sure you read the OAT outline that they have posted on their website that gives you information on what material is tested (requires some digging to find the correct file).

Another piece of advice is to remember the sin/cos/tans as fractions, rather than decimals. You save a lot more time using them as fractions because values often (intentionally) cancel. The calculator you're provided requires you to click, so the less operations you can do using that calculator the better as it will slow you down.

 

[bidiaokais]

you get a problem like this on the exam, it will most likely be easy numbers to work with. But definately know the equation and how to use it/what each variable means