Physics, 26.05.2020 08:57 odetteerivera

You insulate your medium-sized attic with insulation with R=10. Which of the following sets of estimates is most reasonable?

A. The attic is 150 sq ft, the temperature difference on average is 5 degrees C, and electricity costs 1 cent per kWh.

B. The attic is 15,000 sq ft, the temperature difference on average is 10 degrees F, and electricity costs $10 per kWh.

C. The attic is 150 sq ft, the temperature difference on average is 5 degrees F, and electricity costs $1 per kWh.

D. The attic is 1500 sq ft, the temperature difference on average is 50 degrees F, and electricity costs 1 cent per kWh.

E. The attic is 1500 sq ft, the temperature difference on average is 10 degrees F, and electricity costs 10 cents per kWh.

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Answer from: Quest

the first, second, and fifth options

What statements describe acceleration check all that apply?

Answer from: Quest

a and c

c is 100% but i am confused in a

For which optical devices does d sometimes have a positive value

Answer from: Quest

answer: at 24 february, 2014 the population gets twice of its initial population.

explanation:

since we have given that

the population for alpha city, t years after january 1, 2004 is given as

$p(t)=0.3t^2+6t+80$

first we find out the initial population, i.e. t=0,

so, our quadratic equation becomes,

$p(0)=0+0+80=80$

according to question, we have said that if the population has twice its initial population, then it becomes

$p(t)=2\times 80=160$

so, time taken to reach this above mentioned population is given by

$160=0.3t^2+6t+80\\\\160-80=0.3t^2+6t\\\\80=0.3t^2+6t\\\\0.3t^2+6t-80=0\\\\\text{using quadratic formula ,we get }\\\\t_{1,\: 2}=\frac{-60\pm \sqrt{60^2-4\cdot \: 3\cdot \: 800}}{2\cdot \: 3}\\\\t=-29.14\ and\ t=9.149$

now, we know that time can't be negative so, we take t=9.149 years.

hence, at 24 february, 2014 the population gets twice of its initial population.

Answer from: Quest

force is based upon mass and acceleration.the statement is not always true.

it can be explained from newton's second law of motion.

as per newton's second law of motion the rate of change of momentum is directly proportional to the applied force and takes place along the direction of force.

mathematically it can be written as -

$f\alpha \frac{dp}{dt}$

⇒ $f= k \frac{dp}{dt}$

here k is the proportionality constant whose value is 1.

$f=\frac{dp}{dt}$

$f= \frac{d[mv]}{dt}$

$f= m\frac{dv}{dt} +v\frac{dm}{dt}$

$f=ma+v\frac{dm}{dt}$ [a is the acceleration=dv/dt]

hence we see that force not only depends on mass and acceleration but also on velocity and rate of change of mass. force f will depend on mass and acceleration only when mass is constant.

hence the statement that force is based upon mass and acceleration is false.

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