Helplife2.net: I suck at physics

[ríomhaire]

Gah. I got a frickin' A in this subject in secondary school but these university problems are doing my head in. I seem to have lost the ability to do all maths over the summer. The answers are given in the back of the book, but I'm supposed to show my work, of course. HHHHHHEEEEEEEEAAAAAAAAALLLLLLPPPPPPP

Q1: When a certain horizontal force is applied to a box containing two bricks, each of mass 5kg, the acceleration of the box is observed to be 2.4m/s^2. When the same force is applied to the box when it contains 5 such bricks the acceleration is found to be 1.1m/s^2. What are (a) the applied force and (b) the mass of the box?
A1: 30N, 2.7kg

Q2: Electrons in an oscilloscope are accelerated from rest by a constant force of 8x10^16N over a distance of 15mm. Calculate (a) the acceleration and (b) the final velocity of the electrons.
A2: 8.8x10^1 4m/s^2, 5.1x10^6 m/s^2

Q3: A helical spring of spring constant 270 N/m hangs vertically from a fixed support. When a 1.2kg mass is attached to the free end of the sprig the length of the spring is 30cm. What is the unstretched length of the spring? What would the length of the spring if an additional 0.8kg was added?
A3: 26cm, 37cm

Q4: A sodium-23 atom in a solid vibrates with a frequency of 2x10^13 Hz. Assuming that the motion is simple harmonic, estimate the effective 'spring constant' of the force which binds the atom to the other atoms of the solid.
A4: 600 N/m

Q5: Show that if the motion of a body is described the relationship x=Asin(ωt)+Bcons(ωt) the body moves with simple harmonic motion. Derive expressions for (a) the amplitude, (b) the maximum velocity and (c) the phase angle in terms of the constants A and B.
A5: (A^2+B^2)^-2, ω((A^2+B^2)^-2), tan^-1(B/A)

Notes:
Mass of electron: 9.1x10^-31kg
Mass of proton/neutron: 1.67x10^-27kg
Atomic number of sodium: 11

 

[VirusType2]

The fire alarm just went off. I'll post the rest later.

smoke coming out of your ears, or dinner is ready.

 

[Insano]

Q1: When a certain horizontal force is applied to a box containing two bricks, each of mass 5kg, the acceleration of the box is observed to be 2.4ms^2. When the same force is applied to the box when it contains 5 such bricks the acceleration is found to be 1.1ms^2. What are (a) the applied force and (b) the mass of the box?
A1: 30N 2.7kg

Newton's second law: F_res = m . a

Remember F and a are vectors, and we're only looking at the horizontal projections. This means the Gravitational force is left out.

We have two unknown variables (F_unknown and m_box) and two pieces of information:
F_unknown = (m_box + 2 * m_brick) * a_1
F_unknown = (m_box + 5 * m_brick) * a_2

Work out this set of equations and you get the answer.

Q2: Electrons in an oscilloscope are accelerated from rest by a constant force of 8x10^16N over a distance of 15mm. Calculate (a) the acceleration and (b) the final velocity of the electrons.

Once again: Newton's second law.

(a) F = m * a, so 8x10^16N = m_electron * a
You know the mass of an electron, so you can find its acceleration.

(b) x(t) = x_0 + v_0 * t + (a * t^2)/2
v(t) = v_0 + a * t

These are the two equations for the position and movement in function of time. You can take x_0 = 0 and the same for v_0 = 0.

Then you can find the time needed to do 15 mm from the first equation. You'll get two possible answers, the smallest positive number will be the right one.

Then fill in this time in the second equation and find your final velocity.

Cheers.

 

[ríomhaire]

Using F=ma on the electron one keeps giving me an answer 10^32 times larger than the answer given in the book.

 

[The_E]

my head hurts

 

[Reginald]

I could know the answers. But I finished school little more than 3 months ago. I don't want to start thinking again this soon!

 

[jverne]

Using F=ma on the electron one keeps giving me an answer 10^32 times larger than the answer given in the book.

it just might be because 8x10^16 is really 8x10^(-16)? because i doubt a force of 80000000000000000N is applicable on a small electron... my guess

hmm...no that's not the case since the answer is ^14...hmmm

 

[ríomhaire]

it just might be because 8x10^16 is really 8x10^(-16)? because i doubt a force of 80000000000000000N is applicable on a small electron... my guess

Well spotted mate. It seems they left out a minus in the book. Thanks.

 

[jverne]

Well spotted mate. It seems they left out a minus in the book. Thanks.

hmm...so the answer is also ^-14?

Q3:

hook's law

F=k*x (k being the constant, x being the stretch in cm)

a) F1=(1.2kg*10m/s^2)=12N

the the stretched length is 30 cm=x2

unstreched length=x

the difference between x and x2 =x1

x2-x1=x (=26cm)

x1=F1/k (=4.4cm)

don't forget to multiply the answer with 100 to get cm instead of metrers

b)F2=F1+(0.8*10)=20N

the final length with both weights=x4

x3 is the additional length due to the added weight

x3=F2/k (=7cm)

x2+x3=x4 (=37cm)


hmm...i need to double check something here...i think i ****ed up a bit even tough the results are quite close. seems to be correct but i'm bothered a bit by those .4 cm...hmmm
i also cant remember if hooks law works also in relative stretching (as in stretching from 30cm to 37cm or from 0 to 37cm) but i guess the results work...

the b) part bothers me and i don't know why

 

[Seppo]

I get a headache from this

 

[ríomhaire]

hmm...so the answer is also ^-14?

I thought I had noticed something odd about the units in this book. I kept seeing Nm^ 1 and I knew something was wrong but I couldn't place it. All the minus signs in the superscript are missing. What kind of cheap fecking publishers are these guys?

 

[Seppo]

By the way what are you studying in university?

 

[ríomhaire]

By the way what are you studying in university?

Physics. I'm screwed!

 

[jverne]

as for Q4 i don't really know how should it be solved.

but you could start looking for the mass of the atom with

In chemistry and physics, the atomic number (also known as the proton number) is the number of protons found in the nucleus of an atom. It is conventionally represented by the symbol Z. The atomic number uniquely identifies a chemical element. In an atom of neutral charge, atomic number is equal to the number of electrons.

The atomic number, Z, should not be confused with the mass number, A, which is the total number of protons and neutrons in the nucleus of an atom. The number of neutrons, N, is known as the neutron number of the atom; thus, A = Z + N. Since protons and neutrons have approximately the same mass (and the mass of the electrons is negligible for many purposes), the atomic mass of an atom is roughly equal to A. Note that the atomic mass number A of an atom, which is always an integer, is different than the atomic weight of a element, because in general an element consists of a mixture of atoms with the same Z but differing N

wiki

you have the number of protons and their mass. i'm not sure where to get the number of neutrons? maybe out of the sodium molar mass?? i have no idea what i'm talking about

one thing that bothers me is where to get distance with the unit Hz, which is not a composite unit...

god damn it...:hmph:

 

[ríomhaire]

Thanks for your help guys. I'm only stuck on the last one now (TBH for Q4 I just couldn't do it because the only calculator I had was the one on my phone. I borrowed a friend's scientific one and I could do it fine then. And for questions 1 and 3 I was just making stupid slip ups).

For the last one I actually have to understand what the hell all the theory on SHM is and I don't get half of it. Time to reread that chapter methinks. Any help on the final question would be loved.

Edit: jverne, you've clearly never done chemistry
The number of protons in sodium is always 11, and this isotope of sodium is sodium-23, so the number of neutrons is 12 (23-11=12). Not that it matters as the difference in mass between protons and neutrons is negligable, so I can just take the number 23 and multiply it by the mass of one neutron/proton. The mass of the electrons is pretty negligable too, but I left them in the equation anyway. I got an answer of 606N/m, and the answer given is 600, so I'm happy with that.

 

[jverne]

Edit: jverne, you've clearly never done chemistry
The number of protons in sodium is always 11, and this isotope of sodium is sodium-23, so the number of neutrons is 12 (23-11=12). Not that it matters as the difference in mass between protons and neutrons is negligable, so I can just take the number 23 and multiply it by the mass of one neutron/proton. The mass of the electrons is pretty negligable too, but I left them in the equation anyway. I got an answer of 606N/m, and the answer given is 600, so I'm happy with that.

hehe...actually i never paid much attention to chemistry...obviously that was a bad idea

but i'm still wondering how did you get the N/m?

 

[dfc05]

Reading your book for Question 5 should help a lot. It's not really about calculations... just that given that form of equation x=Asin(ωt)+Bcos(ωt) you can get amplitude, period, velocity, phase shift, etc. Those terms should be well defined and the answers would probably be derived in the text.

Basically simple harmonic motion always takes that form. The object oscillates (like a spring bouncing back and forth). A and B describe how big the oscillation is (amplitude), and ω is associated with how fast it oscillates aka how many oscillations per second (notice that it's a multiplier on time). This is why ω is called the angular frequency.

The maximum velocity should be in the middle (at x=0) -- the object slows down, then reaches v=0 when it changes direction, then speeds up in the middle. So I'd suggest first setting x=0 to determine ωt in terms of B & A; then find the expression for velocity, v = dx/dt, and plug in for ωt to get v in terms of ω, B, & A. ****I can't remember a lot of trig identities, so I don't know if this method actually works****

As for phase angle... I can't remember what that is

 

[Insano]

Physics is like sex: sure, it may give some practical results, but that's not why we do it.

Anyhow, any more problems? It seems some people have the same book as you do.

 

[ríomhaire]

hehe...actually i never paid much attention to chemistry...obviously that was a bad idea

but i'm still wondering how did you get the N/m?

I don't know where the units came from, I just put the mass and period into the formula

T=2*(pi)*(M/K)^-0.5

 

[DEATHMASTER]

**** practical physics, its the theory that I like. Though it would be nice if I too understood this stuff so easily and got to understand the real complex stuff that current physicists are trying to figure out (and then figure it out ), but that'll never happen.

 

[jverne]

I don't know where the units came from, I just put the mass and period into the formula

T=2*(pi)*(M/K)^-0.5

hmm...can't really remember this formula, oh well



this book http://www.springer.com/physics/book/978-0-387-95269-7 is awesome i want to buy it someday, i really recommend it

 

[Tagaziel]

This thread wins on, like, so many levels.

 

[Avoidist]

So riomhaire, are you using calculus in physics yet? It makes the SHM stuff a lot easier, it just becomes the solution to the eqn along the lines of x"+(k/m)x=0 for undamped oscillations to give all solns, or if you want to know v(t) given x(t) you just take x'(t)=v(t), same for a(t)=x"(t) etc. I get the feeling (A^2+B^2)^-2 should actually read 1/2 or -1/2. Let me go check Giancoli, I should really revise this stuff.

Edit: Just checked, the coefficients are actually related to amplitude where (let C = amplitude) C=-Asin(phi) and C=Bcos(phi) where phi is a phase constant. The expression for x(t) then becomes x(t)=C*cos(wt+phi). That may help a little.

 

[FieryGoose]

Are you taking mechanics or some higher level of physics because some of the questions look familiar and others don't? I feel your pain though, the physics hw I get is occasionally kicking my ass as well. What book are you using by the way?

 

[The Monkey]

I'm so glad I'm not doing that shit anymore.