# Help for a few questions

Hi guys,

I need some assistance on the following.

1) You need to push a 30kg mass up an inclined slope of 30 degrees from rest. After moving it by 5m and creating 600J of energy due friction in the process, you find that the mass is moving at 1 m/s. Find the work done moving the mass over the distance of 5m.

I have: sin30(30)(10) = 150N.
600 = F x 5 => F= 120N.
So the av. net force is 30N. So 30 X 5 = 150J.
However this is not one of the choices'. Where is the mistake?

2) 2 objects A and B are separated by a finite distance R and act on each other through their g field. If mass A is doubled and distance R tripled, what happens to their gravitational potential?

I'm not sure how to start here. Anyone?

3) Which of the following condtions is not necessary for lgiht waves from 2 sources to intefere?
a) They must have same wavelength
b) They must have same frequency
c) They must have same amplitude
d) They must have a constant phase difference
e) all above are necessary
More than 1 options allowed. I chose amplitude. However, do they need to have a constant phase diff?

4) If the distance of the plates of a capacitor is increased, will the energy stored in it be the same, increase or decrease? Is a capacitor supposed to have small internal resistance or a large one?

I would say the same as there is no relation of the distance of the plates with its energy? For part 2 I'm not sure.

5) Why does the binding energy per nucleon does not depend on the volume of the nucleus?

I only know binding energy is the energy required to separate the individual nucleons. I'm not sure why.

Any help appreciated! Thanks!

1) How did you get an average net force of 30N? Also, remember conservation of energy: you can't get out of a system more than you're putting in, which is what happens with your answer. If you're generating 600J of energy with 150J of work, you may have solved the world's energy problems. ;) What are the choices available?

2) Have you learned the equation for gravitational force yet? You should be able to work it out from that.

3) Not really my area of expertise, but the amplitude is what's affected when two light waves interfere (look up the double slit experiment), so it's unlikely to be that one. Pretty certain there's more than one right answer, though.

4) I'll have to pass, though you could google that one easily enough.

5) See above. :p

3> ya the waves will also have to have a constant phase

4>when the plates of a capacitor are moved apart, then the energy stored increase. see,if u move the plates apart, u have to do some work against the attractive force between the capacitors. where will this work done go? this will be stored in between the plates.

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Hootenanny
Staff Emeritus
Gold Member
app said:
3> ya the waves will also have to have a constant phase
I'm afraid I disagree here. It is not necessary for the two waves to be coherent to interfere. However, the interference pattern is more pronounced if the light is coherent. To al_201314: Is that exactly how the question is phrased or is the words interference pattern mentioned?
app said:
4>when the plates of a capacitor are moved apart, then the energy stored increase. see,if u move the plates apart, u have to do some work against the attractive force between the capacitors. where will this work done go? this will be stored in between the plates.
Again, I must disagree with you here. Capacitance and hence energy stored is inversely proportional to the distance between the plates, thus increasing the plate separation will decrease the capacitance and consequently decrease the energy stored by the plates.

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Hootenanny
Staff Emeritus
Gold Member
al_201314 said:
5) Why does the binding energy per nucleon does not depend on the volume of the nucleus?

I only know binding energy is the energy required to separate the individual nucleons. I'm not sure why.
Okay, your half way there. Think, what is the definition of binding energy? What is the definition of binding energy per nucleon? How would volume affect either of these definitions?

Hootenanny said:
Again, I must disagree with you here. Capacitance and hence energy stored is inversely proportional to the distance between the plates, thus increasing the plate separation will decrease the capacitance and consequently decrease the energy stored by the plates.

Hey, i totally disagree with you. In this case the capacitor is not connected to a battery,is it? No. then how can the energy decrease when we move the plates of a capacitor?where will the work done by you to move the plates go?Let me tell u. if u are thinking that the energy stored is 1/2CV^2 and just because C is decreasing with increase in distance, energy will also decrease, then you are wrong. Just as C is decreasing, V(potential) is increasing when the plates of a capacitor are moved apart. V=Q/C. So, as Q remains same and capacitance C decreases, therefore V will increase. So, total energy stored by capacitor will increase. This energy will be stored in the volume Ad, where A=area of plate and d is the increase in the distance between the plates.
But if battery is connected to the capacitor, then its a different story.If you want i can explain that too. But nowhere does it mention that the battery is connected.

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Hi guys,

First of all thanks for the replies.

For question 1, I figured another way to approach the question. The work done moving the mass can be calculated using C.O.E. The vertical height moved is 2.5m by geometry and so (9.81)(30)(2.5) + (0.5)(30)(1^2) + 600 to give 1365J. Am I right here?

For question 3, yes Hootenanny, the question is phrased in the way I wrote except for the spelling mistakes on my part. I would to say that I got mixed up there is only one option allowed, a, b, c, d, or e. I don't get it myself, I thought phase difference is not necessary as 2 same waves would intefere whenever they meet irregardless of their phase. Right? Phase diff is needed only when observable interference in needed e.g bright fringes at a particular place.

For 4, I've no clue yet still my teacher isn't free but thats the way the question is phrased.

For 5, I also understand that nucleons in the nucleus gain PE due to electrostatic forces of the protons but there is a net decrease in PE as the nucleons have nuclear forces between them. Binding engergy per nucleon is the energy required to seperate them from the nuclues per nucleon. So I would say that its because the radius of a nucleus is directly proportional to the no. of nucleons?
Many thanks!

Hootenanny
Staff Emeritus
Gold Member
app said:
Hey, i totally disagree with you. ...
An elementary error, my apologies.

daniel_i_l
Gold Member
1) I'd be a lot easier to do this with energy!
2) if you need an interferance pattern than they also need to have the same amplidtude.

Thanks. But I suppose the question only wanted any interference?

And I suppose 1365J is correct?