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# Can someone explain the answers to these two multiple choice questions? Tweet

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1. Can someone explain the answers to these two multiple choice questions?

For this one, I can understand that point P would be where the particle is momentarily stationary so kinetic energy would be zero. But if immedietly reverses direction and moves back away from the nucleus, doesn't that suggest its being repelled? And if so, why wouldn't there be electric potential energy?
2. Re: Can someone explain the answers to these two multiple choice questions?
For number 10

Use the equation for a simple pendulum

Substitute
3. Re: Can someone explain the answers to these two multiple choice questions?
(Original post by X1tinkerbellx)
For number 10

Use the equation for a simple pendulum

Substitute
I don't get it, why should the situation be modelled as a simple pendulum?

it doesn't seem remotely similar
4. Re: Can someone explain the answers to these two multiple choice questions?
For the second one, energy is independent of direction, and is only ever positive. As long as the particle has non-zero energy at some point, it has non-zero total energy over the duration of the experiement.
5. Re: Can someone explain the answers to these two multiple choice questions?
1) equate F = GMm/(R^2) to F = m(w^2)R where w = 2pi/T

2)It does have electrical potential energy at P, which is why statement a) it true, so c) has to be false as it contradicts a)
Energy is a scalar quantity so it has to be positive and the particle is stationary at P so b) is also right

It says which one is incorrect btw
Last edited by Antimony; 10-06-2012 at 14:39.
6. Re: Can someone explain the answers to these two multiple choice questions?
(Original post by X1tinkerbellx)
For number 10

Use the equation for a simple pendulum

Substitute
?? Why are you using the pendulum equation???
7. Re: Can someone explain the answers to these two multiple choice questions?
(Original post by Red Richie)
I don't get it, why should the situation be modelled as a simple pendulum?

it doesn't seem remotely similar
you're finding the period of rotation for the planet
8. Re: Can someone explain the answers to these two multiple choice questions?
(Original post by Red Richie)

For this one, I can understand that point P would be where the particle is momentarily stationary so kinetic energy would be zero. But if immedietly reverses direction and moves back away from the nucleus, doesn't that suggest its being repelled? And if so, why wouldn't there be electric potential energy?
I'd recommend that you go with B for every question that you are un-sure about .
If your with AQA, the multiple choice paper is incredibly hard and really hard to revise for because they give you situations that you wouldn't have thought of in a Thousand years
9. Re: Can someone explain the answers to these two multiple choice questions?
(Original post by Antimony)
1) equate F = GMm/(R^2) to F = m(w^2)R where w = 2pi/T

2)It does have electrical potential energy at P, which is why statement a) it true, so c) has to be false as it contradicts a)
Energy is a scalar quantity so it has to be positive and the particle is stationary at P so b) is also right

It says which one is incorrect btw
Thanks, but what is the significance of the phrase in the question, ''(the planet) rotates so rapidly that loose material at the equator only just remains on the surface''? - why does this have to be the case in order to find the time period?
10. Re: Can someone explain the answers to these two multiple choice questions?
(Original post by X1tinkerbellx)
you're finding the period of rotation for the planet
yes, but the rotation of the planet is nothing like a swinging pendulum
11. Re: Can someone explain the answers to these two multiple choice questions?
(Original post by Red Richie)
yes, but the rotation of the planet is nothing like a swinging pendulum
The planet is behaving like a pendulum
12. Re: Can someone explain the answers to these two multiple choice questions?
(Original post by Red Richie)
Thanks, but what is the significance of the phrase in the question, ''(the planet) rotates so rapidly that loose material at the equator only just remains on the surface''? - why does this have to be the case in order to find the time period?
It means that the there is no resultant force on the material from the planet but that can be ignored in the equation
13. Re: Can someone explain the answers to these two multiple choice questions?
(Original post by X1tinkerbellx)
The planet is behaving like a pendulum
how?
14. Re: Can someone explain the answers to these two multiple choice questions?

My maths skills are generally sound... could be incorrect though.

(Original post by Red Richie)
why does this have to be the case in order to find the time period?
Because in that case, there is nearly no resultant force on the material, meaning that the centripetal force due to the circular motion is roughly equal to the gravitational force upon the material.
Last edited by VisualKiddy; 10-06-2012 at 17:19.
15. Re: Can someone explain the answers to these two multiple choice questions?
(Original post by VisualKiddy)

My maths skills are generally sound... could be incorrect though.

Because in that case, there is nearly no resultant force on the material, meaning that the centripetal force due to the circular motion is roughly equal to the gravitational force upon the material.
Thanks for that