Mechanics paper 5 discussion

[SarahKotb]

hi,
i think we can discuss the paper now
guyz can someone remind me of the first question
i remember the answer to the first part but i totally forgot the question

 

[Lifeless]

My paper was horrible ! I just messed up with everything. Btw, how did u do the elasticity question ?

 

[stewing]

I think it was circular motion, did variant 53 though. There were 2 elasticity questions in this paper, I think.

 

[SarahKotb]

i did 52 but i think it is similar to the 51
if u r asking about the elasticity question where it says show
that the particle moves 0.6m in constant acceleration
u had to use v^2=u^2+2as

 

[cocospaniel]

Most horrible exam ever!! It was nothing like the other past papers.

 

[Lifeless]

i did 52 but i think it is similar to the 51
if u r asking about the elasticity question where it says show
that the particle moves 0.6m in constant acceleration
u had to use v^2=u^2+2as

But how did you do the first part? What was the answer ?

 

[SarahKotb]

^^^
i used the principle of conservation of enrgy
my answer was 3.61

 

[MasterOfPuppets]

i did 52 but i think it is similar to the 51
if u r asking about the elasticity question where it says show
that the particle moves 0.6m in constant acceleration
u had to use v^2=u^2+2as

But how did you do the first part? What was the answer ?

First part, it asks for the speed when the acceleration is zero. So the tension in the string must be equal to the weight, to give a resultant force of zero and hence no acceleration.
So you equate the tension with the weight, the weight being 2.4 N (or 0.24g) and the tension being *lambda*x / l, where lambda is the modulus of elasticity, which was 12 N, and l is the natural length, 0.5 m. You equate them and get x, the extension. It is 0.1 m.
Now the motion starts when the extension is 0.3 m and the speed is 3m/s. So you do a normal energy question following that including elastic potential energy, kinetic energy, and gain in gravitational potential energy.

Then for part ii), the acceleration is constant, so the only force is weight, which means the extension of the string is zero. You start another energy procedure to get the speed of the particle when the extension is zero. That's your U. It will continue moving upwards until its speed is zero, so the V is 0. The acceleration is -10. So you use v^2 = u^2 + 2as. After some solving, s should be 0.6.

 

[DDGAL]

can ne1 tell me the total marks for question number 3, 4 , 5 and 6! btw i gave paper 52.

 

[SarahKotb]

well said masterofpuppets
did u get the differential eqn in q5
what about the circular motion question what did u get for part1 and 2
i got 0.518 for part 1
i got 13. something for part 2
for question 2 what did u find h
i got 0.298 or something like that

 

[Lifeless]

i did 52 but i think it is similar to the 51
if u r asking about the elasticity question where it says show
that the particle moves 0.6m in constant acceleration
u had to use v^2=u^2+2as

But how did you do the first part? What was the answer ?

First part, it asks for the speed when the acceleration is zero. So the tension in the string must be equal to the weight, to give a resultant force of zero and hence no acceleration.
So you equate the tension with the weight, the weight being 2.4 N (or 0.24g) and the tension being *lambda*x / l, where lambda is the modulus of elasticity, which was 12 N, and l is the natural length, 0.5 m. You equate them and get x, the extension. It is 0.1 m.
Now the motion starts when the extension is 0.3 m and the speed is 3m/s. So you do a normal energy question following that including elastic potential energy, kinetic energy, and gain in gravitational potential energy.

Then for part ii), the acceleration is constant, so the only force is weight, which means the extension of the string is zero. You start another energy procedure to get the speed of the particle when the extension is zero. That's your U. It will continue moving upwards until its speed is zero, so the V is 0. The acceleration is -10. So you use v^2 = u^2 + 2as. After some solving, s should be 0.6.

Thanks alottt...Could u do all the sums ?? How much are u expecting??

 

[MasterOfPuppets]

well said masterofpuppets
did u get the differential eqn in q5
what about the circular motion question what did u get for part1 and 2
i got 0.518 for part 1
i got 13. something for part 2
for question 2 what did u find h
i got 0.298 or something like that

Yes I got the differential equation after many attempts.
Now the circular motion, I'm pretty sure my answers are the same as yours. And same for the h as well, I think that was 0.2828, so yeah.

 

[SarahKotb]

^^
r u serious u got the same answers or r u just joking

 

[MasterOfPuppets]

Very serious btw

 

[SarahKotb]

i thought i was just making thing up from my mind when answering this question
so thanx
What about the other questions what did u find hard and easy

 

[abuhantash]

For the h, remember how for the CM we had to add the CM distance of the semi circle AND the h ... I didnt do that, but I did the mass and everything else right. AND in (ii) I used the h value I found in (i) which is obviously wrong. How many marks will I lose? =[ I dont have marks to spaare =[ =[ =[

 

[Lifeless]

How much are u guys expecting??