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# Cambridge ENGAA physics questions watch

1. Hi guys i posted my maths related questions in the maths forum, these are my physics ones. If anyone could help me out it would be much appreciated.

Question 14:

Worked out tension in rope as 54N. Unsure as to why the answer is double.

Question 52:

So i equated the change in momentum of the spacecraft and the momentum of the fuel. By rearranging and solving i got the mass of fuel as 4000/19 kg, which i approximated as 200kg, which is E.

However i also put 4000/19 into my calculator and the answer was 210kg which was F. I am not meant to be using calculators for this section and so i was wondering if i have gone wrong somewhere.

if you ignore the acceleration and look at it as a static problem first, you can see that there's 50N tension in the left rope and 50N tension in the right rope... so the coupling has to take 100N before you even start increasing the tension by accelerating the mass.

dunno if you're misreading the question or just not familiar with thinking about pulley systems tbh.
3. (Original post by Joinedup)

if you ignore the acceleration and look at it as a static problem first, you can see that there's 50N tension in the left rope and 50N tension in the right rope... so the coupling has to take 100N before you even start increasing the tension by accelerating the mass.

dunno if you're misreading the question or just not familiar with thinking about pulley systems tbh.
Im just struggling to picture the direction of the tension in each branch.

Does this diagram show the tension forces correctly? I dont know if im making it more complicated than it is.

UPDATE: DONT WORRY IM A SPECIAL KIND OF SPECIAL I KNOW WHAT U ARE ON ABOUT😅
4. (Original post by Shaanv)
Hi guys i posted my maths related questions in the maths forum, these are my physics ones. If anyone could help me out it would be much appreciated.

Question 14:

Worked out tension in rope as 54N. Unsure as to why the answer is double.

Question 52:

So i equated the change in momentum of the spacecraft and the momentum of the fuel. By rearranging and solving i got the mass of fuel as 4000/19 kg, which i approximated as 200kg, which is E.

However i also put 4000/19 into my calculator and the answer was 210kg which was F. I am not meant to be using calculators for this section and so i was wondering if i have gone wrong somewhere.

Assume that the spacecraft is moving to the right relative to the earth, so the fuel is ejected to the left relative to the spacecraft.

As the fuel is ejected to the left at 1425 m/s relative to the spacecraft, the fuel is moving at (7425 – 1425 =) 6000 m/s to the right relative to the earth.

mi is the mass of spacecraft

Δm is the mass of fuel ejected

vi is the initial velocity of the spacecraft relative to earth (+7425 m/s)

vf is the final velocity of the spacecraft relative to earth (+7500 m/s)

uf is the final velocity of the spacecraft relative to earth (+6000 m/s)

Initial linear momentum = mi vi

Final linear momentum = (mi – Δm)vf + Δm uf

mi vi = (mi – Δm)vf + Δm uf

Δm vf – Δm uf = mivfmi vi

Δm = 200 kg

You can also use the relative change in the speed of the spacecraft and the relative speed of fuel ejected to solve the problem in an elegant way.
5. (Original post by Eimmanuel)

Assume that the spacecraft is moving to the right relative to the earth, so the fuel is ejected to the left relative to the spacecraft.

As the fuel is ejected to the left at 1425 m/s relative to the spacecraft, the fuel is moving at (7425 – 1425 =) 6000 m/s to the right relative to the earth.

mi is the mass of spacecraft

Δm is the mass of fuel ejected

vi is the initial velocity of the spacecraft relative to earth (+7425 m/s)

vf is the final velocity of the spacecraft relative to earth (+7500 m/s)

uf is the final velocity of the spacecraft relative to earth (+6000 m/s)

Initial linear momentum = mi vi

Final linear momentum = (mi – Δm)vf + Δm uf

mi vi = (mi – Δm)vf + Δm uf

Δm vf – Δm uf = mivfmi vi

Δm = 200 kg

You can also use the relative change in the speed of the spacecraft and the relative speed of fuel ejected to solve the problem in an elegant way.
I did the change in momentum of the rocket equals the change in momentum of the fuel and solve for the mass of the fuel, which gave me the correct answer.
6. (Original post by Shaanv)
I did the change in momentum of the rocket equals the change in momentum of the fuel and solve for the mass of the fuel, which gave me the correct answer.
how do you do question 51 on the paper?
7. (Original post by bruh2132)
how do you do question 51 on the paper?
Integrate with respect to y to get the area enclosed by the curve and y axis.

Remember to make x=0 to find ur limits for the integration.
8. (Original post by Shaanv)
Integrate with respect to y to get the area enclosed by the curve and y axis.

Remember to make x=0 to find ur limits for the integration.
Thanks a lot- I think I got it now
do you think you could also help with 54?
9. (Original post by bruh2132)
Thanks a lot- I think I got it now
do you think you could also help with 54?
10. (Original post by Shaanv)
thanks, do you also know how to do the last question on the section 2 ENGAA 2016

11. Sorry i dont have much time to explain
12. (Original post by Shaanv)

Sorry i dont have much time to explain
Haha I did that paper two but couldn't finish the whole thing within the time limit so got about 73% but I continued doing it and ended up getting all of them correct according to the answer key- yea we got the same sort of working for the last Q
13. (Original post by Anonymouspsych)
Haha I did that paper two but couldn't finish the whole thing within the time limit so got about 73% but I continued doing it and ended up getting all of them correct according to the answer key- yea we got the same sort of working for the last Q
For the section two i got 15/24 in timed conditions, completely ballsed up section 1 tho, ran out of time.

So tomorrow im gonna goas fast as Usain
14. (Original post by Shaanv)
For the section two i got 15/24 in timed conditions, completely ballsed up section 1 tho, ran out of time.

So tomorrow im gonna goas fast as Usain
I redone both the specimen paper and last years first paper and somehow got over 90% on both I had to go through them very quickly

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