Hard question- Mechanics- ENGAA

Hi guys im unsure how to start to tackle question 4 part d. I will attach my working for the rest of the question. The correct answer is D.

https://www.undergraduate.study.cam.ac.uk/files/publications/engaa_s2_question_paper_2016.pdf

Thanks in advance

(edited 6 years ago)

Reply 1

RDKGames

Study Forum Helper

Original post by Shaanv

Hi guys im unsure how to start to tackle question 4 part d. I will attach my working for the rest of the question. The correct answer is D.

Thanks in advance

What's the question?

Reply 2

Shaanv

Original post by RDKGames

What's the question?

Would help😂

https://www.undergraduate.study.cam.ac.uk/files/publications/engaa_s2_question_paper_2016.pdf

Reply 3

RDKGames

Study Forum Helper

Original post by Shaanv

Would help😂

https://www.undergraduate.study.cam.ac.uk/files/publications/engaa_s2_question_paper_2016.pdf

Treat the cyclist as a projectile being thrown down a slope at an angle

\theta

the slope with speed

V

. Then you want to find the point between B and C when the distance between them and the slope is the greatest.

(edited 6 years ago)

Reply 4

haarithiop

Original post by Shaanv

Thanks in advance

For C) why don't you consider the cyclist accelerating parallel to the slope, if you do you get the answer E which is wrong.

(edited 6 years ago)

Reply 5

Shaanv

Original post by RDKGames

Treat the cyclist as a projectile being thrown down a slope at an angle

\theta

the slope with speed

V

. Then you want to find the point between B and C when the distance between them and the slope is the greatest.

Sorry i an still lost.

Im unsure how to find an expression for the distance of the cyclist above the slope. Having this i could differentiate to find a maximum.

I dont know if this is what im meant to do. Could u give me a further hint

Reply 6

Shaanv

Original post by haarithiop

For C) why don't you consider the cyclist accelerating parallel to the slope, if you do you get the answer E which is wrong.

The way i did it seemed the easiest, so i went with it cus it doesnt accelerate horizontally.

Reply 7

haarithiop

Original post by Shaanv

The way i did it seemed the easiest, so i went with it cus it doesnt accelerate horizontally.

Isn't it accelerating parallel to the slope as it has a component of weight parallel to the slope.

Reply 8

RDKGames

Study Forum Helper

Original post by Shaanv

First of all, you need to work parallel and perpendicular to the slope (atleast this is what I'd find easier). Then write down your SUVAT for parallel and then the perpendicular motion, but before then, keep in mind that

g

affects both parallel and perpendicular motion so you need to resolve the gravitational field force wrt to the slope to get your acceleration parallel and perpendicular to the slope.

Then, you can differentiate your expression for perpendicular displacement and find the TIME when the max point occurs, and sub this time into your expression for the parallel displacement. Form there, you should be good.

Reply 9

Shaanv

Original post by haarithiop

Isn't it accelerating parallel to the slope as it has a component of weight parallel to the slope.

I will try it this way and let u know what i get

Reply 10

Shaanv

Original post by haarithiop

For C) why don't you consider the cyclist accelerating parallel to the slope, if you do you get the answer E which is wrong.

I tried it parallel and i almost got E as my answer. However it was subtly different from E, so i use some trig and managed to reduce it to C.

F0616E91-C063-41F2-B5B1-897EE6355B06.jpg.jpeg

F0616E91-C063-41F2-B5B1-897EE6355B06.jpg.jpeg

Reply 11

haarithiop

Original post by Shaanv

I tried it parallel and i almost got E as my answer. However it was subtly different from E, so i use some trig and managed to reduce it to C.

F0616E91-C063-41F2-B5B1-897EE6355B06.jpg.jpeg

Ah my method was right I just made an algebra mistake when simplifying. I get C now. How did you get the same answer without considering the acceleration parallel to the plane?

Reply 12

Shaanv

Original post by haarithiop

Ah my method was right I just made an algebra mistake when simplifying. I get C now. How did you get the same answer without considering the acceleration parallel to the plane?

Instead of resolving parallel and perpendicular to the slope, i resolved horizontally and vertically.

As there is no horizontal acceleration the problem is much simpler, but resolving perpendicular and parallel helps in the later parts of the question.

Reply 13

Anas Sheikh

Guys can anyone help with question 2c of this paper? What I did was so long that I just gave up midway... it’s crazy hard

Reply 14

old_engineer

Original post by Anas Sheikh

Guys can anyone help with question 2c of this paper? What I did was so long that I just gave up midway... it’s crazy hard

Suggested approach:

1) Redraw the circuit diagram with R1, R1 and R3.

2) Let V2 denote the voltage across the parallel combination of R1 and R3. Obtain an expression for V2 in terms of V, R1 and R3, and simplify as far as possible.

3) P3 = ((V2)^2) / R3.