Name: Cambridge Engineering Interview
Uploaded: 2018-03-08T15:33:04.000Z
Duration: 13 min 31 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

Hi, my name is Madeleine and I've just finished my second year of Engineering at Jesus College,

Cambridge. I applied in 2012 and matriculated in 2013. I'm going to go through 2 interview

questions which are taken from the website I want to study Engineering.org which is a

website which has hundreds of engineering interview type questions with worked through

answers and occasionally videos too. The two questions that I'm going to do are similar

to the kind of questions that I got during my interviews. So hopefully this will help.

So the first question reads as follows. On a clear day, you are on an airplane which

is 38,000 ft above the middle of Pacific Ocean. Taking the radius of the Earth to be 6,400km,

what is the approximate distance between you & the horizon of the Earth? You are also given

that 1 foot is equivalent to 0.3048 meters.

So the first thing that I would do is convert the 30000 feet into meters using the given

conversion. So 38 000 ft times by 0.3084 meters is roughly, using a calculator, 11.6km So

now that you have all of the figures in meters or km, you can then draw a diagram of the

earth with the center here and you can say that the airplane is hereish. So now marking

the distances on, you know that this, the radius of the earth, is 6400km. I'm just

going to say all the distances are in Km. Then the distance from the plane to the Pacific

Ocean is, as we calculated here, 11.6kms. So now we need to think about where the horizon

is. So the horizon is the line of sight from where you are in the airplane to the first

point you can see. By definition or by intuition, you can say that point has to be at right

angles with the radius of the earth because if you are looking at this way and say that

this is your line of sight and this is with the horizon and it's essentially where your

line is horizontal with the circumference of the earth. So if we then draw the radius

of the earth on to this point you know this has to be a right angle and, so from then

on, it's essentially a Pythagoras question. So again this is the radius of the earth and

you are trying to find the approximate distance between you and the horizon. So if we call

this x (that's the distance here). So then if we redraw the triangle, we have x here,

6400km here and, in total, 6411.6 here and so by Pythagoras which in full of course is

this and by rearranging this to get x; as the result, you find the x, the approximate

distance between you and the horizon of the earth is roughly 386km.

The second question which is I'm going to go through goes as follows. A rocket of mass

m is to be launched from the surface of a rogue planet with mass M and radius R and

no atmosphere. By making reasonable assumptions about the distance between the planet and

any nearby galaxies, find the escape velocity required for the rocket to overcome the gravitational

You might be wondering what the reasonable assumptions mentioned in the questions might

mean. And you just really need to think about what the effect of other planets close by

to this planet might be. So if there are planets close to the rogue planet, it's likely that

their gravitational field that have an effect on the motion of the rocket. Therefore, the

assumption that you need to make is the distance between this planet and any nearby planets

is very, very large and, therefore, only the gravitational field of the rogue planet is

important in this question. So for this question, I'll go straight into drawing a diagram

as it might make it clearer as to what you need to do to solve this problem. Say this

is the planet and we can mark on here that this is the radius big R. Now if we draw the

rocket to be here at any moment in time, we can label the distance from the rocket to

the center of the earth as little r. So this is just something that we can define. Say

in another given time, the rocket has now moved. So it's got a little further and

we are going to say that the distance between this instance and this instance is delta r

(just to symbolize a little distance). So we know that as the rocket is moving, there

must be a force due to the gravitational field of this planet acting on the rocket and this

force is going to be in this direction which we can call big F and we know in this instance

it will also be acting obviously with a different value which is given by the formula F equals

big G and then the mass of the planet which is capital M, the mass of the body upon which

the force is acting which is the little m over the distance between the two bodies which

we have to find as r2. Now with kinetic energy questions, you often immediately think of

energy balancing equations. So this might help in this problem. One energy balance equation

that we know is that the work done is equal to force times distance. Although we don't

quite know how we are going to get to kinetic energy through this, it might be worth a try.

We know that the force on the rocket is going to be given by the equation that we just wrote

down and if you don't remember this equation in the interview or you haven't seen it

before just as an example in an interview if you can't think of the equation or you

really don't know it; if you just state it, say “I'm really sorry I don't think

I can't quite really remember the formula of the equation” They will usually give

it to you as it will help you solve the question and they just don't want you to stop in

your tracks so they will help you if you forget things that might be useful. So yes force

times this force times the little distance. So we are working out the work that's done

for the rocket moving from here to here. So we will do times delta r. Now this here is

just an equation for the little amount of work done moving the rocket from this position

to this position which is just an arbitrary small distance. So to get the total amount

of work that will be needed to get the rocket from the surface of the earth as specified

in the question all the way to outside the gravitational field of the planet we will

need to sum all these little works done from bigger to infinity which is where the gravitational

field of the planet will end. Now if this isn't an intuitive step and you don't

get it in the interview, they may again the interviewers may again help you so that you

might be able to proceed further with the question. So don't panic.

Therefore the total work done which I'm going to write as w will be equal to the integral

between big R (so the radius of the planet) and infinity of this. And now you can see

it's just an integral but we need to calculate to get the work done. So to do the integral,

you can rewrite the over r2 as r to the power of -2 which makes it easier and then you can

see that by adding 1, dividing by the new power you get minus … r to -1 which you

can put on the bottom again between r to infinity. Now this when you put the limits in, the first

limit you put in is infinity obviously dividing by infinity is going to give you zero and

then the next step is you are doing is minus and then inserting large R; so you end up

with a minus minus big G big M little m over big R. Now this is just the total work done.

So I've just moved the result of this integral up here to save space for the next bit of

the question. As I have said in the beginning, when you think of kinetic energy questions

you may think of work done and energy balance equations. So now we have the total work needed

to get the rocket from here to outside the gravitational field. Through energy balance,

you know that the work done that is needed to do this must be equal to the initial kinetic

energy that the rocket has when it's at the surface of the planet. Therefore, we can

write that the result of our integral must be equal to half mv squared. From now on,

it's just rearranging to find the v which is the escape velocity, as specified in the

question. So mv2 is 2GFm over big R; therefore, in the end you get v as equal to 2GbigM/R

as the two ms cancel and the whole thing square rooted. And this is the formula for the escape

velocity. So these were the two interview questions. I hope you found them useful. If

you want to see anymore, go to the website that I mentioned earlier, I want to study

Engineering.org. But if I were to give any tips for the interview, I would say try not

to panic I know it's really hard and obviously you are going to feel stressed. But if you

forget anything in a spark of moment, if you misremember an equation or if you literally

can't see where this question is going, don't be afraid to admit that. The interviewers

are there to help and I'm sure teacher or whoever might have been telling you that already.

Subtitles ListPlay Video

Cambridge Engineering Interview

essentially

quote

panic

arbitrary