M4 (1st June 2012, Fri AM)

OP

Can someone help me with June 2009 Q5 c)
In the mark scheme they just dot the vector velocity and the line of centers dived by sqrt(2), where do they get the 1/sqrt(2) bit from?

It's the unit vector in the direction of i - j

Reply 41

Antimony

If you have an equation :" x = e^(-kt)( Acos(kt) + Bsin(kt) )" in damped harmonic motion (x is displacement). It can be turned into the form x = Rcos(kt + CONSTANT), how do you find the period of the motion from this form ?

Also do you need to know the graphs of damped harmonic motion?

Reply 42

Rahul.S

14

had a nap as I had an exam in the morning....now I need to do some m4 :colone:

Reply 43

XiaoXiao1

Original post by Antimony

If you have an equation :" x = e^(-kt)( Acos(kt) + Bsin(kt) )" in damped harmonic motion (x is displacement). It can be turned into the form x = Rcos(kt + CONSTANT), how do you find the period of the motion from this form ?

Also do you need to know the graphs of damped harmonic motion?

k is the angular speed, so T = 2pi/k

Not seen many questions on damped harmonic motion graphs, but it probably can't hurt to know it.

Reply 44

Tamagotchi

1

Could someone explain to me why on Q6)c) from the June 2008 paper, to find the constants for the particular solution, why have they said at t0=0, v=5?
To obtain v, you differentiate the expression for x which measures the displacement so shouldn't v be the rate at which the displacement is changing and not the velocity of the boat since its travelling at a diagonal?

Reply 45

e.corfield

sorry if its a bit late - ive just done this question! if you rearrange the dot product
a.b=|a||b|cosӨ to
a.b/|b| = |a|cosӨ
and let b=i+j (the vector parallel to the line of centres), then |a|cosӨ is the component of the velocity parallel to i+j, allowing you to work out the coefficient of restitution :smile:

you need to divide by root2 because this is the modulus of b

hope this makes sense!

Reply 46

Hudzy

8

Original post by Tamagotchi

Could someone explain to me why on Q6)c) from the June 2008 paper, to find the constants for the particular solution, why have they said at t0=0, v=5?
To obtain v, you differentiate the expression for x which measures the displacement so shouldn't v be the rate at which the displacement is changing and not the velocity of the boat since its travelling at a diagonal?

At t=0 the boat is travelling straight towards to opposite bank as the speed of the river , r=0 (as x=0).

Reply 47

Tamagotchi

1

Original post by Hudzy

At t=0 the boat is travelling straight towards to opposite bank as the speed of the river , r=0 (as x=0).

Oh yeah xD Thanks!

Reply 48

scherzi

4

2011 question 7 - have they made a big error in calculating EPE or am I missing something? They've taken x to be BR, not BR-2a...

Reply 49

Rahul.S

14

Original post by scherzi

2011 question 7 - have they made a big error in calculating EPE or am I missing something? They've taken x to be BR, not BR-2a...

we just want the extension....br is the extension....the other 2a is the horizontal thing- natural length. :biggrin:

Reply 50

scherzi

4

Original post by Rahul.S

we just want the extension....br is the extension....the other 2a is the horizontal thing- natural length. :biggrin:

Ohh the string is attached to A, not R... didn't read the question properly. Thanks

Now just 11 past papers to go... :colone:

Reply 51

Rahul.S

14

Original post by scherzi

Ohh the string is attached to A, not R... didn't read the question properly. Thanks

Now just 11 past papers to go... :colone:

I think im almost done....just going over some stuff....going to check the M2 ms then sleep :biggrin:

GL EVERYONE

Reply 52

Zhy

OP

17

I hope it's a really easy paper. Similar to June 2011 or even better 2008!

Reply 53

scherzi

4

M2 was way harder this time than previous years, I've got a feeling it'll be the same for M4... the last few years haven't really had anything out of left field

Good luck anyway

Reply 54

Tomcrease

Now this get's unlocked just after I make a post!

Well, the worst, weirdest M4 paper I have ever seen. Anyone else?

Reply 55

scherzi

4

sums that up

Never ****ed up an exam so badly - was underprepared, but still some completely unexpected things. Most annoying thing was the need for good M3, which I'd neglected since it's at a later date - past papers/exercises never needed much more than knowing equations for tension/EPE. What's the point in putting M4 before M3 if you're going to do this ffs

Reply 56

Tomcrease

Original post by scherzi

sums that up

Never ****ed up an exam so badly - was underprepared, but still some completely unexpected things. Most annoying thing was the need for good M3, which I'd neglected since it's at a later date - past papers/exercises never needed much more than knowing equations for tension/EPE. What's the point in putting M4 before M3 if you're going to do this ffs

Exactly my response to that paper. 11 people sat it at my college, every single one came out with that "What the f**k" expression.

Low grade boundaries I expect..