# Thread: very random physics qn

1. ## very random physics qn

this kinda sucked damn big time cause my differentiation is atrocious.
and so i need help

newton's second law states that force on an object is given by the rate of change in its momentum correct?

so f=dp/dt

if i expand dp, i get dmv.

f=dmv/dt

then now, i got to break up dmv. i'll let dt go over to F part.

fdt=mdv+vdm

now here is the part that i screw up. wads m and wads v?

i'll throw in some numbers so u all can punch. i got weird values.

assuming a car is travelling initially at 10ms-1, with the mass of the car+fuel being 100kg.

at time 1s later, the car is travelling at 100ms-1, with the mass of car + fuel being 10kg.

what is the force acting on the car during this 1s?

if i take it as Fdt= change of momentum, then i punch in numbers...

i shld get 0 resultant force as the momentum m1v1=m2v2 and hence no force.

yet if u use mdv+vdm.....

100kg (100-10)+ 10ms-1 (10-100)
u get a weird value.

i suspect i punched in wrong numbers for V and M. or my DV and DM is wrong. anyone can advise?

2. ## Re: very random physics qn

Originally Posted by allenleonhart
this kinda sucked damn big time cause my differentiation is atrocious.
and so i need help

newton's second law states that force on an object is given by the rate of change in its momentum correct?

so f=dp/dt

if i expand dp, i get dmv.

f=dmv/dt

then now, i got to break up dmv. i'll let dt go over to F part.

fdt=mdv+vdm

now here is the part that i screw up. wads m and wads v?

i'll throw in some numbers so u all can punch. i got weird values.

assuming a car is travelling initially at 10ms-1, with the mass of the car+fuel being 100kg.

at time 1s later, the car is travelling at 100ms-1, with the mass of car + fuel being 10kg.

what is the force acting on the car during this 1s?

if i take it as Fdt= change of momentum, then i punch in numbers...

i shld get 0 resultant force as the momentum m1v1=m2v2 and hence no force.

yet if u use mdv+vdm.....

100kg (100-10)+ 10ms-1 (10-100)
u get a weird value.

i suspect i punched in wrong numbers for V and M. or my DV and DM is wrong. anyone can advise?
You cannot just bring dt over to f. Differentiation is not like fraction.

f= d(mv)/dt

In this case, u apply product rule.

f= m (dv/dt) + v (dm/dt)

1) By keeping mass of the car constant with varying velocities => m(dv/dt)

2) By keeping the velocity of the car constant with varying mass => v(dm/dt)

For 1), in 1 second, m(dv/dt) = (100)((100-10)/1) = 9000kgm/s^2

For 2), in 1 second, v(dm/dt) = (100)((10-100)/1) = -9000kgm/s^2

3. ## Re: very random physics qn

yeah..f = m.dv/dt + v.dm/dt

4. ## Re: very random physics qn

Originally Posted by chikubang
You cannot just bring dt over to f. Differentiation is not like fraction.

1) By keeping mass of the car constant with varying velocities => m(dv/dt)

2) By keeping the velocity of the car constant with varying mass => v(dm/dt)

For 1), in 1 second, m(dv/dt) = (100)((100-10)/1) = 9000kgm/s^2

For 2), in 1 second, v(dm/dt) = (100)((10-100)/1) = -9000kgm/s^2

For 1), in 1 second, m(dv/dt) = (100)((100-10)/1) = 9000kgm/s^2

For 2), in 1 second, v(dm/dt) = (100)((10-100)/1) = -9000kgm/s^2

this part is the one i need clarification. why is the constant mass 100kg rather than 10kg, or why is the constant velocity 100 rather than 10ms-1? this is the part i dun get it

and it shld be delta t rather than dt. dt is instantaneous. delta is for small changes, so u can still bring delta t over actually. i think its my fault here

5. ## Re: very random physics qn

Glad to see there are physics/maths geeks in the forum. Lol!

6. ## Re: very random physics qn

Originally Posted by koplady11
Glad to see there are physics/maths geeks in the forum. Lol!
yea. until u realsie this is damn basic

ARGH.

7. ## Re: very random physics qn

F = dmv/dt

F = m.dv/dt + v.dm/dt

You'll get the Force acting on the system at the particular "instant". So at the particular instant, there'll be respective values of m and v to use. (Say, if dm/dt = constant, then, at t=0.5s, the m will be 55kg at the instant where t = 0.5s)

If you want to find the average force, you'll need to integrate F over the interval of 1s, then divide by the interval 1s.

Originally Posted by allenleonhart

what is the force acting on the car during this 1s?

8. ## Re: very random physics qn

Originally Posted by Limsgp
F = dmv/dt

F = m.dv/dt + v.dm/dt

You'll get the Force acting on the system at the particular "instant". So at the particular instant, there'll be respective values of m and v to use. (Say, if dm/dt = constant, then, at t=0.5s, the m will be 55kg at the instant where t = 0.5s)

If you want to find the average force, you'll need to integrate over the interval of 1s, then divide by the interval 1s.
ok. let me digest a bit. i think this will work.

so lets say i wanna find at time t0.5. so the respective M value assuming that the rate of fuel burnt is constant, so it will be 55kg. then velocity also. so its 55ms-1.

55kg 90/1 + 55 (-90/1) = 0.

seems correct. thanks a lot

so it is the respective value of M and V at the instant 0.5s.

9. ## Re: very random physics qn

Originally Posted by allenleonhart
yea. until u realsie this is damn basic

ARGH.
Haha. Chill. If I switched on the part of my brain that was once soaked in A levels Physics, I'm sure I could have helped you. But for now, my mind is tuned to H1 Maths coz I'm teaching a kid H1 maths. Initially everything looked so blur but if when I stepped back and relaxed a lil, the stuff pretty easy. No wonder can go thru maths then.

That said, chill bro!! Sumtimes we just tend to overlook the simplest solutions to the worst problems.

10. ## Re: very random physics qn

Originally Posted by allenleonhart
ok. let me digest a bit. i think this will work.

so lets say i wanna find at time t0.5. so the respective M value assuming that the rate of fuel burnt is constant, so it will be 55kg. then velocity also. so its 55ms-1.

55kg 90/1 + 55 (-90/1) = 0.

seems correct. thanks a lot

so it is the respective value of M and V at the instant 0.5s.
By using that formula, u only find the instantaneous force at a particular time. tt's why ur answer is weird.

11. ## Re: very random physics qn

Originally Posted by chikubang
By using that formula, u only find the instantaneous force at a particular time. tt's why ur answer is weird.
hmm. ok. cause i'm trying to play with forces related to varying force and mass at the same time not in a lvl sylabus as far as i know. but i feel pissed if i dun understand the whole logic. its an obsession

12. ## Re: very random physics qn

I also make a mistake. the mass should be the average mass and the velocity should be the average velocity.

I also thought why u ask such physics qns that is not A lvl and u claimed that it's basic. LOL.

13. ## Re: very random physics qn

Originally Posted by chikubang
I also make a mistake. the mass should be the average mass and the velocity should be the average velocity.

I also thought why u ask such physics qns that is not A lvl and u claimed that it's basic. LOL.
as in Mdv the m is avg mass of start and final, Vdm the V is avg of initial and final? or is it at that moment in time?

cause i'm doing physics olympiad competition. needa prep up on this

well. for me the way i study is:
its either 100 marks or 0. either u know or u dunno.

everything is basic. if u cant derive anything, u dunno ur basics.

(i'm a sadist. i like to self pressure a lot)

14. ## Re: very random physics qn

Originally Posted by chikubang
I also thought why u ask such physics qns that is not A lvl and u claimed that it's basic. LOL.
he's liddat one. hi kp! study for your midyrs pls don't go so far T_T

15. ## Re: very random physics qn

Originally Posted by spitfire93
he's liddat one. hi kp! study for your midyrs pls don't go so far T_T
what the. why u suddenly pop up here! SHOO go back and mug your own stuff
thou shalt not disturb poor me who cant solve mdv and vdm.

16. ## Re: very random physics qn

Originally Posted by Limsgp
F = dmv/dt

F = m.dv/dt + v.dm/dt

You'll get the Force acting on the system at the particular "instant". So at the particular instant, there'll be respective values of m and v to use. (Say, if dm/dt = constant, then, at t=0.5s, the m will be 55kg at the instant where t = 0.5s)

If you want to find the average force, you'll need to integrate F over the interval of 1s, then divide by the interval 1s.
Like what Limsgp has said,

Integrate F over interval of 1s and divide by the interval of 1s to find average force.

What I did is to assume is that the force changes evenly, that's why I just took, average of the mass (100+10/2). But this may not be true, depending on the rate of change of F. Am I very confusing? LOL.

17. ## Re: very random physics qn

Originally Posted by chikubang
Like what Limsgp has said,

Integrate F over interval of 1s and divide by the interval of 1s to find average force.

What I did is to assume is that the force changes evenly, that's why I just took, average of the mass (100+10/2). But this may not be true, depending on the rate of change of F. Am I very confusing? LOL.
ah crap. i forgot, rate of change of f also.
and yes. for the numbers i gave u, the rate of change of F shld be 0. cause momentum of system isnt changing (while its going faster, its getting lighter), force is a constant 0.

i think i understand where its weird liao. mine i had the rate of change of velocity = rate of change of mass.with starting mass and final mass of equal magnitude with velocity. if i were to put rate of change of velocity higher i think there will be a resultant force.

18. ## Re: very random physics qn

I just realize I can't teach basic Physics anymore for tuition

19. ## Re: very random physics qn

Originally Posted by wildcat
I just realize I can't teach basic Physics anymore for tuition
huh? nono. to me its basic. something i gotta know. its relative!(aww einstein <3)

dun worry wildcat. i'm sure ur a pro i got the idea for this qn from ICL first year paper so dun worry too much
*zomg i realise i suck at my basics. sigh...*

20. ## Re: very random physics qn

That is like S paper stuff.. normal A level (H2?) shouldn't have it lah..

Originally Posted by wildcat
I just realize I can't teach basic Physics anymore for tuition

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