I need some physics help! (1 Viewer)

[~JiNnIE~]

hi
would you be able to provide a expert style answer for this very famous "car problem" question adapted from Larkin(1980);
A car travelling 25metres per second is brought to rest at a constant rate in 20seconds by applying the brake. How far did it move after the brake was applied?
how would an novicist answer differ from the way an expert would solve it?

 

[+Po1ntDeXt3r+]

i can think of 2 ways...

accel = a = dv/dt
so
velocity = v = at + b = ds/dt -- eqn(1)
then
displacement = s = 1/2 at^2 + bt + c

data
v(0) = 25
v(20) =0

sub into eqn (1)
then a = -1.25 b = 25
for "How far did it move after the brake was applied?" implies distance not displacement so let c= 0

at t=20
distance = 1/2 (-1.25) (20)^2 + 25 (20) = 250m

or..

.. id draw a graph... velocity vs time
.. since its constant... it would be a straight line (making a triangle) and the area underneath.. is 20m x 25m/s x 1/2 = 250m..

i wasnt sure which was 'expert' or which was novice.. but i assume if it were the expert it would be the second one ... cos its smarter..

 

[nit]

a "novicist" (whatever that might be) wouldn't know equations, though, Alvin. Depends on your concept of novicist. Is a novicist someone who cant think for themselves and blindly applies a few equations theyve recently learned, while the expert possesses the unique quality of unparalleled intuiton? Or is this novicist a brilliant empiricist - smart, and practical, while the expert has complete and utter mastery of equations of motion. I suppose there is the distinct possibility that there's a 3rd way of solving this problem. Btw, I haven't read the problem or the solutions as yet

 

[~JiNnIE~]

thankyou so much guys. ^.^

 

[nit]

why "guys" in the plural? My post was utter bs.

 

[acmilan]

why "guys" in the plural? My post was utter bs.

...

 

[xiao1985]

why "guys" in the plural? My post was utter bs.

AHAHAHAHHAH omg taht's gold...

nit, she's just trying to be friendly... otherwise, u'd look like... i dunno some one who doesn't contribute?! =p

 

[+Po1ntDeXt3r+]

why "guys" in the plural? My post was utter bs.

yeah she's sucking up to u before u may one day she needs a real answer .. or meets u in person

i was thinking
mabbe an expert would factor in the
braking systems - ABS? ,Disc? ,Drum?
wheel drive- 4WD, RWD ,FWD.
car aerodynamics,
wind resistance,
tyre traction,
the compentency of the driver and their ability to slow it down at a 'constant rate'..

or from a psych view of this..
the question of why the car needed to be slowed down in the first place? is this part of the greater human psyche or some other desire..

begs the question.. wats an expert?

my question is.. why were u trying to brake at 90km/hr to stop over 20 secs?