IIT JEE , AIEEE Forum - Physics , Mechanics

rakesh61

can anyone tell me the sign convention used

Case 1 when a body is dropped from tower height H wit velocity v , gravity g

Case 2 when body is thrown upward from tower height H wit velocity v , gravity g

Pleas ehelp me i promise to rate for this concept

catch_arnnie

see, the concept is simple, the only thing you hav to keep in mind is choosing your own sign convention & proceeding accordingly..

for example::if you take the downward direction as -ve, then g(acting downward) will also be -ve or displacement (wich is again downward) is also -ve in both cases you've given & so, you'll get the ans. accordingly. also, in 1st case, 'v' will be -ve coz' its acted downward but in 2nd case it will be +ve coz' its acted upwards....

i hope you get the concept....

shreyasnivas

arnnie is correct

LastMinuteGenius

see da, the 'case' does not matter for taking sign convention.

these r the steps to solve sums correctly using sign conventions:

1) fix the +ve and the -ve directions

2) any vector (for eg, velocity vector, acceleration due to gravity vector) should be given the appropriate sign, depending on whether it is pointing towards the +ve or -ve directions, as fixed by you in step 1.

3) use newton's laws of motion equations, using the sign convention obtained for each vector in step 2 and SOLVE.

NOW, IN THE QUESTION ASKED BY U, LETS TAKE CASE 2:

following the steps ive given above,

1) lets say up is -ve & down is +ve (u can take it the other way round also...ok)

2) so now, g is +ve & v is -ve (say that v & g are of magnitude 10)

3) so if you want to find the max height it rises to for example, then

v² = u² + 2gH

0 = (-10)² + 2(10)H

20H = -100

so, H = -5 metres

see, now, just bcoz u r gettin the height as -ve, the answer is NOT wrong:

the reason for this is, u have taken up as -ve & down as positive, keeping the ground as origin, so all the displacements above the ground has to be -ve by this convention.

HOPE I HAVE MADE MYSELF CLEAR and the concept on conventions is clear...

PLS << RATE >> MY EFFORT.......

LastMinuteGenius

this is the concept i have picked up... pls say if i am right (experts + iitians only)

alisha_gupta_27

sign convention that to used in the prob entirely depend on the person

in the first case both the body's velocity and the acceleration due to gravity act in downward direction which you can assume to be positive

while in the second case velocity and the acceleration acts in opposite direction till the body reaches its maximum height and then the body falls with g .Now if suppose we need to find out the height from which the body is thrown upward with velocity v and reaches the ground at time t then we can simply use the formula

h = -vt + gt^2

alisha_gupta_27

RATE ME IF MY EXPLANATION IS ACCEPTABLE

pirate1_from_jee

Look yaar,

At any pt(preferably higher) abve srfce of earth, U frm a co-ordinate axis as pr convntion.Nw imagne a ball drops frm a ht. H.

In ds cse unit vctr of g wl be -j^.

Nw intgrate it --- u wl gt sme unt vctr 4 velcity & displacement since the unit vctrs along the 3 axes in space wl nt change so differentiation or integration vl nt affct it.

Nw apply the sme logic in 2nd case.

kghedriu

Well....it all depends on ur coordinate axis...u can choose it any waay and in any direction.....

it need not be dat the positive direction is up or so on..

but watever b the direction u choose, take care u assign the other quantities their directions acc to coordinate axes...dats all

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