IIT JEE , AIEEE Forum - Physics , Mechanics

chanalicious

am i the only one that hates physics? ok anyway.

please tell me EVERYTHING [basic] you know about projectile motion...

Specific question: rock is thrown from the top of a castle wiht initial velocity of 12 m/s [42 degrees above the horizontal.] rock lands on the other side of the river, 9.5m below the initial level. find time of flight, width of river and velocity at impact (resultant velocity)

i know its hard to picture. dont tell me the answer..just prompt me...

ramyani

You wrote " dont tell me the answer..just prompt me..."
OK
There r 2 components of velocity----x component ( or horizontal component ) and y component ( or vertical component ). u sin

and u cos

.

is given 42 degree.

    Now rock lands 9.5m below the initial level. So u know initial y co-ordinate--9.5. The final y co-ordinate is zero on the ground.
                Use y = ( u sin

) t + 1 / 2 ( a) t² { what is accln here ?? }
So u can find t.
In this time the rock travels horizontally with horizontal component of velocity. And remember in projectile , horizontal component of velocity remains same.
So u can find the width of river ( horizontal distance )
the next is velocity at impact. try.

p.s. why hate physics ?

chanalicious

acceleration is 9.8 m/s2
yes yes i get it now. i was confused because you cant use t= 2v(sin0) / g when the level the ball lands at is different then the original.

for hte x component, why is the only equation you can use d=vt? because v is constant?

why are the x and y components completely independent?

if a rock is dropped in projectile motion and another rock is dropped vertically, howcome they both land at the same time?

p.s. i hate physics because it seems like theres always a big cloud between me and physics concepts. any suggestions?

also, RAMYANI, i see that you are only in 11th...you have already learnt this?

ramyani

in our assam board, this topic is included in the 1st year syllabus.and the first unit test that we gave in this year included this topic.

coolank2

Along x-axis, there is no accleration ....... hence, const velocity .......

Same thing is applicable for the rock which is dropped in projectile motion and the other one which is dropped vertically, they land at same time, as only g is acting downwards on both of them .....

chanalicious

but the ball in projectile motion seems to travel a greater distance...?

also, does acceleration have a direction and if there is how do you graph it on a acceleration time graph?

my textbook says that when something is flying AIR RESISTANCE becomes so high that there is NO more acceleration and the object is travelling at CONSTANT velocity. but...when something is falling, isnt the acceleration= 9.8m/s2???

sinjan.j

http://www.goiit.com/posts/list/mechanics-projectile-motion-greatest-26205.htm

plz have a look actual projectile motion explanation starts from page 2

cheers...............!!!!

sinjan.j

i hope that u have got the answer

i want u to check my answer and tell me if it is corect

height=usin

x t + (gt² / 2)

9.5 = 8.03t + 4.9 t²so u get a quadratic equation and get t's value as 0.8 sec.

width of the river is u cos

x t = 7.13 m.

PLZ rate me if i am correct.

cheers........!!!!

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