Author Topic: All Mechanics DOUBTS HERE!!!  (Read 94966 times)

Offline astarmathsandphysics

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Re: All Mechanics DOUBTS HERE!!!
« Reply #645 on: March 02, 2012, 10:21:24 am »
Didn't read the question properly.
I did for i+j at bottom of page.

Offline WARRIOR

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Re: All Mechanics DOUBTS HERE!!!
« Reply #646 on: March 02, 2012, 10:38:51 am »
Didn't read the question properly.
I did for i+j at bottom of page.

Your answers are right sir ! but how is 12i=3i-6j + c => c= -3i+6j ?????
isnt is supposed to be 9i+6j?
NO secrets to SUCCESS , it is the result of 1.HARD WORK 2.GOOD PREPARATION 3.LEARNING FROM FAILURE
But it ain't how hard you hit; it's about how hard you can get hit, and keep moving forward-Balboa

Offline astarmathsandphysics

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Re: All Mechanics DOUBTS HERE!!!
« Reply #647 on: March 02, 2012, 10:48:47 am »
Now I get 9i+18j after checking

Offline WARRIOR

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Re: All Mechanics DOUBTS HERE!!!
« Reply #648 on: March 02, 2012, 10:55:03 am »
Now I get 9i+18j after checking

I got the same at first , but i checked the answer at the back it was exactly like this

( 3t(sq) + 9) i  + ( 4t(cubed) -10t + 6 ) j

Very weird book !!!!

anyway thanks a million for your answers and explanations !  + rep!
NO secrets to SUCCESS , it is the result of 1.HARD WORK 2.GOOD PREPARATION 3.LEARNING FROM FAILURE
But it ain't how hard you hit; it's about how hard you can get hit, and keep moving forward-Balboa

Offline astarmathsandphysics

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Re: All Mechanics DOUBTS HERE!!!
« Reply #649 on: March 02, 2012, 10:59:49 am »
BOOK is correct!!!
When I did it the first time I ignored the 12i
and the second time I got 12i+6j=18j

Offline WARRIOR

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Re: All Mechanics DOUBTS HERE!!!
« Reply #650 on: March 02, 2012, 11:26:05 am »
BOOK is correct!!!
When I did it the first time I ignored the 12i
and the second time I got 12i+6j=18j

HOW :o im so slow , i still dont understand how 12i= 3i + ( 4 -10 ) j could be 12i+6j=18j
NO secrets to SUCCESS , it is the result of 1.HARD WORK 2.GOOD PREPARATION 3.LEARNING FROM FAILURE
But it ain't how hard you hit; it's about how hard you can get hit, and keep moving forward-Balboa

Offline astarmathsandphysics

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Re: All Mechanics DOUBTS HERE!!!
« Reply #651 on: March 02, 2012, 11:32:16 am »
i and j are not like terms so 12i+6j is not equal to 18j
Shall I make a video the this?
theeducationchannel.info

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Re: All Mechanics DOUBTS HERE!!!
« Reply #652 on: March 02, 2012, 03:54:10 pm »
At time t seconds , a particle P has position vector r with respect to a fixed origin O , where
  r= [  (3t(sq) - 4) i +  ( 8 - 4t(sq) ) j ]

a Show that the acceleration of P is a constant .
b Find the magnitude  of the acceleration of P and the size of the angle which the acceleration makes with j .

 I get magnitude of acceleration = 10

But size of angle is 143.1  HOWWWWWWWWWWWWWWW
NO secrets to SUCCESS , it is the result of 1.HARD WORK 2.GOOD PREPARATION 3.LEARNING FROM FAILURE
But it ain't how hard you hit; it's about how hard you can get hit, and keep moving forward-Balboa

Offline astarmathsandphysics

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Re: All Mechanics DOUBTS HERE!!!
« Reply #653 on: March 02, 2012, 08:57:20 pm »
Differentiate twice get a=6i-8j
This is at an angle tan^(-1) (8/6)=53.13 below the x axis (plot (6,-8))
add 90 cos it is the angle with the j axis

Offline LekhB

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Re: All Mechanics DOUBTS HERE!!!
« Reply #654 on: March 03, 2012, 07:52:21 pm »
Thanks for the previous answers. Now, I got stuck with 3 different questions while I was doing my homework.


1/. A car is moving along a straight road with uniform acceleration. The car passes a check-point A with a speed of 12 m/s and another check-point C with a speed of 32 m/s. The distance between A and C is 1100m.
(i) Find the time, in s, taken by the car to move from A to C.
(ii) Given that B is the mid-point of AC, find in m/s to 1 decimal place, the speed with which the car passes B.


2. A, B and C are 3 points on a straight line, in that order, and the distance AB and AC are 45m and 77m respectively. A particle moves along the straight line with constant acceleration 2 m/s². Given that it taked 5 seconds to travel from A to B, find the time taken to travel from A to C.

3. An aircraft accelerates along the runway with constant acceleration a m/s². 3 points on the runway are A, B, C, where the distances AB and AC are each 120m. The speed of the aircraft as it passes A is u m/s. The aircraft takes 6s to cover the distance AB and 4s to cover the distance BC. Find the values of a and u.


It would be awesome if anyone could help. Cheers.

Offline astarmathsandphysics

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Re: All Mechanics DOUBTS HERE!!!
« Reply #655 on: March 03, 2012, 09:37:03 pm »
Have done these question but need to get scanner working

Offline astarmathsandphysics

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Re: All Mechanics DOUBTS HERE!!!
« Reply #656 on: March 04, 2012, 08:10:05 am »
Those 3 suvat questions

Offline LekhB

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Re: All Mechanics DOUBTS HERE!!!
« Reply #657 on: March 04, 2012, 08:02:07 pm »
Thank you a lot!

Attached 3 more questions. The connecting particles chapter is really getting on my nerves.





Offline astarmathsandphysics

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Re: All Mechanics DOUBTS HERE!!!
« Reply #658 on: March 04, 2012, 09:42:30 pm »
In the last suvat question I got a sign wrong at the end so the answer is wrong but the method is correct.
I made a video.

Offline astarmathsandphysics

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Re: All Mechanics DOUBTS HERE!!!
« Reply #659 on: March 04, 2012, 09:45:56 pm »
For the connecting particle questions I will make some videos tomorrow