# findin avg acc

A particle moves along a semicircular path of radius R in time t with constant speed. For the particle calculate average acceleration.

answer given is : 2Î R/t2

why (2 x avg speed/time) is taken n not (2x avg velocity/time) ?

1
paromita89 paromita ·

actually i ws thinking why its not =4R/t ...

1
paromita89 paromita ·

yes, i understud. thnx.

now i undrstnd, my main prob ws, dat i cudnt find initial and final velocities.

62
Lokesh Verma ·

Does this make things any simpler?

Or even before that.. did u understand this?

62
Lokesh Verma ·

You are absolutely correct

See what is happening is that initially the ball is travelling upwards

later it is travelling downwards

If you see closely.. the ball is travelling in the opposite direction at both these times!

We find the speed.: Total distance travelled by the ball is the arc length = pi. R

Time = t

Speed = pi.R/t

Now if the top direction is given by j

then initial velocity is pi.R j
Final velocity is - pi.R j

1
paromita89 paromita ·

no sir not at all... evn i kw this is going to help me..

vectors hv direction n magnitude.

sir if i take the origin at A then position vectors wud b

1) 0
2) 2R

62
Lokesh Verma ·

see sorry for asking this.. do u understand what a vector is?

cos i want to know where you are getting it wrong!

Can you solve these!

1) What is the initial Position
2) What is the position at time t?

Dont get demotivated..... That is not what i am trying.

I am just trying to find out where u are missing the concept!

1
paromita89 paromita ·

sir... i dont kw..

wats it sir ?

62
Lokesh Verma ·

ok.. lets do this like this.. So that we can find out where you are wrong.. right?

1) What is the initial Velocity
2) What is the velocity at time "t"

Answer these two first .. ok?

1
paromita89 paromita ·

no actually aftr posting dat.. it came in my mind that its writen in qn dat speed is constant... but the main thing i ws wondering is dat one above..
nn now i feel totally confused.. hehe..

62
Lokesh Verma ·

well paromita..

Speed is constant.. (So speed change is zero)

I did not understnad you.. dont shy away from this one.. i think we need to do some fixing of basics here!

1
Rohan Ghosh ·

average acceleration=

V2-v1/t

where V2 and v1 are the final and initial velocities respectively

as they havvve same magnitude with opposite directions

v2-v1 = 2v = 2(Ï€R/t)

average acceleration = avg velocity /time = 2Ï€R/t2

62
Lokesh Verma ·

Paromita:

Avg Velocity= "Change in Displacemtn"/"Change in Time"

Avg Acceleration= "Change in Velocity"/"Change in Time"

1
Rohan Ghosh ·

but average acceleration = v2-v1/t

not average velocity by time

1
paromita89 paromita ·

rohan..
i think u need to see the diagram then u will a ble to visualise..

now displacement is 2R,
avg vel = displacemnt/ time = 2R/t

1
Pavithra Ramamoorthy ·

wats d difference between speed n velocity???

1
Rohan Ghosh ·

HEY average velocity is not 2R/T check it again

1
paromita89 paromita ·

1st of all i undrstnd u wrote dat,1st line, by mistake.
but sumhow i feel .. either u r not able to undrstnd wat i m asking (may b becoz i m nt clr on my part) or i m jus not undrstnding wat u mean to say... becoz wat u r saying isnt the answer of the qn i hd asked...

1
Rohan Ghosh ·

average velocity = average acceleration/(time taken)

remember it is always a vector

1
paromita89 paromita ·

in this qn, avg speed,v= Î R/t
avg velocity= 2R/t
acc= 2v/t...

dats y i asked.. why acc= avg speed/t n not avg vel/t ?

1
Pavithra Ramamoorthy ·

rohan...dat cld be right.....:-)