simple yet good ..

An infinite plane inclined at an angle θ to the horizontal . A point mass m is placed on the plane at very large height .

the mass is given a push with 'v' velocity in the horizontal direction along the plane .
i.e. the direction perpendicular to the one pointing straight down the plane.

there is friction between mass and plane . frictional coefficient = tanθ

Assuming gravitational force doesnt change with height

find the velocity of the block after infifnite time .

remember velocity means both dirn and magn so you have to give both with some explanation .

28 Answers

1
satan92 ·

your solution is not clear ..

9
Celestine preetham ·

terrific work rohan

but you must have waited for another day so that we try to solve

62
Lokesh Verma ·

awesome work Rohan :)

1
satan92 ·

well here is the two liner solution ..

as we know friction opposes relative motion , friction will always act opposite to the body's velocity

magnitude of frition = μN(as it is moving)=tanθmgcosθ=mgsinθ

finally as subash and asish have pointed out .. the velocity would be straight down the plane ..

because at each moment

let velocity be oriented at an angle of β to the horizontal clockwise .

ax=-Fcosβ

ay=Fsinβ + mgsinθ

as the object moves its velocity along x decreases and y increases therby increasing β

as tiime passes we see that at sufficient β ay surpasses ax and after a long time ax will almost have reduced to zero
(as β increases at infinite time we would have β =90 and thus the net force if you calculate comes to be zero)

thus the object will have a constant veocity at infinite time .

further observe that -

acceleration along velocity = -F+mgsinθsinβ
acceleration along Vy = -Fsinβ +mgsinθ

so

md(vy + v)/dt=-mgsinθ + mgsinθsinβ -mgsinβsinθ +mgsinθ =0

thus the quantity v + vy always remains constant ..

at t=0 the above quantity is = V0 + 0
at infinitely long time when v is vy (no vx)

=v + v

so we have

v+v= V0 + 0

thus v=V0/2

1
satan92 ·

Problem forgotten??

62
Lokesh Verma ·

subash keep trying ... you will enjoy if you solve this one yourself :)

11
Subash ·

bhaiyya! can you provide solution or will we have to wait

im not making much progress here

1
satan92 ·

yes subash and asish the direction given by you is correct ..

106
Asish Mahapatra ·

this is what i know of concept but clueless how to convert it to equations...

--> friction acts in a direction opposite to the resultant velocity..
--> initially friction acts opposite to horizontal direction.
--> as time increases and speed down the incline increases... the frictional force continually changes direction and becomes more inclined towards the downward velocity
--> After infinitely long time the frictional force will be directed up the incline.
--> kaymant sir's hint is i think the best way.. let me try it.. now only understood his hint

11
Subash ·

is the direction given by me correct?

66
kaymant ·

yes satan, you are right again.. now the answer to your original question can be easily guessed [1]. but i suggest ppl to first try it themselves before pondering over the answer given by satan

11
Subash ·

the direction of the final velocity would be down the plane

because no other force other than friction in the direction A(in your figure)

magnitude still thinking........

11
virang1 Jhaveri ·

This is the figure for my solution

1
satan92 ·

how can u write tana=V/gtsinθ ?

friction will affect both horizontal and vertical velocities !!

1
satan92 ·

FINALLY WAS ABLE TO UPLOAD ..

the direction to which i am referring is that one along A (along the plane)

11
virang1 Jhaveri ·

See

Velocity due to gravity u = tgsinθ

The V and u are at right angle

Resultant velocity is
R2 = V2 + u2

Force due to friction is tanθmgcosθ = mgsinθ
therefore acc due to friction is gsinθ

The resultant velocity = R - gsinθt
Velocity = √(V2 + t2g2sin2θ) -tgsinθ

Limit both of them for t==>∞

1
satan92 ·

it will be

this is for kaymant sir ..only

v/(1+cosθ)

66
kaymant ·

good question satan...
Additionally, you guys may try to find the speed of the particle as function of the angle between the instantaneous velocity vector and the line of greatest slope (which is the line perpendicular to the direction of A in figure of #2)

1
satan92 ·

remember the concepts virang ..

friction opposes relative motion

so intially friction will act opposite to the velocity .. and gravity will act down the plane and hence there will be downward motion ..

11
virang1 Jhaveri ·

For motion downwards Forces are gravity and friction

Noraml = mg
Friction is tanθmgcosθ = mgsinθ

Force due to gravity
= mgsinθ

Since friction is equal to gravitational pull There will be no downwards motion.

The Body will only move horizontally on the plane and come to rest after T = V/gtanθ

1
satan92 ·

cmon !! more people should try it!!!

1
Ritika ·

Thank god!! Well, i'm tryin it...

1
satan92 ·

nothing at all .. didnt i mention its simple ..

well there may be big ways to do it as u mentioned but the method i am talking about doesnt need these ..

1
Ritika ·

Infinite time....something to do with limits here? Or integration? Then i might as well leave the job to you or anyone else coz i'm hopeless at that.

1
satan92 ·

yes ritika ... ...

1
Ritika ·

hmm....do we take the components of 'g' ? And then friction also will oppose horizontal as well as vertical motion..

1
satan92 ·

remember friction opposes relative motion .. :)

21
tapanmast Vora ·

mayb the friction force will also hav to b resolved into 2 components

Your Answer

Close [X]