Ball rolling and falling on the ground(what's new about that)

As shown in the figure, a spherical ball of radius r r and mass m m is pure rolling on a rough horizontal platform which is at height h h from the ground with the velocity of centre of mass as v v

After some time it falls of the edge on the ground. Find the horizontal range on the ground marked from the edge from where it fell that is from point M.

Details and Assumptions

  • m = 2 m m=2 m , r = 0.63 m r=0.63 m , v = 1.8 m / s v=1.8 m/s , g = 10 m s 2 g= 10 m{s}^{-2} , h = 2.126 m h=2.126 m
  • Assume friction to be sufficiently high on the platform.


The answer is 1.30.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

1 solution

Mvs Saketh
Sep 28, 2014

Since there is sufficient friction, the ball at any instant has its instantaneous axis of rotation as the point of contact with the ground,hence when it reaches the cliff, it first starts rotating about the cliff tip as usual , however the centripetal acceleration (or centrifugal force based on your frame) tends to cause it to lose contact , when this becomes equal to the component of gravity directed towards sharp contact of cliff it will leave the cliff, and based on the velocities at that instant we can treat subsequent motion as projectile, however, as has been warned in the question itself, the height it has to fall is not h h but h R + R c o s ( θ ) h-R+Rcos(\theta) where R R is the radius and θ \theta is the angle of departure from the cliff with the vertical,then it can be evaluated to find the range, however another critical point here, the distance obtained here is actually the distance traversed by the com from departure, but we need to find distance from M, so add R s i n ( θ ) Rsin(\theta)

Solving you get 1.2974 m 1.2974 m

The calculations are complex and the numbers are weird so i will just write the equations to be used.

m g c o s θ = m ω 2 r mgcos\theta = m{ \omega}^{ 2 }r and

m g R ( 1 c o s θ ) = I p ω 2 / 2 I p ω o 2 / 2 mgR(1-cos\theta )={ I }_{ p }{ \omega }^{ 2 }/2-{ I }_{ p }{ \omega }_{ o }^{ 2 }/2

Solving these two we get :

c o s θ = 0.8 cos\theta = 0.8

Further we use this to find ω R \omega R which is V c o m {V}_{com} and use the equations

v t + g t 2 2 = h R ( 1 c o s θ ) = 2 vt+g\frac { { t }^{ 2 } }{ 2 } = h-R(1-cos\theta )=2

u t = d ut= d

where v v and u u are the vertical and horizontal velocities of c o m com at that instant

Now solving we get d = 0.9194 d = 0.9194

then since we have to find distance from M M , we add 0.6 R 0.6R and get result as 1.2974 1.2974 .

and yes thanks, i have been waiting for more of your awesome mechanics problems

Did the same but forgot to add 0.6R to it :(

Aniket Sanghi - 5 years, 1 month ago

why is the latex font so small,, please fix it @rounakaggarwal

Mvs Saketh - 6 years, 8 months ago

Log in to reply

I have fixed your latex you can use it for your reference, also you don't need to enclose text within latex and place \quad behind each word, just use latex for only math expressions as I have done here.

Also can you please reshare my question so that others can try this question too. Also please enter only one comma where you are putting two commas

Ronak Agarwal - 6 years, 8 months ago

Log in to reply

Got it... i use app for latex so i just put as much as possible in latex..

Mvs Saketh - 6 years, 8 months ago

How can I calculate v and u the horizontal and vertical velocity just after tipping.

Ishan Dixit - 4 years, 2 months ago

Log in to reply

Its simple. Find the theta when normal reaction is zero and then find at that moment the w and with it find centre of mass velocity and take its components .

Aniket Sanghi - 4 years, 2 months ago

Log in to reply

@Aniket Sanghi Got it thnx again

Ishan Dixit - 4 years, 2 months ago

Why did you ignore the normal reaction when it is rotating about the edge?

Tushar Gopalka - 6 years, 8 months ago

Log in to reply

At the instant of departure.. it is just about to lose contact with the edge. Hence is is practically no longer in contact or the normal reaction has become 0.... so as for the force analsys normal reaction tends to 0 as the instant of departure approaches...

now why did i ignore work done by normal reaction? Simple... ... the primary one is in all situations involving rigid immovable walls and ground.. the motion of the point of contact is essentially parallel to the surface and hence angle between normal reaction and displacement is 90 degree... so work done is 0. I also ignored work done by friction because the point of contact is not moving as the ball is pure rolling about the edge.. So work done by friction is 0... and as friction is proportional to normal reaction. So at instant of departure friction too becomes 0

Mvs Saketh - 6 years, 8 months ago

Awesome Solution..Will you be giving NSEP this year..from which state?

Tushar Gopalka - 6 years, 8 months ago

Log in to reply

Yes,, from west bengal :)

Mvs Saketh - 6 years, 8 months ago

Log in to reply

Me too...prepared and confident?

Tushar Gopalka - 6 years, 8 months ago

Log in to reply

@Tushar Gopalka How's the question, do you like it.

Ronak Agarwal - 6 years, 8 months ago

Log in to reply

@Ronak Agarwal Awesome.....yet again....

Tushar Gopalka - 6 years, 8 months ago

@Tushar Gopalka Well i might be ready, but i would still rather work on my skills and make the most in the time remaining. Best of luck

Mvs Saketh - 6 years, 8 months ago

I also did Same but in the end I used equation of Trajectory !! And Ronak really you frame an very nice question ! I enjoyed very much it in solving it !!

Deepanshu Gupta - 6 years, 7 months ago

Log in to reply

Log in to reply

Try this question! A very edgy problem

Kishore S. Shenoy - 5 years, 8 months ago

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...