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A solid metallic sphere of mass m m and radius R R is free to roll (without sliding) over the inclined surface of a wooden wedge of equal mass m m . Wedge lies on a smooth horizontal floor. When the system is released from rest, find the force f f between sphere and wedge.

If f = h k m g f = \dfrac{h}{k} mg , give your answer in the form of k h + h + k \dfrac{k}{h}+h+k


The answer is 15.5.

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2 solutions

Kishore S. Shenoy
Sep 29, 2015

Forgive me for placing extra-large image right next to a tiny one; I don't know how to change it.


Equating forces on the sphere along the perpendicular of the surface of the wedge , N ( 1 + sin 2 θ ) = m g cos θ + f cos θ sin θ 34 N 25 = 12 f 25 + 4 m g 5 ( 1 ) N(1+\sin^2 \theta) = mg\cos \theta + f\cos \theta\sin\theta\\\Rightarrow \dfrac {34N}{25}= \dfrac{12f}{25} + \dfrac{4mg}5~~\cdots(1)

Also, α = 5 f 2 m R m a = m R α = 5 f 2 \alpha = \dfrac{5f}{2mR}\\ma = mR\alpha = \dfrac{5f}2

Taking forces along the surface of the wedge, N sin θ cos θ + m g sin θ f ( 1 + cos 2 θ ) = 5 f 2 ( 2 ) N\sin\theta\cos\theta + mg\sin\theta - f(1+\cos^2\theta) = \dfrac{5f}2~~\cdots(2)

Substituting ( 1 ) (1) in ( 2 ) (2) , 144 f + 48 × 5 m g + 510 m g = 3519 f f = 750 3375 m g = 2 9 m g 144f+ 48\times5mg+510 mg=3519f\\f = \dfrac{750}{3375}mg=\dfrac 29 mg

f = 2 9 m g \Huge\therefore \boxed{f=\dfrac29mg}

Would you please give the whole solution on how you got those numbers?

Miguel Vásquez Vega - 5 years, 8 months ago

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That will be tough. There are slightly large numbers (4 digit) involved. Are you sure you want it?

Kishore S. Shenoy - 5 years, 8 months ago

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Definitely! ¨ \displaystyle\Large\ddot\smile

Miguel Vásquez Vega - 5 years, 8 months ago

4 digit numbers ... how ?

Anirban Mandal - 5 years, 5 months ago

Brilliant problem!! really enjoyed it

Prakhar Bindal - 5 years, 5 months ago

@Miguel Vásquez Vega and @Anirban Mandal ... here's my solution :)

Kishore S. Shenoy - 5 years, 5 months ago

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Why have you stopped posting ur cool mechanics problems? and you haven't posted more binomial summations also i am waiting for them :)

Prakhar Bindal - 5 years, 4 months ago

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I had {Exams} + {No Internet} (internet connectivity problem). So I had to stop :).

Kishore S. Shenoy - 5 years, 3 months ago

Hey can you tell me a way to learn latex? your latex skills are awesome!

Prakhar Bindal - 5 years, 5 months ago

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Just read the Wikipedia Page of Latex. It's awesome!

Kishore S. Shenoy - 5 years, 5 months ago

Thanks! Question: can you apply Newton's second law on the sphere w.r.t. the non- inertial frame the wedge is on?? As long as I understand, you would have to consider another inertial force acting on the sphere due to the wedge's acceleration...

Miguel Vásquez Vega - 5 years, 5 months ago

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Well, that's why I included pseudo accelerations... see those f cos θ f\cos\theta and N sin θ N\sin\theta acting in opposite directions to the corresponding accelerations on the wedge.

Kishore S. Shenoy - 5 years, 5 months ago

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@Kishore S. Shenoy I totally agree now! Nice problem and solution! Thank you very much!

Miguel Vásquez Vega - 5 years, 4 months ago

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@Miguel Vásquez Vega You're Welcome!

Kishore S. Shenoy - 5 years, 4 months ago

Nice problem! Your solution makes the problem look easy even if it isn't.

Aditya Kumar - 5 years, 1 month ago

I really liked your style of applying the pseudo force directly, i.e. the opposite forces.

Keep up the good work

Mayank Singh - 5 years, 3 months ago

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Thank you! I appreciate that you appreciate my work!

Kishore S. Shenoy - 5 years, 3 months ago

how did you get this N ( 1 + s i n 2 θ ) = m g c o s θ + f c o s θ s i n θ 34 N / 25 = 12 f / 25 + 4 m g / 5 N(1+sin^2θ)=mgcosθ+fcosθsinθ⇒34N/25​=12f/25​+4mg/5​

Aryan Bansal - 1 year, 8 months ago

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Umm... what's the value of 1 + sin 2 θ 1+ \sin^2 \theta ?

Kishore S. Shenoy - 1 year ago

Dude what grade where you in when you solved this ,this is insane .Nice job .

Keep it Dope - 7 months, 1 week ago

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Thanks. I was in my junior year of high school (grade 11).

Kishore S. Shenoy - 1 month, 1 week ago
Aryan Bansal
Oct 13, 2019

t h a t a s h o w i d i d i t thatas how i didit

Spelling mistake zone ahead.

Kishore S. Shenoy - 1 year ago

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