did i miss something XD (try this)

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 ratio m g f \frac{mg}{f} , here f f is the friction between the sphere and the wedge.

The wedge is a right triangle

4 3.5 5.5 4.5

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2 solutions

Karan Chatrath
Oct 27, 2019

The force m a 1 ma_1 acting on the sphere is a pseudo force.

The equations governing the motion of the sphere and wedge are:

m g + N 2 cos θ + f sin θ = N 1 ( 1 ) mg + N_2\cos{\theta} + f\sin{\theta} = N_1 \ \dots (1) N 2 sin θ f cos θ = m a 1 ( 2 ) N_2\sin{\theta} - f\cos{\theta} = ma_1 \ \dots (2) m g cos θ = N 2 + m a 1 sin θ ( 3 ) mg\cos{\theta} = N_2 + ma_1\sin{\theta} \ \dots (3) m a 1 cos θ + m g sin θ f = m a 2 ( 4 ) ma_1\cos{\theta} + mg\sin{\theta} - f = ma_2 \ \dots (4) f R = ( 2 5 m R 2 ) α ( 5 ) fR = \left(\frac{2}{5}mR^2\right)\alpha \ \dots (5) a 2 = R α ( 6 ) a_2 = R\alpha \ \dots (6)

  • Equation 1 balances forces along the vertical for the wedge.

  • Equation 2 is the application of Newton's 2nd law for the wedge along the horizontal.

  • Equation 3 is a force balance in a direction perpendicular to the incline for the sphere.

  • Equation 4 is the application of Newton's 2nd law for the sphere along the incline.

  • Equation 5 is the torque equation τ = I α \tau = I\alpha about the COM of the sphere.

  • Equation 6 is the pure rolling constraint.

Taking cos 3 7 o = 0.8 \cos{37^o}=0.8 and solving the six equations in six unknowns gives:

f = 2 9 m g f = \frac{2}{9}mg N 2 = 2 3 m g N_2 = \frac{2}{3}mg

The resultant force between the wedge and the sphere is:

F = f 2 + N 2 2 F = \sqrt{f^2 + N_2^2}

Apologies for the messy diagram. I have denoted f f as the friction force between the sphere and the wedge.

Aryan Bansal
Oct 13, 2019

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