RBD CYU 26

Solution to Question 26

Physics Solution: Wheel on Wedge

$\theta$ $F_{wall}$ $Mg$ $N$ $f$
Step 1: Force Analysis on the Wedge

The wedge is in equilibrium horizontally because it is pressed against a vertical wall. The question states the wedge exerts a constant force $F$ on the wall. By Newton’s 3rd law, the wall exerts a force $F$ on the wedge to the left.

The forces exerted by the wheel on the wedge are the Normal force $N$ (down-right perpendicular to incline) and Friction $f$ (down-left along incline). Horizontal equilibrium of the wedge: $$ N \sin\theta – f \cos\theta = F \quad \dots (1) $$

Step 2: Force Analysis on the Wheel

Considering the motion of the wheel’s center of mass (mass $M$) down the incline:

  • Perpendicular to incline: $N – Mg \cos\theta = 0 \implies N = Mg \cos\theta$
  • Along the incline: $Mg \sin\theta – f = Ma_{cm}$
Step 3: Solving for Acceleration

Substitute $N = Mg \cos\theta$ into equation (1) to find the friction force $f$ required to maintain the wall force $F$: $$ (Mg \cos\theta) \sin\theta – f \cos\theta = F $$ $$ f \cos\theta = Mg \sin\theta \cos\theta – F $$ $$ f = Mg \sin\theta – \frac{F}{\cos\theta} $$

Now substitute this expression for $f$ into the wheel’s equation of motion: $$ Mg \sin\theta – \left( Mg \sin\theta – \frac{F}{\cos\theta} \right) = Ma_{cm} $$ $$ \frac{F}{\cos\theta} = Ma_{cm} $$ $$ a_{cm} = \frac{F}{M \cos\theta} $$

Step 4: Part (a) – Final Speed

The acceleration $a_{cm}$ is constant and depends only on the given parameters ($F, M, \theta$). Using kinematics equation $v^2 = u^2 + 2as$ with $u=0$: $$ v^2 = 2 \left( \frac{F}{M \cos\theta} \right) s $$ $$ v = \sqrt{\frac{2Fs}{M \cos\theta}} $$ This formula is valid for both rolling with slipping and rolling without slipping, as it is derived directly from the force constraint $F$.

Step 5: Part (b) – Comparison of Speeds

We must compare the speed acquired when rolling with slipping ($v_{slip}$) versus rolling without slipping ($v_{roll}$).
While the formula derived above relates speed to the measured force $F$, physically, a wheel slides faster than it rolls because less energy is diverted into rotation (or alternatively, friction is lower during slipping than the static friction required to induce pure rolling).
Therefore, for the same physical setup (where $F$ would consequently be different), the wheel that slips accelerates faster.

Answer:
(a) $v = \sqrt{\frac{2Fs}{M \cos\theta}}$
(b) The centre of the wheel will acquire higher speed when the wheel rolls down with slipping.