COM CYU 19

Solution 19: Prisms and Cylinder

Physics Solution: System of Prisms and Cylinder

1. Analysis of Motion and Constraints

Let the velocity of prism A (mass $m_A$) be $v_A$ to the left, and the velocity of prism B (mass $m_B$) be $v_B$ to the right. Let the cylinder (mass $m$) move with velocity components $v_x$ (horizontal, to the right) and $v_y$ (vertical, downwards).

Since the surfaces are frictionless and the system starts from rest with no external horizontal forces, the horizontal momentum of the entire system is conserved.

A α v_A B β v_B m v_y v_x

Constraint Equations (Contact Condition)

For the cylinder to maintain contact with the prisms, the velocity components perpendicular to the contact surfaces must match.

Contact with Prism A (Angle $\alpha$):
The relative velocity perpendicular to the face must be zero. $$v_y = (v_A + v_x) \tan \alpha \quad \dots(1)$$

Contact with Prism B (Angle $\beta$):
Similarly, for prism B: $$v_y = (v_B – v_x) \tan \beta \quad \dots(2)$$

From (1) and (2), we equate $v_y$ to find $v_x$:

$$(v_A + v_x) \tan \alpha = (v_B – v_x) \tan \beta$$ $$v_x (\tan \alpha + \tan \beta) = v_B \tan \beta – v_A \tan \alpha$$ $$v_x = \frac{v_B \tan \beta – v_A \tan \alpha}{\tan \alpha + \tan \beta} \quad \dots(3)$$

2. Conservation of Momentum

Since the initial horizontal momentum is zero:

$$P_{initial} = 0$$ $$P_{final} = m_B v_B – m_A v_A + m v_x = 0$$ Substituting $v_x$ from eq (3): $$m_B v_B – m_A v_A + m \left[ \frac{v_B \tan \beta – v_A \tan \alpha}{\tan \alpha + \tan \beta} \right] = 0$$ Multiply by $(\tan \alpha + \tan \beta)$ to clear the denominator: $$m_B v_B (\tan \alpha + \tan \beta) – m_A v_A (\tan \alpha + \tan \beta) + m v_B \tan \beta – m v_A \tan \alpha = 0$$ Group terms by $v_A$ and $v_B$: $$v_B [ m_B (\tan \alpha + \tan \beta) + m \tan \beta ] = v_A [ m_A (\tan \alpha + \tan \beta) + m \tan \alpha ]$$ $$\frac{v_A}{v_B} = \frac{m_B (\tan \alpha + \tan \beta) + m \tan \beta}{m_A (\tan \alpha + \tan \beta) + m \tan \alpha}$$

3. Calculation

Given values:

  • $m_A = 9.0 \text{ kg}, \quad \alpha = 53^\circ \Rightarrow \tan 53^\circ = 4/3$
  • $m_B = 16.0 \text{ kg}, \quad \beta = 37^\circ \Rightarrow \tan 37^\circ = 3/4$
  • $m = 25 \text{ kg}$

Calculate the term $(\tan \alpha + \tan \beta)$:

$$\tan \alpha + \tan \beta = \frac{4}{3} + \frac{3}{4} = \frac{16+9}{12} = \frac{25}{12}$$

Numerator (coefficient of $v_B$):

$$\text{Num} = 16\left(\frac{25}{12}\right) + 25\left(\frac{3}{4}\right) = \frac{400}{12} + \frac{75}{4} = \frac{625}{12}$$

Denominator (coefficient of $v_A$):

$$\text{Den} = 9\left(\frac{25}{12}\right) + 25\left(\frac{4}{3}\right) = \frac{225}{12} + \frac{100}{3} = \frac{625}{12}$$ $$\text{Ratio } \frac{v_A}{v_B} = \frac{625/12}{625/12} = 1$$

Answer:

The ratio of the speed of prism A to that of prism B is 1.