Motion of an Object on Perpendicular Belts

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Dynamics Advanced Non-inertial reference

Source: Physics Competition - Mechanics

Problem

Two conveyor belts are in the same horizontal plane, moving perpendicularly to each other. Belt 1 moves at speed $v_1$, and belt 2 moves at speed $v_2$. An object of mass $m$ is delivered from belt 1 onto belt 2. The coefficient of kinetic friction between the object and the belts is $\mu$. The point where the object lands on belt 2 is the origin O of a stationary coordinate system, with belt 1's motion along the x-axis and belt 2's motion along the y-axis. The belts are sufficiently wide.

  1. Find the coordinates (x, y) of the object in the stationary frame when it stops sliding relative to belt 2.
  2. Find the total time the object slides on belt 2.

[Q1] $(x, y) = (\frac{v_1 \sqrt{v_1^2 + v_2^2}}{2 \mu g}, \frac{v_2 \sqrt{v_1^2 + v_2^2}}{2 \mu g})$ [Q2] $t = \frac{\sqrt{v_1^2 + v_2^2}}{\mu g}$

The initial velocity of the object is $\vec{v}_0 = v_1 \hat{i}$. The velocity of belt 2 is $\vec{v}_{belt} = v_2 \hat{j}$. The object stops sliding when its velocity becomes equal to that of belt 2, so the final velocity is $\vec{v}_f = v_2 \hat{j}$.

The initial relative velocity of the object with respect to belt 2 is:

$$\vec{v}_{rel} = \vec{v}_0 - \vec{v}_{belt} = v_1 \hat{i} - v_2 \hat{j}$$

The kinetic friction force $\vec{f}$ opposes this relative motion. Its magnitude is $f = \mu mg$. The acceleration of the object is constant, with magnitude $a = f/m = \mu g$, and its direction is opposite to $\vec{v}_{rel}$.

The change in the object's velocity is $\Delta \vec{v} = \vec{v}_f - \vec{v}_0 = v_2 \hat{j} - v_1 \hat{i}$. The time $t$ taken for this change is given by $\vec{a}t = \Delta \vec{v}$. Taking the magnitude of both sides:

$$|\vec{a}|t = |\Delta \vec{v}|$$ $$(\mu g)t = \sqrt{(-v_1)^2 + (v_2)^2}$$ $$t = \frac{\sqrt{v_1^2 + v_2^2}}{\mu g}$$

For motion with constant acceleration, the displacement can be found using the average velocity. The final coordinates $(x,y)$ are the components of the displacement vector $\Delta\vec{r}$.

$$\Delta\vec{r} = \vec{v}_{avg} t = \frac{\vec{v}_0 + \vec{v}_f}{2} t$$ $$x\hat{i} + y\hat{j} = \frac{(v_1 \hat{i}) + (v_2 \hat{j})}{2} t$$

Substituting the expression for $t$:

$$x = \frac{v_1}{2} t = \frac{v_1 \sqrt{v_1^2 + v_2^2}}{2 \mu g}$$ $$y = \frac{v_2}{2} t = \frac{v_2 \sqrt{v_1^2 + v_2^2}}{2 \mu g}$$