A bar of mass and length is lying on a horizontal frictionless surface. One end of the bar is pivoted at a point about which it is free to rotate. A small mass is moving on the same horizontal surface with speed on a path perpendicular to the bar. It hits the bar at a distance from the pivoted end and returns back on the same path with speed . After this elastic collision, the bar rotates with an angular velocity . Which of the following statement is correct?
Answer: A. and
Or read the worked solution below. Jump to the steps ↓
Written solution
Step 1: Calculate the moment of inertia of the bar.
The bar is pivoted at one end , so its moment of inertia about the pivot is given by the formula for a uniform rod pivoted at its end.
Substitute the given values and .
Step 2: Apply conservation of angular momentum.
Since there are no external torques acting on the system about the vertical axis through pivot , the angular momentum is conserved.
Rearrange the equation to express in terms of .
Substitute the values , , , and .
Step 3: Apply conservation of kinetic energy.
Since the collision is elastic, the total kinetic energy before the collision equals the total kinetic energy after the collision.
Substitute the known values into the energy equation.
Step 4: Solve for the final velocity and angular velocity .
Substitute the expression for from Step 2 into the kinetic energy equation.
Expand and simplify the terms to form a quadratic equation.
Solve for using the quadratic formula.
Substitute back into the expression for .
Answer: and