How Do Rotating Disks Measure Bullet Speed?

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The discussion focuses on using a device with two rotating disks to measure bullet speed, with specific parameters provided, including distance and angular speed. The key equations involve angular displacement, angular velocity, and the relationship between distance and time. Participants express confusion about calculating the time it takes for the bullet to travel between the disks and how to apply these equations effectively. Clarification is sought on integrating the time calculation into the overall solution for bullet speed. The conversation highlights the need for a clear understanding of the equations and their application in this context.
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Homework Statement



A device can be used to measure the speed of a bullet. The device consists of two rotating disks, separated by a distance of d = 0.850 m, and rotating with an angular speed of 95.0 rad/s. The bullet first passes through the left disk and then through the right disk. It is found that the angular displacement between the two bullet holes is 0.240 rad. From these data, determine the speed of the bullet.

Homework Equations

Angular displacement = arc length/radius
Angular velocity = angular displacement/time

The Attempt at a Solution


I attempted to set up an equation, but I simply couldn't.
 
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What is the angular displacement of the first disk during the bullet's flight between the disk? Of the second one?
 
The definition of Velocity is distance/time

The distance is known so you just need to work out the time it takes to travel the distance between the discs.

The definition of angular velocity is

Angular velocity = angle/time

So rearrange that to give an equation for the time. Plug that into the first equation.
 
CWatters said:
The definition of Velocity is distance/time

The distance is known so you just need to work out the time it takes to travel the distance between the discs.

The definition of angular velocity is

Angular velocity = angle/time

So rearrange that to give an equation for the time. Plug that into the first equation.

Please clarify? I've been stumped for a long time.
Following your advice, time = angle/angular velocity
But how do I plug it in?
 

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