Tangential speed problem with a hard drive disk

[Spartan301]

Homework Statement

A computer hard drive disk with a diameter of 3.5 inches rotates at 7200 rpm. The “read head” is positioned exactly halfway from the axis of rotation to the outer edge of the disk. What is the tangential speed in m/s of a point on the disk under the read head?

Objective: Find tangential velocity on disk.

Homework Equations

Battle plan:
Convert inches to meters
Find the distance to the axis
Find the angular velocity by dividing the angular displacement by the time.
Convert from an angular speed in rad/s to a tangential speed in m/s by multiplying the angular speed by the radius of rotation.

The Attempt at a Solution

Outcome:
0.0889m/4 = 0.022225 m
7200r/min x 60 = 432000 r/s
432000 r/s x 0.022225 m = 9601.2 m/s :(

The key says the answer is 17 m/s.

I would so appreciate any help on this problem. Thanks!
-Tom

 

[G01]

Homework Statement

A computer hard drive disk with a diameter of 3.5 inches rotates at 7200 rpm. The “read head” is positioned exactly halfway from the axis of rotation to the outer edge of the disk. What is the tangential speed in m/s of a point on the disk under the read head?

Objective: Find tangential velocity on disk.

Homework Equations

Battle plan:
Convert inches to meters
Find the distance to the axis
Find the angular velocity by dividing the angular displacement by the time.
Convert from an angular speed in rad/s to a tangential speed in m/s by multiplying the angular speed by the radius of rotation.

The Attempt at a Solution

Outcome:
0.0889m/4 = 0.022225 m
7200r/min x 60 = 432000 r/s
432000 r/s x 0.022225 m = 9601.2 m/s :(

The key says the answer is 17 m/s.

I would so appreciate any help on this problem. Thanks!
-Tom

Hi Tom,

Your "battle plan" is just fine. The only problem is that you should divide by 60. (The units of minutes are in the denominator.) Also, on the last line, there is a factor of $2\pi$ need to put the angular speed in radians/s.

 

[Spartan301]

Thank you!