Discussion in 'Physics' started by s2pk, Aug 5, 2011.

  1. s2pk

    Thread Starter Member

    Jul 10, 2010
    what is the difference rpm and speed???
  2. aeroguy


    Sep 16, 2009
    If you define "speed" as the magnitude of a velocity vector, you can associate an infinite number of "speeds" with (for example) a rotating disc.

    A disc rotating at a fixed RPM has lower wheel speeds near the axis of rotation, and higher wheel speeds as you move away from the axis of rotation.

    Speed is proportional to radius * RPM.
    s2pk likes this.
  3. elec_mech

    Senior Member

    Nov 12, 2008
    Hi s2kpk,

    I don't know in what context you're asking this, but RPM stands for rotations per minute. Very simply put, RPM is rotational speed. If you're asking the difference between the RPM of a drive motor on a car, for instance, and the speed the car can travel at, then you're asking for the difference between RPM (rotational speed) and linear speed.

    If that is the case, both are speed and they are related by the diameter of the wheel.

    As an example, say you have a motor rated at 1000 RPM and you attach a wheel directly to the shaft. Let's say the wheel diameter is 20 inches.

    Your linear speed is found by multiplying your rotational speed by the circumference of your wheel: RPM x ∏D => 1000 RPM x 3.14 x 20in =

    62800 inches per minute


    5233.3 feet per minute


    0.9912 miles per minute


    59.5 miles per hour

    Keeping the RPM the same, if you increase the wheel diameter, your linear speed goes up and vice versa.

    Hope this helps.
    s2pk likes this.
  4. strantor

    AAC Fanatic!

    Oct 3, 2010

    speed is distance over time. RPM is revolutions over time

    "C" is a shaft. it completes 100 rotations every minute, so 100RPM

    "A" is a wheel. It's diameter is 2ft, so it's circumference (the distance around the outside of it) is 6.28ft. since it is mounted to a shaft which is spinning 100RPM, the speed measured at the outside of it would be 628Feet Per Minute, or 7.14 Miles Per Hour.

    "B" is also a wheel, mounted to the same shaft, spinning at the same 100 RPM. It's diameter is 4ft, circumference 12.56ft. The speed measured at the outside of wheel "B" would be 14.27 Miles Per Hour.
    s2pk likes this.
  5. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    If your question is in reference to the common displays everyone seems to have in the automobile they drive.

    RPM is the number of revolutions the drive shaft of your car is making per minute.

    Speed is the linear velocity of the vehicle.

    The RPM can be some random value, say 3000, for several different car speeds. This happens because of the GEARS in your transmission. Different gears give different rates of rotation to the wheels of the car for the same number of RPMs of the car's drive shaft.
  6. BillO

    Distinguished Member

    Nov 24, 2008
    They are both speed. Which is just a scalar measure of how fast something is going.

    Linear speed is a general linear scalar quantity, ... so many distance units per so many time units. Specific examples would be miles per hour, or meters per second, or inches per year, etc.

    Angular speed is the rotational analogue of linear speed, ... so many rotational units per so many time units. Revolutions per minute (RPM) is one specific example. Other examples might be 'degrees per hour', or 'radians per second', etc.
    Last edited: Aug 6, 2011
  7. fredrikkaplan

    New Member

    Sep 25, 2011