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Homework Help: Friction problem-how does speed affect the frictional force

  1. May 13, 2012 #1
    If we consider an object sliding with constant velocity, and frictional force is not neglected, then the magnitude of the force applied will be equal to the magnitude of the frictional force. In one of the experiments I did, I had two do 3 different runs and needed to measure the frictional force, while the normal force (the load) was kept the same, and the surfaces were the same (measured on a horizontal surface). The only thing which was changed in each run was the velocity-the direction was kept the same, but the speed was increased ( for example, run 1-5 m/s, run 2-10 m/s). The velocities were kept constant for the time the motion is measured in each run. Then, the frictional force measured was increasing with the speed. What can I conclude out of this since the formula for the frictional force is μN(μmg) and doesn't implicate any connection of the frictional force with the speed? Are my results due to possible errors in measuring?

    I was thinking that the force needed to set the object in motion is bigger when the velocity is bigger, as in since the initial velocity was zero, it takes more force to increase for example the velocity from 0 m/s to 10 m/s than from 0 m/s to 5 m/s. Since this applied force has to be the same as the friction force ( constant velocity, no acceleration), then the friction force would therefore be bigger for the run with a bigger velocity. Can this be a reason, is it even correct?
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  3. May 13, 2012 #2
    When you accelerate an object against friction, the force needed is composed of two components. One is the frictional force and the other is the force due to Newton's Second law, F=ma. Once the speed is reached and acceleration is zero, then only frictional force applies.

    10 meters/second is a pretty good clip. Depending on the frontal area of the object you are moving, have you considered the possibility of air resistance affecting your measurements?
  4. May 13, 2012 #3
    This what i found in Mathematics for Physics Students(pg. 137).

    The faster the object is going, the more molecules per second it will encounter that will slow its motion. Hence, in this situation, a fast moving object will slow down by a greater number of meter per second than the same object moving at a slower speed on the same surface.
  5. May 13, 2012 #4
    The thing is, we actually had a 2-body system, the first body was a motorized cart with a sensor, and it has 3 speeds-min, medium, max; that cart is connected to the object on which we observe the friction with a string which mass is negligible...now since the cart will always be moving with a constant speed, and the sensor measures the tension force, that tension force will be always equal to the friction force...

    Now, as I have said, for the first run we used the const speed let's say 2 m/s, for the second run 5 m/s, and for the third run 10 m/s. We neglected the air resistance initially, but since the friction force measured for those 3 runs kept increasing, I am not sure whether it is due to air resistence and some other things we neglected but that might have an effect, or is it actually due to the effect of speed? SO basically, what I am asking is, does speed effect the force of friction when the object is moving with a constant velocity?
  6. May 13, 2012 #5


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    sliding or kinetic friction between 2 surfaces that are moving relative to each other does not change with speed, but skin friction and drag is dependent on the speed, which you and others have noted: the faster the speed, the greater the 'drag'. What is the mass and size and shape (particularly frontal area exposed to the moving air) of your object? The tension force must be great enough to match the sum of the sliding friction force and the drag forces.
  7. May 14, 2012 #6
    The mass is about 1100 g, it basically looks like a cube, each side about 20 cm long...but the shape and the formal area you have asked for are not required for our observation at all, at least they are not mentioned anywhere. But I think you are right, the drag force is the one depending on the speed, so if I take that into the observation, that is why I probably have the frictional force increasing. I have also seen other people's results, a lot of them don't have any kind of consistent increase or decrease in their results, so I guess that all we have to mention is our results and that frictional force doesn't depend on the speed.

    Thank you all for the answers :)
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