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?