Friction: Stick or Slip?

Going back to my last entry on the sliding car, it’s worth commenting a bit more on the nature of friction here. When a car goes round a corner, what prevents it from sliding is the friction between the tyres and the road. Tyres are unsurprisingly designed to be able to give a high frictional force when in contact with the road. If you’ve got access to a car tyre that’s not  attached to a car, try putting it up vertically (as when mounted on a car) and pushing it sideways across the road. Not at all easy, which is quite reassuring, really.

Friction is a complicated beast. We usually separate discussion into ‘kinetic’ friction and ‘static’ friction. Kinetic friction is what happens when an object is moving; static when the object is stationary. Kinetic friction can often be described nicely by ‘the coefficient of kinetic friction’; in this case the frictional force (which of course acts against the direction of movement) is given by the coefficient of kinetic friction times the normal force that the surface exerts on the object. From Newton’s second law, the normal force exerted on an object on a flat surface will equal the object’s weight (but that’s not true on an inclined surface) and so the heavier an object is, the greater the frictional force on it when it’s sliding. That should pretty well tie in with your personal experience, I’m sure. Pushing that filing cabinet is so much easier when you take the files out first.

The coefficient of friction itself depends on the nature of the two surfaces – so rubber on asphalt has a pretty high coefficient of friction, but steel on ice (ice-skate style) is extremely low.

Things are similar but slightly more complicated when an object doesn’t move. We use a coefficient of static friction now, but this time we have to say that the frictional force is less than or equal to the coefficient of static friction times the normal force. (If that force isn’t sufficient to hold the object in place, it will start sliding.) So, the larger the coefficient of static friction is, the steeper the ramp you need before an object starts sliding down it.

Now, things often get interesting with friction because the coefficient of kinetic friction can be considerably less that the coefficient of static friction. What this means is that an object can be hard to get moving, but, once it is moving, sliding it becomes much easier. An example is shifting furniture around our new house by pushing it across the carpet. The difficult bit is to get the chest of draws to move to start with – but once it is moving, maintaining its movement isn’t so tricky.

A great example of the interplay between kinetic and static friction is with bowing a violin string. The string moves in a ‘slip-stick’ manner. It will stick to the bow, and move with the bow, until the restoring force on it is large enough to get it to move across the bow (the ‘slip’) which it will then do very easily, returns towards its original position and overshoots (like simple harmonic motion). Restoring forces then bring down the velocity of the string, and, once the velocity of the string is reduced, it sticks again and the cycle continues. A nice little animation and comprehensive explanation can be found here.


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