Introduction You already know what friction is, from your everyday experience. Some people call it "traction". If you try to slide a box across a floor, you know that it'll take some effort to get it going, and keep it going, because of the friction between the box and floor. It's easier if the floor is smooth, and easier still if the box has wheels. You may have also noticed that usually it's easier to keep the box moving, once it's moving, than it was to get it started.
Here's a song about friction.
What causes friction? That isn't a trivial question to answer. First, realize that two surfaces that are touching are not really touching across the entire area that appears to be touching, because the surfaces are rough.There is no such thing as a perfectly flat surface. Seen through a high-power microscope, a typical surface looks like the following pictures. (These are, left to right: a typical metal surface, polished metal, wood, and porcelain.)
So, two surfaces in contact look like this, on a microscopic level:
Notice that the two surfaces are only really touching at a few places. At these places of true contact, friction can happen for a couple of reasons:
Activities & Practice
1. Play with this simulation of what's happening at the microscopic level that causes friction.
Static Friction vs Kinetic Friction If
two surfaces are in contact and are not sliding, then the chemical bonds
have enough time to
and the upside-down-mountains in the upper object are fully seated in
the valleys between the mountains of the lower object. So, for most materials,
the friction needed to break free from a stopped position is greater
the friction needed to keep moving after movement starts. The frictional
force when the surfaces are not sliding relative to each other
is called static
friction, FFS. If you push lightly
on a box resting on the floor, it won't move. Push a little harder, and
it still won't move.
Apparently, the static frictional force can adapt — it can
be any value from 0 Newtons, up to some maximum value, which we'll call
the surfaces are sliding,
we call that friction kinetic friction, FFK.
2. Play with this simulation of Motion and Forces, working through all four sections. In each case, turn ON the display of the force vectors using the checkboxes.
The Role of the Normal Force In both
cases, static and kinetic, the harder the two surfaces are pressed into each
other (in other words, the normal force, FN), the greater the
The Role of the Materials How much
friction there is also depends on what the two objects are made out of.
A hockey puck experiences less friction sliding over ice than it would
if, say, the rink were coated with rough sandpaper. It is convenient
to characterize the "slipperyness" or "stickyness" of two materials rubbing
against each other with a number. A fancy word for "number" is coefficient.
Putting the factors together mathematically. Recapping the above discussion...
"Apparently, the static frictional force can adapt — it can be any value from 0 Newtons, up to some maximum value, which we'll call FFS,Max."
"In both cases, static and kinetic, the harder the two surfaces are pressed into each other (in other words, the normal force, FN), the greater the frictional force." In other words, the maximum static friction, and the kinetic friction, are both proportional to the normal force. The proportionality constants are, respectively, the coefficient of static friction (µs) and the coefficient of kinetic friction (µk). The greater the µ, the more "sticky" the surfaces are.
"The friction needed to break free from a stopped position is greater than the friction needed to keep moving after movement starts."
3. Push a filing cabinet in this friction simulation. Click-n-drag the cabinet to apply force to it, and see the static friction oppose you until you push hard enough. There is a live free-body diagram at the upper-right corner. Turn on the applied force and other graphs. Experiment.
Here are a few specific questions:
(a) How much force does it take to get the refrigerator moving?
(b) How much force does it take to keep the refrigerator moving at a constant speed?
(c) If you double the mass of the refrigerator from 200 kg to 400 kg (use the "Additional Controls" button at the lower right), what happens to your answers to (a) and (b)?
Coefficients of Friction
The coefficients of static and kinetic friction are determined by experimentation. Take two objects, measure the normal force between them and the applied force needed to (a) start the sliding, and (b) keep the sliding going at constant velocity.
Coefficients of friction for myriad materials can be found in standard reference books used
by scientists, engineers and designers.
Example A 25-kilogram cardboard box full of books is sitting on a level concrete floor. A person pushes the box sideway with a force of 47 Newtons.
(a) What is the coefficient of kinetic friction?
(a) The box weighs F = m · g = 25kg · 9.8N/kg = 245N.
(b) The box now is only 14 kg in mass, and its weight is Fg = m · g = 14kg · 9.8 N/kg = 137 N. The result of part (a) says that, for this particular combination of cardboard and concrete, the friction will be 19% of whatever the normal force is.
Implications for Your Life
If you are ever trying to push a box across a floor, don't let your applied force have a downward component. Doing so is inefficient, for two reasons. First, the downward component of your push just presses the box harder into the floor. In other words, it increases the normal force, which isn't your goal. Secondly, that increased normal force increases the friction. Instead, push perfectly horizontally. Better yet, pulling with some upward component of force will reduce FN and therefore reduce the friction.
Additional Activities & Practice
4. A particular box, weighing 20. N, is being pushed on a horizontal floor. If requires a 7.0-Newton horizontal push to "break" the static friction, and 5.0 Newtons to keep it sliding with a constant velocity. Solution
5. Car tires are designed to have high coefficients of friction. Let's say a particular tire, on typical asphalt, has µk = 0.70 and µs = 0.80. Let's say four of these tires are on a car that is 1200 kg in mass. Solution
6. A book with mass 0.30 kg is pressed into a wall by Mr. Regester, pushing purely horizontally. The coefficient of static friction is 0.15. The book is not moving. Solution
7. A 50.0-kilogram wooden box is placed on the floor. You push sideways on the box. At first it doesn’t move, but when you push hard enough (140 Newtons) it starts to slide. To keep it sliding with a constant velocity, you only have to push sideways with 120 Newtons of force.
8. What are the names of the two kinds of friction we have discussed?
What kind of friction is there between the tires and the road, when a car is skidding?
What kind of friction is there between the tires and the road, when the car is starting or stopping normally, without skidding?
Which will stop you in a shorter distance in an emergency? (A) stepping hard on the brake pedal, but just short of skidding, or (B) stepping on the brake pedal as hard as you can, causing the car to skid.
9. A 950-kg car is driving on a level road. The coefficient of static friction between the tires and the road is 0.85, and the coefficient of kinetic friction is 0.75. The car is travelling at 22 m/sec.
(c) What is the magnitude of the friction force?
(d) What is the resulting acceleration of the car?