In this science article we investigate the force of friction and how it opposes the forward motion of solids.In Forces of Drag we investigate friction in fluids, including friction in water and air.
Aristotle (384BC – 322 BC), the ancient Greek scientist and philosopher, studied the physics of motion.Aristotle believed that some external force had to be applied to an object to keep it in motion.As far as Aristotle was concerned this chariot could only stay in motion so long as it was being pulled by horses.
Remove the pulling force of the horses, the ‘violent’ force as Aristotle called it, and the chariot would come to a halt.
While Aristotle understood the principle of applied forces, including pulling and pushing forces, he failed to identify forces which oppose forward motion. These opposing forces include friction and air resistance.
It was not until the 15th century that friction was finally identified as a force which opposes forward motion.
Leonardo da Vinci (1452-1519)…..
…measured the angle at which a block of wood started sliding down an inclined plane from a position of rest.
He also measured the weight needed….
….to pull a block of wood across a horizontal surface (a ‘flat plane’).
Another Italian scientist Galileo Galilei (1564–1642) carried out experiments with inclined planes.
He observed how a ball released down a rough surface failed to reach the same height on the other side.
Galileo concluded that it was the opposing force of friction, the interaction between ball and surface of the ramp, which prevented the ball rising to the same height.
As a ball runs down an inclined plane, under force of gravity, it is in contact with the surface.
The surface of the ball and inclined plane appear smooth to the naked eye. However when examined under an electron microscope both surfaces appear rough and bumpy….
….just like the surfaces of these ‘smooth’ metal pipes!
We call this interaction between ball and inclined plane the force of friction.
Friction is the contact force which slows down the forward motion of the ball as it rolls down an inclined plane.
If the same ball is rolled down an inclined plane with a smooth surface…..
….friction is greatly reduced. The reduced friction of a smooth surface means that, on the upslope, the ball will roll higher.
We now investigate friction when force is applied to an object on a horizontal surface.
The first thing to say is that there can be no friction if there is no force or attempted force applied to an object.
This man just watches the crates; he makes no attempt to apply any force to the crates; the crates are at rest (stationary) so there can be no friction.
The man now applies a pushing force of 158 newtons onto the crates; each crate has a mass of 50 kgs. A pushing force of 158 newtons is not enough to start the crate moving. The crates remain at rest.
The forces are in balance; the applied force equals the opposing frictional force. So long as the crates remain at rest the frictional force is static (ie not moving). It is static friction which keeps the crates at rest.
The crates still remain stationary when then man pushes harder with a force of 250N. The opposing frictional force is also 250N.
It is only when the man applies a pushing force of 251N that the crates start moving. The applied force of 251N is now greater than the friction force of 250N.
The crates are now in motion. Static (stationary) friction now becomes kinetic (moving) friction.
The man now pushes the crates even harder applying a force of 281N; the crates now accelerate.
The applied force (281N) exceeds the friction force which remains the same at 250N. The ‘sum of forces’, the difference between the applied force and the friction force, is now 31N.
As the man pushes harder the crates accelerate faster.
The forces are now very unbalanced with the applied force (368N) greatly exceeding the opposing friction force (250N).
In this example the value of kinetic (moving) friction remains the same at 250N no matter what the velocity of the crates. This is because the surface the crates are being pushed across is unchanging.
When the man applies a lesser force of 250N the forces are in balance. The crates now move forward at a constant velocity and neither speed up nor slow down.
Notice how it takes a pushing force of 251N to get the crates into motion. Contrast this with the pushing force (250N) required to keep the load moving at a constant velocity.
Starting the crates in motion, when static friction has to be overcome, requires more force than keeping the crates in motion, when kinetic friction has to be overcome.
When the applied force (123N) is less than the frictional force (250N) the crates slow down. The forces are unbalanced once again.
If the load is increased it will need an applied force of 444N to move the box from rest and overcome static friction.
From this example you can see that a heavier load creates a greater opposing frictional force.
The answer lies in the interaction between the surface of the ground and the bottom of the crate.
This diagram represents the weight of a light load pressing down onto the ground…
….while this diagram represents the weight of a heavy load pressing down onto the ground.
The downwards force of the heavier load becomes more adhesive and sticks to the ground more. Because the gaps between the two surfaces are narrower, the heavier load will need more applied force to push it across the ground.
It is the weight of the load that increases friction…
….not the surface area of the load in contact with the ground.
Assuming the man can grip the surface with his shoes (which he couldn’t if the surface was really frictionless) it would take very little force to start the box in motion.
Once in motion the crates will move forward at a constant speed; they will stay in motion forever and the man will no longer need to push the crates unless an unbalanced force (such as a change in surface friction, air resistance or an obstacle etc) stops the crates’ forward motion.
It will still only take 1N of applied force to set a really heavy load in motion at a constant speed across a frictionless surface!
We need to remember that friction always comes in ‘pairs’; the strength of the opposing frictional force depends on the nature of the two surfaces that are rubbing together.
We compare the frictional force between different pairs of surfaces using the co efficient of friction. The co efficient of friction describes the force of friction in a mathematical way between two bodies which are attempting to move against each other or which are in motion against each other.
The co efficient of friction is represented by the Greek symbol μ (mu).
Co efficient of friction values usually range between 0 μ (frictionless) and 1 μ (very high frictional force between two surfaces.)
Ice and metal have a very low frictional co efficient of 0.03μ ; metal ice skates slide easily across ice. To describe it another way there is a very low resistance to the surfaces sliding across each other.
Rubber tires on dry road surfaces have a high frictional coefficient of 0.7μ. That means tires don’t slide easily when you apply the brakes or turn your vehicle into a corner. To describe it another way there is a very high resistance to the surfaces moving against each other.
Rubber tyres on wet road surfaces have a lower co efficient of friction at 0.4μ. There is a lower resistance to the surfaces moving against each other.
All the above are examples of kinetic friction where objects are already in motion.
Remember how it takes more force to start an object in motion than it does to keep that object in motion? This fact is reflected in the value given to the co efficients of friction. Static friction co efficients are greater than kinetic friction coefficients.
For example it takes more force to start moving a chair with wooden legs across a wood floor (friction co efficient of 0.5μ static ); it takes less force to keep the same chair moving across the wood floor (friction co efficient of 0.2μ kinetic ).
The static friction value of bone moving across bone in human joints is an incredibly low 0.01μ s The kinetic friction value is 0.003μ k
Such low values result from the lubrication of joints by naturally occurring fluids present in cavities between bones.
Moving parts of machinery are often lubricated with oil to reduce friction.
Oil reduces friction between materials that have a high co efficient of friction. The co efficient value of steel moving across steel is 0.74μ s and the kinetic value is 0.57μ k
Circular and spherical shapes are very efficient at reducing friction; these ball bearings reduce friction…
…at the center of a bicycle wheel hub.
1) Friction can be useful such as:
2) Friction can be harmful such as:
Explore friction and play this fun science game at Phet.colorado.edu
See how friction can affect us in our everyday lives!
Now you know why there are no square tyres on cars- creates too much friction!