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Friction forces

Here is a low-friction penguin 🙂

penguin low friction GIF

Try to slide a heavy object across a floor:
at first it may not move.
Friction holds the object in place:
the object is static (not moving.)

When you apply enough force the object begins moving:
it is kinetic (in motion)
Now we need to apply less force to keep it moving.

Conclusion 1: static friction is a strong force.
At first, the more force you apply, the more friction pushes back.
Keeps the object locked into place.

Conclusion 2: Once you overcome a certain barrier,
most friction forces is broken:
Whatever causes static friction forces can’t be
re-established while the object is moving.
So while moving, friction will always be much less.

friction lab 2

Let’s see the results of an experiment – a block is on a rough surface.

Attach a spring scale to it – and pull the block.

The scale lets us see how much force we are applying to the block as we pull it.

As we pull more and more, we see the force increase – yet the block doesn’t move (it’s static)

Suddenly, the block breaks loose and starts moving – and at that instant the amount of force on the scale drops!

As long as we keep the block moving, the force is less than it was, when it was standing still.

This visually shows that  [static coefficient of friction] >[kinetic coefficient of friction]

Experiment: Add mass to the object. You’ll need to push (or pull) harder to get it started – and also need to push or pull more to keep it moving.

Experiment: If you oiled the floor then its easier to get the object moving. Conclusion: some fluids can disrupt the formation of whatever creates friction.

Here is a cat – notice that the coefficient of friction between the cat’s paws and the table is very low.

Low friction cat on table

What is the molecular origin of friction?

The surfaces are rough. When you get an object moving, you must (slightly) raise the object until it can skip along – with just the tips of the surface hitting, or breaking off the points.

Notice that friction always opposes the motion.


The friction force is proportional to the squeezing force

The harder the surfaces are pushed together (gravity) the more force is needed to move them.

Source of friction?

* adhesive forces between the surface molecules of the two objects:

* adhesion varies with the type of surface

* this is a complicated aspect of surface physics.

Once an object is moving, there are fewer points of contact, so less force required to keep it moving.

* At small speeds friction is nearly independent of speed.

(text above adapted from ABE Advanced Level Physics, Derived from College Physics by OpenStax College)

The molecular origin of friction





Excerpted from:


* many frictional forces.

* point opposite the direction of motion, and parallel to the surfaces of contact.

* this symbol is written as a cursive ‘F’ in some books, but we’ll use ‘f’.

* we focus on friction due to contact between two surfaces.

* we use Greek letter mu μ for coefficient of friction.

The coefficient of friction is a percentage of the normal force.

* found through experimentation
* depends on the nature of the materials in contact.
* Occasionally there is no friction (like being on slick ice)

Top material

Bottom material


Static coefficient of friction


Kinetic coefficient of friction




– – –


on dry wood




on clean iron




on clean ,dry, oak

– – –



on steel



on clean glass

0.9 – 1.0

– – –

Rubber tire

on asphalt




Static friction = the two surfaces not sliding across each other.

Kinetic friction = the two surfaces are sliding across each other.

The standing girl on the slide, (right) has static friction between her shoes and the slide.
These two surfaces are not moving relative to each other.

The girl moving down (left) is experiencing kinetic friction between her pants and the slide.
This is because the pants are moving relative to the slide’s surface.

In both cases friction opposes the direction of motion, and is parallel to surface.
The free body for each girl would look like the one below.
(other forces dimmed to highlight the friction)

Friction acts parallel to the surfaces that are in contact, and in the direction opposite to the motion of the object.

Friction depends on the nature of the materials, and their smoothness.

Kinetic (sliding) friction is less than static friction.

Kinetic friction is pretty much independent of speed.

Friction is practically independent of the area of contact. (this is not intuitive)

Friction is proportional to the force pressing the surfaces together.

Drawing the frictional force on a free body diagram.

Notice that the frictional force’s vector is (1) parallel to the force, (2) opposite the direction of motion -or intended motion.


     Tension exists in any body that is pulled by to opposing forces.  Typically we talk about ropes and chains as being in tension but any body can be in put in tension.


A block of mass m = 0.75 Kg is pulled along a table at constant velocity. If it takes 2.0 N of force to maintain constant velocity, calculate the coefficient of friction for this system.

Solution: First draw a diagram of the situation. The 0.75 Kg mass exerts a force (F = mg) on the table, and the normal force is equal to it. I have not bothered to make these two forces of opposite sign but, strictly speaking, they are opposite vector forces. In these cases of constant velocity, there is no acceleration, so all forces must be balanced. Therefore it doesn’t matter how fast the block is moving, only that it is moving at constant velocity. Then the friction force Ff is equal to the pulling force of 2.0 N (but again of opposite sign, which I’ll ignore).

Now it’s a simple matter of rearranging the friction equation and using the normal and friction forces to calculate the coefficient of friction. Notice that in calculating μ, the units cancel.

Example 2

A 4.5 Kg mass is pulled up a 30˚ ramp at constant velocity by a falling 8.0 Kg mass, as shown. Calculate the coefficient of friction.

Solution: Here’s the diagram.

 All of the relevant calculations are shown above. Notice that this is another of those problems that makes μ easy to find because the force up the ramp (22.0 N) is equal to the frictional force. The normal force of 38.2 N gives the result:


Learning Standards

2016 Massachusetts Science and Technology/Engineering Curriculum Framework

3-PS2-1. Provide evidence to explain the effect of multiple forces, including friction, on an
object. Include balanced forces that do not change the motion of the object and
unbalanced forces that do change the motion of the object.

HS-PS2-1. Analyze data to support the claim that Newton’s second law of motion is a
mathematical model describing change in motion (the acceleration) of objects when
acted on by a net force… Forces can include contact forces, including friction.

A FRAMEWORK FOR K-12 SCIENCE EDUCATION: Practices, Crosscutting Concepts, and Core Ideas
What underlying forces explain the variety of interactions observed?

All forces between objects arise from a few types of interactions… Any two objects in contact also exert forces on each other that are electromagnetic in origin. These forces result from deformations of the objects’ substructures and the electric charges of the particles that form those substructures (e.g., a table supporting a book, friction forces).

Massachusetts Science and Technology/Engineering Curriculum Framework (2006)

1. Motion and Forces. Central Concept: Newton’s laws of motion and gravitation describe and predict the motion of most objects.
1.6 Distinguish qualitatively between static and kinetic friction, and describe their effects on the motion of objects.

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