Here is a low-friction penguin 🙂
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.
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.
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
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
PS2.B: TYPES OF INTERACTIONS
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).
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.