type of friction and motion?

Question

What kind of friction is the Friction between 2 surfaces that are not moving past each other?

What kind of friction occurs when solid objects slide over each other.

What kind of motion is the force exerted by air or liquids (water, oil)

Answer 1

Hope this answers your questions. Best of luck.

Sliding friction
When two solid objects are in contact and a force is applied to slide one object against the other, sliding friction force resists the motion. If F is the force pushing on an object and Fr is the force of friction, the relationship between F and Fr will determine whether the object will slide or not move at all.

Kinetic friction
If force F is greater than friction Fr (written as F > Fr), then the object will slide or move. The friction is considered kinetic friction, which means moving friction.

Pushing force greater than friction force

Static friction
If F is less than Fr (written as F < Fr), there is no motion and the objects remain static with respect to each other. In this case, the friction is considered static friction, which means it is not moving.

Static > Kinetic
What is interesting is that the static friction that holds an object in place is greater than the kinetic friction that slows down a moving object. In other words, once you start an object moving, the friction decreases from the static friction holding the object in place.

You have seen this in trying to slide a heavy box across the floor. It may be very difficult to move, but once it starts sliding, it is easier to push.

Causes of sliding friction
The causes of sliding friction are surface roughness of the materials, molecular attraction or adhesion between the materials, and deformation resistance in the case of soft materials.

Rolling friction
When a ball or wheel is in contact with a solid object and a force is applied to the wheel, it will start to roll due to the friction at the point of contact with the other surface. This is the starting friction for a wheel.

Once the wheel starts rolling, there is a resistive force that slows the wheel's motion on the other surface. This is called rolling friction.

Starts rolling
When a force is applied to a wheel is not enough to overcome the static force of friction, the wheel will start to roll. If the force is greater than the static resistance, the wheel will slide or spin. It will also roll, but not at the same rate as with static friction.

A good example of this is accelerating an automobile on wet pavement. Pushing on the accelerator peddle too hard will cause the wheels to spin, and the car will not move forward as fast as when you push on the gas peddle less.

Slows rolling
Once the wheel is rolling, friction at the point of contact with the other surface slows down the motion of the wheel. Typically, rolling friction is much less than sliding friction. A wheel can roll for some distance before slowing down and stopping.

But there are situations where rolling friction can be large. Trying to ride a bicycle in loose dirt is an example of friction greatly slowing down the rolling motion.

Less than sliding
An advantage of rolling friction is that it is much less than sliding friction.

When the Great Pyramids were being build in ancient Egypt, they used logs as rollers under the giant blocks of granite instead of trying to slide the rocks along the ground.

Rollers reduced friction when moving granite block

Causes of rolling friction
The causes of rolling friction are similar to that of sliding friction. They are surface roughness of the materials, molecular attraction or adhesion between the materials, and deformation resistance in the case of soft materials. Included in surface roughness is the existence of treads on the wheel or tire.

Fluid friction
When a solid object is in contact with a fluid, such as a liquid or gas, and a force is applied to either the object or the fluid, there is a friction force that resists the motion. Examples where fluid friction occurs are water flowing through a hose, an airplane flying through the atmosphere, and oil lubricating moving parts.

Static and kinetic
If the viscosity or thickness of the fluid is great, there may be no movement due to static friction. One example is trying to move heavy grease through a hose. You need to apply a great pressure to finally break the static friction and start the grease moving.

Once a fluid moves through a hose or an object is moving along a fluid, the resistance is considered kinetic friction. The grease will still move much slower than a fluid with low viscosity, like water.

Note that it is also possible to have fluid friction with moving one fluid in contact with another fluid. That subject is usually classified as part of Fluid Dynamics and is not within the scope of our lessons.

Causes of fluid friction
Causes of fluid friction are turbulence effects from surface roughness and deformities, molecular attraction or adhesion between the materials, and deformation resistance of the fluid.

In conclusion
Friction is a force that resist the motion of the object that is in contact with another object or material. The forms of motion determine the types of friction, which are sliding, rolling and fluid friction. Friction be static when the objects don't move or kinetic when they are moving. The cause of friction is a combination of surface roughness, molecular adhesion and deformation effects.

Answer 2

Static friction is the friction that keeps something in place, for example: Try pushing a car, it is very hard to do because the force of static friction between the wheels and the ground is high. Kinetic friction is the friction that opposes the motion of a moving object, for example: The friction between the tires and the ground while a car is driving. If you combine the two, you see that you must put in a certain amount of force to OVERCOME the static friction and cause the object to start moving. Static friction's coefficient of friction is set as a range between 0 and its maximum, producing a range of force you can apply to an object before it moves. Kinetic friction's coefficient of friction is a single number and it represents one surface's friction with the second surface. Hope this helps!