Brief Summary
Alright, so this lecture gives a good overview of friction, covering its types, how it behaves, and where it's useful or not. We look at external vs. internal friction, static vs. dynamic friction, and fluid vs. solid friction. Key takeaways include understanding when friction is helpful (like walking or braking) and when it's a pain (like in engines). Also, we learn about Coulomb's laws of dry friction and how to apply them, plus a heads-up on a problem we'll tackle next time.
- Friction can be both beneficial and detrimental depending on the application.
- There are different types of friction: external (static and dynamic) and internal (fluid and solid).
- Coulomb's laws of dry friction provide a basis for understanding and calculating frictional forces.
Introduction to Friction
So, we're starting with friction, which, you know, usually has a negative vibe. But friction is actually quite important. Like, you need it to walk, otherwise you'll be slipping all over the place like on a glass floor. Even simple machines like 'atta chakki' use friction with belt drives. And brakes? Totally rely on friction to stop your car. Basically, friction is needed for a lot of things we take for granted.
Undesirable Effects of Friction
But, friction can also be a problem. Think about engines – all that oil is there to reduce friction between the piston and cylinder. Journal bearings use oil to keep metal parts from touching. In workshops, you see oil being used everywhere to lubricate machines. Some systems even flood parts with oil to minimize friction. So, yeah, sometimes you really want to get rid of friction.
Reactions on Rough Surfaces
When we look at supports, like pin joints, they stop movement in two directions, leading to two unknown reactions. Same goes for frictionless thin bearings. Now, with rough surfaces, you've got a normal force and a frictional force. Figuring out the direction of the frictional force is the tricky part. If motion is about to happen, you can use friction equations. Otherwise, you gotta rely on free body diagrams to figure things out.
Classifications of Friction
Friction can be broadly classified into external and internal types. External friction includes static and dynamic friction, while internal friction includes fluid and solid friction. External friction is the interaction between surfaces of two solid bodies in contact. Static friction occurs when surfaces are at rest with a tendency for relative motion, while dynamic friction occurs when surfaces are in relative motion. The term kinetic friction is now generally referred to as dynamic friction.
Fluid Friction
Fluid friction happens between fluid layers moving at different speeds, especially when lubricating oil separates metal surfaces. The friction force here is proportional to the relative velocity and the fluid's viscosity. Unlike dry friction where force is proportional to acceleration, fluid friction is all about velocity. This is useful in pipes, lubricated surfaces, and even shows up as frictional drag in cars and high-speed trains, where streamlined designs minimize it. Even athletes' clothing is designed to reduce this drag. In vibration studies, it's called viscous damping, like in a dash-pot.
Solid Friction
Solid friction comes into play when solid materials are under cyclic loading, dissipating energy inside the material. Even a spring oscillating in a vacuum will eventually stop due to this. The energy lost per cycle doesn't depend on frequency but is proportional to the square of the vibration's amplitude. Interestingly, the frictional force here is proportional to displacement. This is different from fluid friction (proportional to velocity) and regular friction (proportional to acceleration).
Dry Friction
Now, let's talk dry friction. If you have a block and no force is acting on it, no friction develops. Friction only kicks in when there's a tendency for movement. As you increase the applied force, the frictional force increases too, up to a limit. Once you hit that maximum, the object is about to slide. If you increase the force further, dynamic friction takes over, and the frictional force actually drops. This is why it's easier to keep pushing something once you get it moving.
Coulomb's Laws of Dry Friction
Coulomb's laws, building on Amonton's work, state that the frictional force is directly proportional to the normal load between surfaces. This coefficient of friction is for a pair of materials, not just one. Also, the maximum static frictional force is independent of the apparent area of contact. Coulomb added that the kinetic or dynamic frictional force is independent of sliding velocity. The frictional force is less than or equal to µsN, where µs is the coefficient of static friction and N is the normal load.
Factors Affecting Static Friction
The coefficient of static friction (µs) depends only on the contacting surfaces. Experiments show that even if you vary the load significantly (like from 1 gram to 1 ton), µs remains the same. Similarly, µs stays constant even if you change the contact area by a large factor (like 250 times). So, for practical purposes, we can say it's independent of both the contact area and the load.
Experimental Determination of Friction Coefficient
The simplest way to find the coefficient of static friction is using an inclined plane. You put a block on the plane and slowly increase the angle until the block slips. The angle at which it slips is related to the coefficient of friction. Changing the surface area of the block doesn't affect the angle. The angle at which the block is about to slide is called the friction angle (φs), and µs = tan(φs). This isn't dependent on mass or gravity.
Dynamic Friction and Stick-Slip Phenomenon
At higher speeds, dynamic friction (µd) comes into play. For metal-to-metal contact, µd decreases with increasing velocity, especially in poorly lubricated conditions. This can lead to stick-slip, causing those eerie creaking sounds you hear from poorly lubricated doors. Well-lubricated surfaces, on the other hand, increase µd with sliding velocity, minimizing these oscillations.
Selection of Appropriate Frictional Laws
To choose the right friction laws, consider the surface conditions: dry, greasy/partially lubricated, or fully lubricated. For dry or partially lubricated surfaces, use Coulomb's laws. If there's a film separating the surfaces, use viscous friction laws. In dry friction, the maximum frictional force is proportional to the normal load and occurs just before slipping. Remember, you can't just assume frictional force = µN; you need to check if motion is impending. Dynamic friction is always less than the maximum static friction.
Applications of Friction and Problem Introduction
Dry friction principles are useful for studying wedges, power screws, partially lubricated bearings, brakes, clutches, and belt drives. To understand these concepts better, we'll solve a problem next time. We'll look at a block with a weight of 200 Newtons and different applied forces, and we'll figure out the block's condition and reaction forces. The coefficient of static friction is 0.3, and the dynamic coefficient is 0.25.
Summary of Key Points
So, in this class, we covered the types of friction: external (static and dynamic) and internal (fluid and solid). We learned that frictional force can be proportional to displacement (solid friction), relative velocity (fluid friction), or acceleration (dry friction). Remember, frictional forces only develop when there's a tendency for relative motion, and they increase with external forces until motion is impending. Only then can you use the equation frictional force = µ times the normal force.
