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• CBSE Physics Class 8 Notes

# Increasing and Reducing Friction

The force that acts between two bodies which are sliding or trying to slide against each other is known as friction. For example, when we push a box along a rough floor, friction is responsible for making the task difficult.

Friction is also known as an opposing force since it always acts in the opposite direction of a body that is moving or trying to move. A moving body is slowed down due to the virtue of friction. At times, friction is useful since it stops car tires from skidding on the road and also helps us to walk on the pavement without slipping. While walking, the friction caused between the tread on shoes and the ground prevents us from slipping.

Sometimes, too much friction is unnecessary, and we want to reduce friction. For example, friction between machine parts reduces the efficiency of the machine and in order to reduce this friction, we oil the machine parts. Oil helps to separate the surfaces and this helps to reduce the friction between them.

### Factors affecting Friction

There are many factors that affect the frictional conditions at the interface between two surfaces in relative motion. These factors are as follows:

• Surface Finish- The frictional coefficient is drastically affected by the roughness, number, and even the directional contact points of the asperities on the surfaces.
• Temperature- The overall level of cold or heat or cold in an environment can affect friction. For example, temperature determines whether an anti-wear or extreme pressure additive will be effective in certain applications.
• Operational Load- Friction varies directly with the load. A load that exceeds the designed capacity will drastically increase the frictional coefficient of friction.
• Relative Speed- Increasing the speed beyond the specified safety level will dramatically increase friction.
• Nature of the Relative Motion between the Surfaces- The frictional coefficient is also affected by the sliding motion versus the rolling motion.

### Methods of Increasing Friction

Method 1: Create an uneven or rugged or adhesive point of contact. When two or more bodies either slide or rub against each other, there are three things that may happen: small irregularities, nooks, and crannies on the surfaces can catch on each other; one or both the surfaces can deform due to motion; and lastly, the atoms within each surface can interact with each other. Practically, all three of these effects do the same thing: generate friction. An adhesive interaction with other surfaces (like tacky glue, etc.) is an easy way to increase friction.

Method 2: Pressing the two surfaces together harder. A fundamental principle of basic physics is that the friction experienced by a body is directly proportional to its normal force. This implies that we can increase the friction between two surfaces can be increased if we press the surfaces into each other with a greater force.

Method 3: Stopping any relative motion. That is, if one body is in motion with respect to another body, stop it. Until now, we have focused on sliding friction, which is also known as kinetic friction-the friction that occurs between two bodies as they slide against one another. In fact, this friction is different from static friction, which occurs when a body just starts to move against one another. The friction between two bodies is the highest right when they start moving against one another.  This friction decreases, once they are gradually in motion. This is one of the most important reasons why it’s harder to start pushing a heavy body than it is to keep it moving.

Method 4: Remove lubrication between the two surfaces.  Oil, grease, petroleum jelly, etc. are lubricants that can greatly reduce the friction between two objects or surfaces. This is due to the fact that friction between two solids is much higher than the friction between those solids and the liquid between them. To increase friction, we need to remove any lubricants from the scenario, using only dry and un-lubricated parts to generate friction.

Method 5: Increase the fluid viscosity. Besides solid objects, fluids (liquids) and gases (like air) can also generate friction. The amount of friction generated by a fluid as it passes against a solid depends on several factors. One of the easiest of these to control is the fluid viscosity since the greater the viscosity of the liquid, the greater is the friction between the fluid and the solid. The highly viscous fluids (ones that are “thick”, “gooey”, etc.) generate more friction than fluids that are less viscous (ones that are “smooth” and “liquid”).

Method 6: Increase the area exposed to air. As noted in the previous point, fluids like water and air can generate friction as they move against solid objects. The frictional force that an object experiences as it moves through a fluid is called drag. One of the most important properties of drag is that objects with bigger surface area, to the fluid as they move through it â€” have a greater drag.

Method 7: Use a shape that has a greater drag coefficient. A variety of shapes interact with fluids in a variety of ways as they pass through them â€” this implies that some shapes can have greater drag than other shapes that are made out of the same amount of material. The drag coefficient is the quantity that measures the relative amount of drag a shape makes hence shapes with high drags are said to have high drag coefficients.

Method 8: Use a less permeable material. Some materials are permeable to fluids. In Laymanâ€™s terms, they have holes in them that allow the fluid to pass through them. This readily reduces the area of the object that the fluid is able to push against and this lowers the force of drag. This property holds even if they are microscopic holes â€” as long as the holes are large enough to let some of the fluid pass through the object, the drag will be reduced. This is why parachutes which are designed to produce lots of drag to slow the speed of the user’s fall, are made out of strong, light silk or nylon and not cheesecloth or coffee filters.

Method 9: Increase the speed of the object. It does not matter what the shape of an object it is or how less/more permeable the material itâ€™s made from is, the drag which it creates will always increase as it goes faster. The faster a body moves, the more fluid it has to move through, and, thus, the greater drag it experiences. Bodies moving at very high speeds will experience very high friction due to drag, so these objects must be streamlined or else they will fall apart under the force of the drag.

### Methods of Reducing Friction

Method 1: Objects that move in fluids such as boats, planes, cars, etc, the shape of their body must be streamlined in order to reduce the friction between the bodies of the objects as the fluid.

Method 2: Friction can be reduced by polishing the surface of a body as polishing makes the surface smooth and even.

Method 3: Lubricants such as oil or grease must be applied to machine parts regularly to reduce the friction between them.

Method 4: Suppose an object is rolled over a surface, the friction between the rolled object and surface can be reduced by using ball bearings.

Method 5: Friction between two surfaces can also be reduced by reducing the contact between the surfaces.

Method 6: Ball bearings are used in manufacturing vehicles, bicycles, and vehicles to reduce friction.

### Sample Questions on Friction

Question 1: What happens when you decrease the amount of friction?

Less friction means it is harder to stop. The low friction scenario occurs to cars when it rains. That’s why there are often so many accidents. Even though the friction of the brakes is still present, the brakes may be wet, and the wheels are not in as much contact with the ground surface.

Question 2: What are 3 ways friction can be reduced?

Polishing the rough surface. Adding bearings or wheels between the moving parts of a machine or vehicles reduce friction and allow smooth movement as rolling friction is less than sliding friction.

Question 3: Can we reduce friction to zero?

Friction can be reduced to a great extent by polishing surfaces or by using a large amount of lubricants such as oil, water, or grease but we cannot reduce friction to zero. We cannot completely eliminate friction as it is necessary for any motion without it we cannot imagine any motion as there is no motion on a smooth surface.

Question 4: On what factors friction depends?

Friction mainly depends on two factors: 1) the material that is in contact with the body 2) the force which is pushing the two bodies together

Question 5: Does friction depend on mass?

Friction DOES NOT depend on the mass of the body. It only depends on the normal force and the roughness of the surface in contact as it is generated during actual contact only.

Question 6: What do we call the substance that is used to reduce friction?