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Updated on 06th October, 2023 , 7 min read
When we attempt to move an object across a surface, the movement is impeded by a connection between the object and the surface. This connection is due to the electromagnetic forces between the charged particles present at the surface and the object's surface that is in contact with it. This opposition is denoted by a force called friction. The focus of our discussion is specifically on 'kinetic friction.' As we delve deeper into this topic, we will explore the definition of kinetic friction, the work accomplished by friction, the distinction between static and kinetic friction, and the three types of kinetic friction. In addition, we will provide visual aids to aid in comprehending each concept thoroughly.
Kinetic friction, also known as dynamic friction, is the force that opposes the movement of an object relative to a surface. When an object slides, rolls, or moves in any other way over a surface, kinetic friction acts in the direction opposite to the motion of the object, making it harder to move. This force is caused by the irregularities in the surfaces of both the object and the surface it's in contact with, which create interlocking points and prevent the object from sliding smoothly. Kinetic friction depends on the type of surfaces in contact, their roughness, and the force pressing them together. It can be calculated by multiplying the coefficient of kinetic friction, which represents the interaction between the surfaces, by the normal force, which is the force pressing the object and surface together.
The formula for kinetic friction can be expressed as:
where Fk is the force of kinetic friction, μk is the coefficient of kinetic friction, and N is the normal force, which is the force perpendicular to the surface that the object is in contact with. The coefficient of kinetic friction is a constant that depends on the type of surfaces in contact and their roughness. It is a dimensionless quantity that ranges between 0 and 1, where 0 represents no friction, and 1 represents a strong frictional force. The unit of kinetic friction is the same as that of force, which is Newtons (N).
The force of kinetic friction is given by:
Fk = ma
where m is the mass of the object and a is the acceleration of the object. Since the object is moving with constant velocity, a = 0.
Fk = 0
However, this does not mean that there is no force acting on the object. There is a force of kinetic friction acting in the opposite direction to the motion of the object.
F - Fk = 0
where F is the external force applied to the object.
Fk = F
F = ma
Fk = ma
Fk = μkN
μkN = ma
a = μk (N/m)
where N/m is the force per unit mass, which is equal to the acceleration due to gravity, g.
Therefore, we get:
a = μk g
This is the derivation of the formula for kinetic friction, which relates the acceleration of an object to the coefficient of kinetic friction and the acceleration due to gravity.
There are three types of kinetic friction:
Sliding Friction: This type of friction occurs when an object slides over a surface. For example, when a box is dragged across the floor, sliding friction opposes its motion.
Rolling Friction: This type of friction happens when an object rolls over a surface. For example, when a wheel rolls on a road, rolling friction acts against its motion.
Fluid Friction: This type of friction occurs when an object moves through a fluid, such as air or water. The resistance to the object's motion is due to the viscosity of the fluid, which causes the fluid to stick to the object's surface and create drag. For example, when a swimmer moves through water, the fluid friction of the water opposes their motion.
The type and magnitude of kinetic friction depend on the properties of the surfaces in contact and the speed and direction of the object's motion.
Kinetic friction has many practical applications in our daily lives, some of which are:
Brakes: The braking systems in vehicles use kinetic friction to stop the motion of the vehicle. The brake pads press against the wheels, creating friction and reducing the speed of the vehicle.
Tires: The tires of vehicles use rolling friction to provide traction and enable the vehicle to move forward.
Walking: When we walk, the soles of our shoes experience kinetic friction with the ground, allowing us to move forward and maintain balance.
Sports: In sports like soccer, basketball, and tennis, the ball experiences kinetic friction when it rolls or bounces on the ground, which affects its speed and trajectory.
Manufacturing: The production of many goods, such as sandpaper, adhesive tapes, and grinding wheels, requires the use of abrasive materials that generate kinetic friction to shape or polish surfaces.
Energy conversion: Kinetic friction is also used in many energy conversion devices, such as generators and wind turbines, where mechanical energy is converted into electrical energy by inducing a rotating motion that experiences kinetic friction with the surrounding surfaces.
Understanding the properties and behavior of kinetic friction is essential in designing and optimizing these applications.
There are actually four laws of kinetic friction:
The force of kinetic friction is proportional to the normal force: As the normal force increases, the force of kinetic friction also increases proportionally.
The force of kinetic friction is independent of the velocity: The force of kinetic friction is generally assumed to be independent of the velocity of the object. However, in reality, the force of kinetic friction can sometimes vary with the speed of the object.
The force of kinetic friction is dependent on the nature of the surfaces in contact: The coefficient of kinetic friction, which determines the magnitude of the force of kinetic friction, depends on the nature of the surfaces in contact and their roughness.
The force of kinetic friction is independent of the contact area: The force of kinetic friction is independent of the contact area between the object and the surface, as long as the normal force and coefficient of kinetic friction remain constant.
These four laws describe the basic behavior of kinetic friction and can be used to calculate the force of kinetic friction in different situations. However, it is important to note that the force of kinetic friction can vary depending on several other factors, such as the temperature, the presence of lubricants, and the surface quality, among others.
There are four main types of friction:
Kinetic Friction: This is the force that opposes the motion of an object that is already in motion. It is the force that needs to be overcome to keep an object moving at a constant velocity.
Static Friction: This is the force that resists the motion of an object that is at rest and prevents it from moving. It is the force that needs to be overcome to start the motion of an object.
Rolling Friction: This is the force that resists the motion of an object that is rolling along a surface. It is typically lower than kinetic friction, allowing objects such as wheels to roll smoothly.
Fluid Friction: This is the force that resists the motion of an object through a fluid (e.g. air or water). It is also known as drag and can be either laminar or turbulent depending on the speed of the object and the properties of the fluid.
Type of Friction | Definition | Example | Direction of Force |
Static Friction | Force that resists the motion of an object that is at rest and prevents it from moving | Trying to push a heavy box that won't budge | Opposes the direction of the applied force |
Kinetic Friction | Force that opposes the motion of an object that is already in motion | Sliding a box across a surface | Opposes the direction of motion |
Rolling Friction | Force that resists the motion of an object that is rolling along a surface | Rolling a ball across a surface | Opposes the direction of motion |
Fluid Friction | Force that resists the motion of an object through a fluid | Swimming through water | Opposes the direction of motion |
Some solved examples of Kinetic Friction are listed below:
Solution: The formula for kinetic friction is Fk = μkN, where Fk is the force of kinetic friction, μk is the coefficient of kinetic friction, and N is the normal force. Since the block is moving at a constant speed, the force of kinetic friction is equal and opposite to the applied force, which is the force needed to overcome the friction. Therefore, Fk = ma = 0, and the force of kinetic friction is:
Fk = μkN = 0.5 x (mg) = 0.5 x (10 kg x 9.81 m/s^2) = 49.05 N
Solution: The force of kinetic friction is equal to the force needed to overcome the resistance of the tires and keep the car moving at a constant speed. Therefore, the force of kinetic friction is:
Fk = μkN = 0.4 x (mg) = 0.4 x (1200 kg x 9.81 m/s^2) = 4704 N
Solution: The net force acting on the box is the applied force minus the force of kinetic friction. Therefore, the net force is:
Fnet = Fa - Fk = 40 N - (0.3 x 10 kg x 9.81 m/s^2) = 7.53 N
Using F = ma, the acceleration of the box is:
a = F/m = 7.53 N / 10 kg = 0.753 m/s^2
Solution: The net force acting on the crate is the applied force minus the force of kinetic friction. Therefore, the net force is:
Fnet = Fa - Fk = 200 N - (0.2 x 50 kg x 9.81 m/s^2) = 98.1 N
Using F = ma, the acceleration of the crate is:
a = F/m = 98.1 N / 50 kg = 1.96 m/s^2
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Kinetic friction is the force that opposes the motion of a moving object on a surface, which is caused by the interactions between the surfaces and the atoms or molecules in contact.
The formula for kinetic friction is Fk = μkN, where Fk is the force of kinetic friction, μk is the coefficient of kinetic friction, and N is the normal force.
The coefficient of kinetic friction depends on several factors, including the types of surfaces in contact, the roughness or smoothness of the surfaces, and the temperature of the surfaces.
Kinetic friction occurs when an object is already in motion, while static friction occurs when an object is at rest and needs to be overcome to start moving.
The weight of an object affects its kinetic friction indirectly, as it affects the normal force (N) acting on the object, which in turn affects the force of kinetic friction (Fk) through the formula Fk = μkN.
Kinetic friction does negative work on a moving object, as it acts in the opposite direction of motion and reduces the object’s kinetic energy.
The coefficient of kinetic friction does not vary significantly with velocity, as it depends more on the types and properties of the surfaces in contact.
Rolling friction is the force that resists the motion of a rolling object on a surface, which is caused by the deformation and adhesion of the surfaces in contact.
Fluid friction is the force that opposes the motion of an object through a fluid, such as air or water, which is caused by the viscosity and turbulence of the fluid.
The force of kinetic friction can be reduced by using lubricants between the surfaces in contact, making the surfaces smoother or more polished, or reducing the normal force acting on the object.
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