Friction is the force of opposition between two surfaces that are in relative contact. Friction is very important for moving, running machines and stopping them. If there is no friction we cannot move, cannot apply brakes on vehicles. Thus, it impacts all types of moving systems. As soon as the body is pushed or pulled, friction enters. The friction may be static or kinetic. If objects stop moving, the friction is static.
If objects move, the friction is kinetic which may be in form of sliding, skidding, or rolling. Sometimes friction is taken as an energy–transforming effect. As in a braking vehicle, the moving energy is converted into heat.
No surface is completely smooth. The contact points between the two surface areas form a sort of cold welds. Due to these cold welds, there is friction in almost every moving surface. The higher the pressing force higher the friction between the moving surface areas.
Friction is equal to the applied force that tends to move a body at rest. The maximum value of friction is referred to as the force of limiting friction (F). The ratio between friction and Reaction (R) is constant which is called the coefficient of friction. And we can get the value as Fs = µ mg.
Have you ever observed why a moving ball stops? Why cycle stop when a bicyclist stops pedaling?
Naturally, there should be some force that opposes moving things. That force not only moves objects but also stops them.
Definition of Friction
Friction is a tangential resistant force between two surfaces that oppose the motion of moving objects.
Friction is a force that enters the action as soon as a body is pushed or pulled over a surface. In the case of solids, the force of friction between two bodies relies on lots of factors such as the nature of the two surface areas in contact and the pressing force between them.
Rub your palm over different surfaces such as table, carpet, polished marble surface, brick, etc. You will discover smoother is the surface, the easier it is to move over the surface. Plus, the more forcibly you push your palm over the surface, the harder would it be to move.
Why friction oppose motion?
No surface is completely smooth. A surface area that appears smooth has pits and bumps that can be seen under a microscope. A magnified view of two smooth surfaces in contact reveals the spaces and contacts between them. The contact points between the two surface areas form a sort of cold welds.
These cold welds withstand the surfaces from sliding over each other. Including weight over the upper block increases the force pushing the surfaces together and hence, increases the resistance. Therefore, the higher the pressing force higher the friction between the moving surface areas.
Formula of Friction
Friction is equal to the applied force that tends to move a body at rest. It increases with the applied force. Friction can be increased to a particular optimum value. After this friction does not increase. The maximum value of friction is referred to as the force of limiting friction (F). It depends upon the reaction (pressing force) between the two surface areas in contact. The ratio between the force of limiting friction F and the normal reaction R is constant. This constant is called the coefficient of friction and is denoted by µ.
If m is mass then for the horizontal surface
R = mg
Then by putting the value of R in the equation, we get
The wheel is one of the most essential creations in the history of humanity. The first thing about a wheel is that it rolls as it moves instead of to slide. This significantly minimizes friction.
However, you ever thought about why this happens?
When the axle of a wheel is pushed, the force of friction between the wheel and the ground at the point of contact supplies the reaction force. The reaction force acts at the contact points of the wheel in an opposite direction to the applied force.
The wheel rolls without bursting the cold welds. That is why the rolling friction is much smaller than moving friction. The fact that rolling friction is less than moving friction is used in ball bearings or roller bearings to decrease losses due to friction. The wheel would not roll on pressing it if there would be no friction between the wheel and the ground. Hence, friction is desirable for wheels to roll over a surface area.
It is dangerous to drive on a wet road because of the friction between the roadway and the tyres is little. This increases the possibility of slipping the tires on the roadway. The threading on tires is designed to increase friction. Thus, threading enhances road grip and make it safer to drive even on damp roadway.
A bicyclist applies brakes to stop his/her bike. As quickly as brakes are applied, the wheels stop rolling and start to slide over the road. Given that sliding friction is much greater than rolling friction. For that reason, the cycle stops very quickly.
Advantages & Drawbacks of Friction
Friction has plus points in addition to downsides.
Friction can be a helpful force because it prevents our shoes from slipping on the pavement when we walk and stops vehicle tires from skidding on the road. A slippery ground offers very little friction.
For this reason, anybody who attempts to run on slippery ground might have an accident. So, friction is necessary. We can not write if there is no friction between the paper and a pencil. Birds might not fly if there is no air resistance. The reaction of pushed air makes it possible for the birds to fly.
Friction is unfavorable when moving at high speeds because it opposes the motion and therefore restricts the speed of moving things. The majority of our useful energy is lost as heat and noise due to the friction between various moving parts of machines. In machines, friction also triggers wear and tear of their moving parts.
Methods of Reducing Friction
The friction can be decreased by:
- making the moving surfaces smooth.
- making the fast-moving objects as streamlined (fish shape) such as trains, aircraft, and so on. This causes the smooth circulation of air and thus minimizes air resistance at high speeds.
- Oiling and greasing the moving surfaces.
- Utilizing ball bearings or roller bearings. Because the rolling friction is lesser than the sliding friction.
MCQs about Friction
- What is friction?
- A) The force of attraction between two surfaces
- B) The force of opposition between two surfaces in relative contact
- C) The force that causes objects to move
- D) The force that keeps objects in motion
- Answer: B) The force of opposition between two surfaces in relative contact
- What type of friction occurs when objects stop moving?
- A) Kinetic friction
- B) Static friction
- C) Rolling friction
- D) Sliding friction
- Answer: B) Static friction
- Which force opposes the motion of moving objects?
- A) Gravitational force
- B) Frictional force
- C) Magnetic force
- D) Centripetal force
- Answer: B) Frictional force
- What is the maximum value of friction referred to as?
- A) Force of static friction
- B) Force of kinetic friction
- C) Force of limiting friction
- D) Force of rolling friction
- Answer: C) Force of limiting friction
- What is the formula for friction?
- A) F = ma
- B) F = μR
- C) F = mg
- D) F = mv
- Answer: B) F = μR
- What does the coefficient of friction represent?
- A) The ratio of friction to reaction
- B) The ratio of friction to mass
- C) The ratio of friction to velocity
- D) The ratio of friction to acceleration
- Answer: A) The ratio of friction to reaction
- Which type of friction is smaller, rolling or moving friction?
- A) Rolling friction
- B) Moving friction
- C) They are equal
- D) Depends on the surface
- Answer: A) Rolling friction
- Why does rolling friction typically have less resistance than moving friction?
- A) Because of the smoothness of the surfaces
- B) Because of the way cold welds form
- C) Because of the way wheels roll instead of slide
- D) Because of the direction of the applied force
- Answer: C) Because of the way wheels roll instead of slide
- What happens to friction on a wet road?
- A) It increases
- B) It decreases
- C) It remains the same
- D) It becomes static
- Answer: B) It decreases
- Why is friction necessary for walking on pavement?
- A) To increase speed
- B) To decrease speed
- C) To prevent slipping
- D) To maintain balance
- Answer: C) To prevent slipping
- What does friction cause between various moving parts of machines?
- A) Wear and tear
- B) Lubrication
- C) Cooling
- D) Strengthening
- Answer: A) Wear and tear
- How can friction be reduced between moving surfaces?
- A) By increasing weight
- B) By making surfaces rougher
- C) By applying force parallel to the surface
- D) By making surfaces smoother
- Answer: D) By making surfaces smoother
- What shape helps reduce air resistance and friction at high speeds?
- A) Spherical
- B) Cuboid
- C) Streamlined (fish shape)
- D) Cone
- Answer: C) Streamlined (fish shape)
- What can be used to reduce friction in moving parts?
- A) Oiling and greasing
- B) Increasing weight
- C) Using rough surfaces
- D) Decreasing speed
- Answer: A) Oiling and greasing
- Which type of friction occurs when brakes are applied to a bicycle?
- A) Static friction
- B) Rolling friction
- C) Kinetic friction
- D) Sliding friction
- Answer: D) Sliding friction
- Why are ball bearings or roller bearings used in machines?
- A) To increase friction
- B) To decrease friction
- C) To maintain friction
- D) To change the direction of friction
- Answer: B) To decrease friction
- What is the ratio between the force of limiting friction and the normal reaction?
- A) Coefficient of velocity
- B) Coefficient of acceleration
- C) Coefficient of friction
- D) Coefficient of resistance
- Answer: C) Coefficient of friction
- What happens to friction as the pressing force between two surfaces increases?
- A) It decreases
- B) It remains constant
- C) It becomes zero
- D) It increases
- Answer: D) It increases
- Why does a bicycle stop quickly when brakes are applied?
- A) Because of decreased friction
- B) Because of increased friction
- C) Because of constant friction
- D) Because of frictionless surfaces
- Answer: B) Because of increased friction
- Which type of friction occurs between a wheel and the ground while rolling?
- A) Static friction
- B) Kinetic friction
- C) Rolling friction
- D) Sliding friction
- Answer: C) Rolling friction
- What does the ratio between friction and reaction represent?
- A) Coefficient of friction
- B) Coefficient of velocity
- C) Coefficient of acceleration
- D) Coefficient of resistance
- Answer: A) Coefficient of friction
- How is friction related to the force applied to move a body at rest?
- A) It is equal to the force applied
- B) It is proportional to the force applied
- C) It is inversely proportional to the force applied
- D) It is not related to the force applied
- Answer: B) It is proportional to the force applied
- What is the main purpose of threading on tires?
- A) To decrease friction
- B) To increase friction
- C) To change the direction of friction
- D) To maintain friction
- Answer: B) To increase friction
- What does friction transform moving energy into?
- A) Light
- B) Heat
- C) Sound
- D) Electricity
- Answer: B) Heat
Friction, the force of opposition between two surfaces in relative contact, is crucial for various aspects of daily life, including movement and machine operation. It can be static or kinetic, manifesting in forms such as sliding, skidding, or rolling. Friction transforms energy into heat, as seen in braking vehicles. Surfaces are never completely smooth, and microscopic irregularities contribute to frictional forces.
The friction formula involves the coefficient of friction (µ) and the normal reaction (R), with friction (F) equaling µ times R. Understanding friction is essential for comprehending why objects stop moving and why certain forces oppose motion. For instance, the rolling motion of a wheel is made possible by the friction between the wheel and the ground.
While friction has advantages, such as preventing slipping and enabling functionality like writing and flying, it also has drawbacks. It can restrict speed and lead to energy loss and wear and tear in machines.
Methods to reduce friction include smoothing surfaces, streamlining fast-moving objects, lubrication with oil or grease, and using ball bearings or roller bearings. These methods aim to minimize resistance and improve efficiency, particularly in high-speed scenarios.
Overall, friction plays a significant role in everyday activities and technological advancements, shaping how we interact with the world and innovate solutions to optimize performance and functionality.