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Static friction can be defined as the friction experienced when individuals try moving a stationary object on a surface, without influencing any relative motion between the body and the surface it is on. Static friction refers to a kind of force that makes an object stay at rest. The formula of Static Friction is: Fs = μs N
Here,
- Fs = Static Friction Force,
- μs = Static Friction Coefficient,
- N = Normal Force (Contact Force)
Friction is a force that prevents fluid layers, solid surfaces, and material elements sliding against each other from moving in the same direction. Friction is a force that exists whenever two objects rub against each other. Friction is of various types such as:
- Fluid Friction
- Lubricated Friction
- Skin Friction
- Internal Friction
- Dry Friction
- Kinetic Friction
- Static Friction
| Table of Content |
Key Terms: Static Friction, Limiting Friction, Dry Friction, Kinetic Friction, Fluid Friction, Velocity, Area
What is Static Friction?
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Static Friction can be defined as:
| “The friction that occurs when people attempt to move a stationary object on a surface without causing any relative motion between their bodies and the surface they are on.” |
- Simply, the frictional force between the surfaces of two substances when they are not in motion with regard to each other or the force that holds an object at rest is known as Static Friction.
- It is a frictional force that helps balance the applied force for the duration of the body's stationary state.
- The static frictional force is self-regulating, meaning it will always be equal to and opposing the applied force.
- Let us assume we are pulling a box.
- If we gradually increase the force with which we pull the box, the graph shows that the friction force increases with our force up to a critical value fL, at which point the box begins to move, and as soon as it does, a smaller force is required to maintain its motion because friction is reduced in motion.
- Hence, static friction is defined as the friction value between 0 and fL, which balances the external force on the body and keeps it from sliding.
Static Friction Formula
The formula of Static Friction is:
| Fs = μs N |
Where,
- Fs = Static Friction Force,
- μs = Static Friction Coefficient,
- N = Normal Force (Contact Force)
Also Read: Types of Friction & Examples
Types of Friction
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There are several types of Friction, apart from Static Friction. Some include:
| Types of Friction | Description |
|---|---|
| Fluid Friction | The friction existing between the layers of a liquid is called Fluid Friction. It usually arises among the various layers of viscous fluid. |
| Lubricated Friction | The frictional force between a lubricant, example include oil, and the two surfaces of a solid is called Lubricated Friction. |
| Skin Friction | When a liquid is seen to move across a solid’s surface, then it gives rise to a frictional force between the liquid and the solid surface. This is known as a Skin Friction. It is also known by the name of “drag”. |
| Internal Friction | The frictional interaction between the adjacent portions in the interior of a given substance as a result of viscous deformation, thus causing in the production of heat. |
| Dry Friction | The frictional force which occurs when two solid surfaces come into contact. It can be further divided into two categories, Static Friction and Kinetic Friction. |
| Kinetic Friction | The dry frictional force found between two moving surfaces, that slide over each other or rub against one another, is called Kinetic Friction. It is also often referred to as Dynamic Friction or Sliding Friction. |
| Static Friction | The frictional force found between the surfaces of two substances when they aren’t in motion with respect to one another or the force that holds an object at rest is known as Static Friction. |
Static Friction Examples
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The form of friction that prevents an object from moving on its own without the assistance of another force is known as static friction. It's one of the most powerful forms of friction, and it's at work all around you. In everyday life, there are several examples of static friction.
A Car Stopped on a Level Surface
When you're driving and you want to come to a complete stop, you must apply friction.
- You're observing static friction if you're on a flat surface and can take your foot off the brake and the car doesn't move.
- Static friction is what keeps your automobile from moving on a flat road or driveway.
- The static friction between your tires and the ground protects the car from rolling by holding it in place.
- When you remove your foot off the brake or drive out of a park and your automobile rolls, a distinct type of friction is created.
Car stopped on a level surface due to static friction
Furniture on the Ground
Because your furniture is heavy, you can't move it around with even the lightest touch when it's on the ground.
- Static friction is the reason your furniture doesn't move around on the floor for no apparent reason.
- You can't move a piece of furniture without exerting some force yourself.
- The force that prevents your furniture from just sliding over the floor is known as static friction.
- Anything on your house's floor might otherwise move around with the slightest touch or bump.
Furniture on the ground due to Static Friction
Stone on the Ground
A stone resting on the ground is not going to move until acted upon by an external force.
- The stone will not move if a force, like that of a gush of wind, tries to shift its position.
- It is due to the static friction with respect to the ground.
- Static friction depends on the mass of the object.
- Thus, the heavier the stone, the tougher it is going to be to move it.
Laws of Static Friction
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There are several laws of Static Friction. Some include:
- The direction of static friction force is always the opposite of the direction in which the body moves or tends to move.
- Friction is a self-adjusting force that grows in proportion to the applied force, eventually equaling and opposing it until the motion is about to begin.
- The force of friction between the surfaces in contact is greatest when one body simply tends to move. Limiting friction is the force of friction.
- The static friction is determined by the nature of the surfaces in contact but is unaffected by their area.
- The normal reaction N is comparative to the static force, i.e. with an increase in the normal force, the maximum external force endured by an object without having to move also increases.
- The frictional force between the two surfaces is unaffected by their relative velocities.
- The maximal static friction force is independent of the contact area.
Also Read:
| Topic Related Concepts | ||
|---|---|---|
| Aristotle Fallacy | Rolling Friction | Sliding Friction |
Coefficient of Static Friction
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The coefficient of static friction is calculated as follows:
| μs = F / N |
Where,
- μs is the static friction coefficient
- N is the normal reaction force
- F is the static frictional force
What is Limiting Friction?
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When an object is about to slide over the surface of another object, the highest amount of static friction comes into play. This is known as Limiting Friction.
- The body begins to move when an external force is applied that is greater than the limiting friction. It's the product of the normal force and the limiting friction coefficient.
- The mathematical representation is as follows:
| F = μs N |
Where,
- Limiting friction is denoted by the letter F,
- μs is the limiting friction coefficient
- N stands for normal force
Motion occurs when a force applied to the two surfaces exceeds the limiting friction. Kinetic friction is a new sort of friction that enters in the picture. The limiting friction is the product of the normal reaction force and the coefficient of limiting friction for two dry surfaces.
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Laws of Limiting Friction
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The laws of Limiting Friction are:
- The normal reaction between the two surfaces determines the size of limiting friction.
- The material, the nature of the contacting surfaces, and their uniformity all influence the limiting friction.
- As long as the normal reaction is the same for any two given surfaces, the quantity of limiting friction is independent of the form or area of the surfaces in contact.
- The direction of the limiting frictional force is always the opposite of the motion direction.
- Friction is reduced by operating tangentially on the two surfaces that are contacting.
Things to Remember
- The friction that occurs when an object is placed on a surface is known as static friction.
- The static friction coefficient is a scalar quantity symbolized by the letter μs. As a result, μs = limiting force/normal force
- The magnitude of static friction fs ≤ μsN when the objects are stationary, where μs is the coefficient of static friction and N is the size of the normal force (the force perpendicular to the surface).
- Up to its utmost limit, static friction is a responsive force that grows to be equal and opposing to whatever force is applied.
- The object will move once the applied force reaches fs(max). As a result, fs(max) = μsN.
- The friction that is formed between two static surfaces that come into contact with each other is known as limiting friction. It's the product of the limiting friction coefficient and the normal force.
Also Read: Derivation of Equation of Motion in Physics
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Sample Questions
Ques. What are the features that characterize static friction? (1 mark)
Ans. Contact force and responsive force that reach a maximum value before kinetic friction takes control are characteristics that constitute static friction.
Ques. Define maximum static friction. (1 mark)
Ans. Limiting friction is another term for maximum static friction. It is, as previously stated, the greatest value that static friction can achieve. When an object is about to move, this is the value of friction.
Ques. Explain how static friction is a force that adjusts on its own. (2 marks)
Ans. Static friction is a force that adjusts on its own. To stop an object from moving, the static frictional force adjusts itself. It will try to equal the applied force until the limitation force is achieved. However, beyond a certain point (limiting friction), it transforms into kinetic friction.
Ques. Is static friction always the same? (2 marks)
Ans. Until the magnitude of limiting friction is reached, static friction is proportional to the force applied. In other words, static friction increases until a limit is reached and then decreases. The kinetic friction value remains relatively constant after reaching the limiting friction value.
Ques. Calculate the normal force on a body with a coefficient of friction of 5 and limiting friction of 50 N. (2 marks)
Ans. As per the given data, we know that F = μs N
So, N = F/μs
Hence, N = 50 / 5
= 10 N
Ques. If the coefficient of friction is 0.2, calculate the minimal force required to move a 5 kg box. (3 marks)
Ans. The value of friction force owing to a frictional surface is the minimum force because if we apply greater force than friction force, the box will move.
As per the formula, N = m × g
So, N = 5kg × 10
= 10 N
Since, F = μ × N
= 0.2 × 10
= 2 N
Hence, the minimal force will be of 2N
Ques. What three examples of static friction? (3 marks)
Ans. Here are some examples of static friction:
- Lifting and moving a heavy box against the ground is tough.
- A nightstand perched atop a lamp table.
- Friction toys that, in a static manner, simulate the behavior of force in the case of vehicles.
Ques. What is Static Friction and write its formula? (3 marks)
Ans. The friction that occurs when we try to move a stationary object on a surface without producing any relative motion between the body and the surface it is on is known as static friction.
Static friction (F) = μsN
where μs is the static friction coefficient and N is the normal force.
Ques. The normal force of an object is 50 N, but the static frictional force is 80 N. What is the static friction coefficient? (3 marks)
Ans. According to the given question, N= 50 N and F = 80 N.
We know that the formula for static friction coefficient μs = F / N.
So in that case, μs = 80/50 = 1.6
Hence, the static friction coefficient is 1.6.
Ques. An object weighing ten kilograms is put on a smooth surface. 30 N represents the static friction between these two surfaces. What is the static friction coefficient? (3 marks)
Ans. As per the given question, we collect the data such as m = 10 kg and F = 30 N. We have to find the static friction coefficient.
Normal force N = m x g
N = 10 × 9.81 = 98.1 N
We also know that the formula for static friction coefficient μs = F / N.
So in that case, μs = 30 / 98.1 = 0.305
Hence, the static friction coefficient is 0.305
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