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## Class 11 Physics (India)

### Course: Class 11 Physics (India)>Unit 9

Lesson 9: Friction

# Friction review

Review the key concepts, equations, and skills for friction, including how to find the direction and magnitude of the friction force.

## Key terms

Term (symbol)Meaning
Friction (${F}_{f}$ or $f$)A contact force that resists sliding between surfaces.
Kinetic friction (${F}_{f,k}$ or ${f}_{k}$)Friction when an object slides along a surface. Direction is opposite the object’s sliding direction and is parallel to the contact surface.
Static friction (${F}_{f,s}$ or ${f}_{s}$)Friction that prevents an object from sliding along a surface. Direction stops the object from sliding against another surface and is parallel to the contact surface.
Coefficient of friction ($\mu$)A number typically between $0$ and $1$ that describes the roughness between two surfaces, where $0$ is slippery and $1$ is very rough. A unit-less ratio of the frictional force to the normal force. The static friction coefficient ${\mu }_{s}$ is for surfaces that are not sliding, while kinetic ${\mu }_{k}$ is for sliding surfaces.

## Equations

EquationSymbol breakdownMeaning in words
$|\stackrel{\to }{{F}_{f,k}}|={\mu }_{k}|\stackrel{\to }{{F}_{N}}|$${F}_{f,k}$ is kinetic friction, ${\mu }_{k}$ is coefficient of kinetic friction, ${F}_{N}$ is normal forceKinetic friction magnitude is directly proportional to the normal force magnitude and the roughness between the sliding surfaces.
$|\stackrel{\to }{{F}_{f,s}}|\le {\mu }_{s}|\stackrel{\to }{{F}_{N}}|$${F}_{f,s}$ is static friction, ${\mu }_{s}$ is coefficient of static friction, ${F}_{N}$ is normal forceStatic friction magnitude is directly proportional to the normal force magnitude and the roughness between the sliding surfaces.
$\mu =\frac{|\stackrel{\to }{{F}_{f}}|}{|\stackrel{\to }{{F}_{N}}|}$${F}_{f}$ is friction, $\mu$ is coefficient of friction, ${F}_{N}$ is normal forceThe coefficient of friction is the ratio of the magnitude of frictional force divided by the normal force magnitude.

## How to find direction of friction force

### Static friction prevents sliding

Static friction is the force holding an object in place on an incline, such as the cheese in Figure 1. The friction force points against the direction that the object would slide without friction. Static friction keeps gravity from pulling the cheese down the incline.
Another example of static friction comes when objects are moving. When you are walking, static friction pushes in the direction of you are trying to move (see Figure 2 below). The foot pushes on the ground, and without friction the foot would slide backwards (like walking on ice). Static friction pushes in the direction that prevents your foot from sliding, which results in forward motion.

### Kinetic friction acts opposite the object’s sliding direction

Kinetic friction always opposes the object’s sliding direction. As seen in Figure 3 below, if an object is moving up an incline, the force of friction ${f}_{k}$ points down the incline. If the object is moving down an incline, the ${f}_{k}$ points up the incline.

## How to determine magnitude of friction force

Friction is determined by the two surfaces in contact, and how tightly the two surfaces are pushed together (normal force ${F}_{N}$).
• Coefficient of friction ($\mu$): this describes the roughness between two surfaces. A high coefficient of friction produces more friction.
• Normal force (${F}_{N}$): squeezing surfaces together more tightly increases the friction. This is one reason why heavy objects are harder to slide across the ground.
These factors of friction are reflected in its generalized equation:
$|\stackrel{\to }{{F}_{f}}|\le \mu |\stackrel{\to }{{F}_{N}}|$
Friction for a given object isn’t always just one value though, it can change. Let’s learn how by imagining a person pushing a refrigerator as shown in Figure 4 below. When we push an initially resting refrigerator with an external applied force ${F}_{\text{app}}$ and start it moving, both static and kinetic friction push back on the object at different times.

### Static friction

Initially static friction ${F}_{f,s}$ prevents the refrigerator in Figure 4 from moving. But as we continue applying more and more force ${F}_{\text{app}}$, eventually the refrigerator begins sliding. This is because static friction has a maximum value that it can reach before it lets an object begin sliding. As long as $|{F}_{\text{app}}|\le |{F}_{f,{s}_{max}}|$, then the refrigerator remains at rest. This is described by the equation below:
$|\stackrel{\to }{{F}_{f,s}}|\le {\mu }_{s}|\stackrel{\to }{{F}_{N}}|$

### Kinetic friction

Once an object begins to slide, kinetic friction ${F}_{f,k}$ acts with a constant amount to resist the sliding motion:
$|\stackrel{\to }{{F}_{f,k}}|={\mu }_{k}|\stackrel{\to }{{F}_{N}}|$

## Common mistakes and misconceptions

People often mistake the coefficient of friction $\mu$ for the frictional force ${F}_{f}$. The coefficient of friction is a number that describes the interactions between the surfaces, it is not a force. In order to find the friction force, $\mu$ has to be multiplied by the normal force on the object.

For deeper explanations of friction, see our video comparing static and kinetic friction.
To check your understanding and work toward mastering these concepts, check out the friction exercise.

## Want to join the conversation?

• is there a video for calculating the net force on an inclined plane with the components of friction included in the problem?
• Yes, please let us know if there is a video regarding this. It would be very helpful. Thank you!
• Is there a certain limit of what the coefficient of friction can be? Does it always have to be less than 1 or is it possible for the coefficient of friction to be any real number?
• yes aluminium-aluminum has coff-friction ranging from 1.05-1.35.
• Do you have forces on incline
• which force act on an object at rest on a moving object. For example, a box on moving truck?
• That force is friction. The friction between the box and bed of the moving truck causes the box to accelerate with the truck. Hope this helps!
• is it possible for an applied force to have a magnitude less than the magnitude of the kinetic friction?
(1 vote)
• No, because in order to move an object, you have to overpower the maximum static friction. And since kinetic friction is always less than the maximum static friction, it would mean that the force can never be less than the kinetic friction
• Can you do a video about calculating forces on 3 different objects connected at angles?
• "Suppose two children push horizontally, but in exactly opposite directions, on a third child in a wagon. The first child exerts a force of 75.0 N, the second a force of 90.0 N, friction is 12.0 N, and the mass of the third child plus wagon is 23.0 kg." in this situation since there is two people pushing in opposite direction is there going to also be two frictions in opposite directions and are they both 12n or 6n each?
(1 vote)
• This kind of problems are tricky. Since there are two forces in opposite directions with friction, which way should the frictional force point? The way I like to think of this kind of thing is to take it in steps. First, find the net force if there was no friction. The frictional force will be in the opposite direction of this net force. From there, it is kind of self-explanatory. Hope this helps!
(1 vote)
• how do you find the range of Force that can be applied to a object in rest and still remain motionless
(1 vote)
• Any number of forces can be applied on an object at rest and still remain at rest ie. equilibrium. The only condition being that the vector sum of all the forces should equal zero, or else the object starts accelerating which puts it into motion