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Frictional Force: Static and Kinetic Friction

Frictional Force (Friction)

Learn about the force of friction and how to solve problems that involve friction or the coefficient of friction.

Friction Learning Targets

  • I know the difference between static, kinetic sliding, and kinetic rolling friction
  • I can solve for the force of friction
  • I can solve for the coefficient of friction
  • I can determine net force and acceleration
  • I can use the overall net force to determine other variables in the acceleration equations

Friction force (Ff)

  • Force created when surfaces rub against each other
  • Reason: surfaces are rough at the microscopic level
  • Created in the opposite direction of motion
frictional force direction

Types of Friction

Static Friction: frictional force caused by an applied force at rest (before motion).

Kinetic Sliding Friction: frictional force when an object is sliding in motion

Kinetic vs. Static Friction
Rolling Friction

Kinetic Rolling Friction: frictional force when an object is rubbing surfaces while rotating

From here, we will call kinetic sliding friction just kinetic friction and kinetic rolling friction just rolling friction.

  • Until motion, surfaces have static friction
  • Once moving, surfaces have kinetic friction
    • The type of kinetic friction depends on if the object is sliding or rolling

Kinetic friction is less than static friction

  • A General Rule: Kinetic (sliding) friction is less than static (not moving) friction
  • Because moving surfaces are bouncing off each other with little less surface area interaction
  • Rolling Friction would have less friction (opposing force) than static or kinetic sliding friction
Force Applied Static Friction Animation
Kinetic Friction Animation

Q1: What time of friction do you have when an object is not yet moving?

Static Friction

Q2: Place the following in order from greatest possible force of friction to least

(Rolling Friction, Static Friction, Kinetic Friction)

  1. Static Friction (greatest)
  2. Kinetic Friction
  3. Rolling Friction (least)

Frictional Force Equation

Friction Equation

Ff : Force of friction (N)

  • Force opposing motion due to rough surfaces

μ : Coefficient of friction (no unit)

  • Number that relates the amount of friction to normal force. The coefficient of friction (μ) will change depending on the surfaces in contact

FN : Normal Force (N)

  • Force of the ground pushing against weight (perpendicular to the surface)

On a horizontal surface, the magnitude of normal force equals weight

  • Weight (FW) in Newtons equals to the magnitude of normal force (FN)
  • If given mass (m) in kilograms, solve for weight (Fw = mg) with g = 10 m/s2 to determine the magnitude of normal force (FN)
Normal Force and Weight

Different Coefficient of Friction (µ) Variables

  • µs represents the coefficient of static friction
  • µk represents the coefficient of kinetic sliding friction
  • µr represents the coefficient of kinetic rolling friction (wheel rolling)

Engineers will use tables to determine material to use in equipment depending on it's purpose and intention to stick, slide, or roll.

different coefficients of friction

More Facts About Coefficient of Friction (µ)

  • Surfaces have different roughness and therefore oppose motion at different rates
  • The bigger the coefficient of friction (µ) the more friction or resistance to motion
  • When the same surfaces are in contact, coefficient of kinetic friction (µk) is almost always less than the coefficient of static friction (µs)
  • It’s harder to get an object moving than keep it moving
Rough-Surfaces

Q3: Which coefficient of friction would have the greatest resulting force of friction?      

µ1 = 0.8    µ2 = 0.2    µ3 = 0.5

µ1 = 0.8

Calculating the force of static friction

  • Any force applied before motion equals the magnitude of static friction
  • No motion means you have static equilibrium
  • Net force equals 0
  • Because: FA = -Ff

Notice in the animation that the force of static friction increases until there is motion

1 N static friction with a spring scale

Q4: What is the force of static friction when 3.5 N of force is applied horizontally and the object is not moving?

3.5 N

If still not moving the force applied equals the static friction

Calculating coefficient of static friction

  • Measure the maximum force of static friction (equal to the force applied necessary to first move the block)
  • You use this maximum static friction (Ff) to solve for the coefficient of static friction (µs)
1 N static friction with a spring scale

Notice in the animation that the spring scale gets to 1.0 N before the block moves and force decreases.  Therefore, the maximum force while still static (not moving) is 1.0 N.

Q5: A 0.4 kg block is sitting on a surface.  What is the coefficient of static friction between the block and surface when it takes 1.0 N of force before the block begins to move?

1

Calculating the force of kinetic friction in a lab setting

  • Create a situation of dynamic equilibrium
  • Drag the block at a constant velocity so there is no net force
  • The force applied (FA)to drag the block is equal to kinetic friction (Ff)

equilibrium

testing kinetic friction

Q6: What is the force of kinetic friction when 1.3 N of force is applied horizontally to keep the object moving at a constant velocity?

1.3 N

Constant velocity means no acceleration

No acceleration means FA equals the magnitude of Ff

The object is moving so it has kinetic friction

Calculating coefficient of kinetic friction in a lab setting

  • Measure the force required to keep the block moving at a constant velocity
  • This will be the equal to the frictional force (Ff) you use to determine the coefficient of kinetic friction (µk)
testing kinetic friction

Q7: A 0.4 kg block on a horizontal surface.  What is the coefficient of kinetic friction between the block and surface when it takes 0.9 N of force to keep the block moving at a constant velocity?

Solution

(Q8-Q11) It takes 45 N of force horizontally to move a wood block with a constant velocity across a desk.

Q8: What is the applied force?

45 N

Q9: What is the force of friction?

45 N

Q10: What is the net force?

0 N

Q11: What is the acceleration?

0 m/s²

Non-zero net force causes acceleration

  • When the force applied is greater than the force of static friction, you have a net force and acceleration

Fnet = ma

Q12: What minimum force do you have to apply horizontally to get a 1.5 kg block of wood at rest to move on a wood surface?

Solution

coefficient of friction table

Q13: How much force would it take to keep the 1.5 kg block of wood moving at a constant velocity after its moving?

Solution

Q14: Would a 15 kg brick at rest on a wood floor accelerate when 120N of horizontal force is applied?

Solution

Q15: What would a 15 kg brick on a wood floor accelerate at when 120N of horizontal force is applied?

Solution

With acceleration you can determine other variables depending on the information given

F=ma Equations

Q16: An 81 kg person runs and slides.  The coefficient of kinetic friction between the person and ground is 0.72.  What is the magnitude of frictional force?

Solution

 

Q17: An 81 kg person runs and slides.  The coefficient of kinetic friction between the person and ground is 0.72.  What was the person’s initial speed if it took 2.0 seconds to come to rest?

Solution

When a force is at an angle to the horizontal plane, the block will move on:

  • The opposite side will decrease the normal force of the ground
  • The adjacent side is in the direction of motion and will be used to determine the applied force forward
Normal force and force horizontal when pulling at an angle

Q18: A 5 kg box is pulled at an angle of 25° from the horizontal with a force of 75 N if the coefficient of kinetic friction is 0.25.  What is the normal force?

Solution 18

Q19: A 5 kg box is pulled at an angle of 25° from the horizontal with a force of 75 N if the coefficient of kinetic friction is 0.25.  What is the horizontal applied force?

Solution 19

Q20: A 5 kg box is pulled at an angle of 25° from the horizontal with a force of 75 N if the coefficient of kinetic friction is 0.25.  What is the force of friction?

Solution 20

Q21: A 5 kg box is pulled at an angle of 25° from the horizontal with a force of 75 N if the coefficient of kinetic friction is 0.25.  What is the net force?

Solution 21

Q22: A 5 kg box is pulled at an angle of 25° from the horizontal with a force of 75 N if the coefficient of kinetic friction is 0.25.  What is the acceleration of the box?

Solution 22

Q23: If you lift the side of a desk creating an incline until the moment an object first starts moving. What would happen next keeping that exact same angle and why?

A. The block would stop

B. The block would move at a constant rate down the incline

C. The block would accelerate down the incline

( Choice C ) The block would accelerate down the incline

Because: Kinetic friction is less than static friction.  So as soon as there is motion the force of kinetic friction drops below the force of static friction.  While the force downhill that caused the motion stays the same.  Therefore there is a net force which causes acceleration and not just motion

(See the animation of the forces interacting below)

Increasing incline slope

Notice how frictional force uphill and parallel force downhill increase with the incline until motion occurs.  Because of motion, static friction changes to a lesser kinetic friction.  Therefore the force uphill (friction) is less than the force downhill.  At that moment the net force is downhill.  So the net force downhill creates an acceleration.

Increasing incline slope

Friction Quiz

1 / 7

What type of friction do you have while an object is at rest?

Static Friction: at rest

Kinetic Friction: moving and can be sliding or rolling which would have a lower coefficient of friction than sliding.

2 / 7

What is the magnitude of frictional force when 162 N of force is required to keep an object in constant motion?

What is the magnitude of frictional force when 162 N of force is required to keep an object in constant motion?

If there is constant motion there is no acceleration and the net force is zero.  The frictional force must equal the force applied in the opposite direction so 162 N as well

3 / 7

What is the magnitude of net force when 162 N of force is required to keep an object in constant motion?

Constant motion means no acceleration.  Net force (sum of overall forces on an object) must be zero if there is no acceleration.  

4 / 7

Which is almost always greater

When an object is at rest (static) the surfaces between objects are settled and closer together.  With surfaces closer together there is more interaction and therefore more static friction friction than kinetic. 

friction

 

5 / 7

You use a spring scale to measure the amount of force required to first start an object moving.  You would use this value to determine the coefficient of ___________ friction.

Coefficient of static friction.  The moment the object first starts moving is also almost equivalent to the maximum force provided by friction while the object was at rest or static.

 

Force applied equals the magnitude of force of friction in constant motion.  If you find the force of friction (Ff), you know the Force applied (FA)

6 / 7

coefficient of friction table

What is the force required to keep a 0.5 kg brick in constant motion on a wood surface?

 

Kinetic Friction Wood on Brick

7 / 7

coefficient of friction table

What is the minimum force required to start a 0.5 kg brick moving on a wood surface?

 

Brick on Wood Static Friction

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Unit 1: One Dimensional Motion
Unit 2: 2D Motion
Unit 3: Newton’s Laws and Force
Unit 4: Universal Gravitation and Circular Motion
Unit 5: Work, Power, Mechanical Advantage, and Simple Machines
Unit 6: Momentum, Impulse, and Conservation of Momentum
Unit 7: Electrostatics
Unit 8: Current and Circuits
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Unit 10: Intro to Waves
Unit 11: Electromagnetic Waves
Unit 12: Nuclear Physics

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