Kinetic Friction

​FRICTION

​What is Friction

• ​the resistance that one surface or object encounters when moving over another.

• ​You can have static friction and kinetic friction

• ​

​Static Friction

​Static friction can prevent an object from sliding down a sloped surface.

• The coefficient of static friction, typically denoted as μs, is usually higher than the coefficient of kinetic friction.

• ​note the equation in green

​

​F_f

​F_N

​Wt = mg

​F_a

​Static Friction

​F_f = µ_s F_N

​

​Kinetic Friction

• ​Kinetic friction is defined as a force that acts between moving surfaces.

• A body moving on the surface experiences a force in the opposite direction of its movement.

• ​note the equation in green

Multiple Choice

Friction is a force that acts in an ___________ direction of movement.

• similar

• opposite

• parallel

• north

Multiple Choice

What type of friction is not moving?

• static
• sliding
• fluid
• rolling

Multiple Choice

Rougher surfaces have ...

• greater friction
• less friction
• the same level of friction

Multiple Choice

If you have the following Forces: F_N=100N, F_f=20N . What is the coefficent of friction?

• μ=5.0

• μ=0.2

• μ=1.2

• μ=0.1

Multiple Choice

 What is the force of friction between a block of ice that weighs 950 N and the ground if μ = .12? 

• 65 N
• 226 N
• 175 N
• 114 N

Multiple Choice

What is the coefficient of static friction if it takes 44 N of force to move a box that weighs 86 N? 

• 0.78
• 0.51
• 0.78 N
• 0.58 N

Multiple Choice

Which would NOT be a way of reducing friction?

• making the surfaces smoother
• using lubricants such as waxes, oils, or grease
• changing from sliding friction to rolling friction using ball bearings
• Using kitty litter on snow and ice to help your car gain traction

​Finding Friction using the Normal [Real] force

• ​When a system is balanced the Normal force and Gravity Force are equal

• ​When a system is balanced the Friction and applied force are equal

• ​A balanced system means that the object is at rest, or moving at a constant velocity [no acceleration]

Multiple Choice

When looking at friction problems, if the net force in the x-direction on an object is zero, what can we say about its acceleration?

• it is positive

• it is negative

• it is zero

• not enough information is given

Multiple Choice

Normal force is

• Always opposite the weight

• Always parallel and down the incline

• Always perpendicular to the surface

• It depends on the direction of motion

Multiple Choice

• 1

• 2

• 3

• 4

Multiple Choice

• 1

• 2

• 3

• 4

Multiple Choice

Calculate the resultant force.

• 60N, Left

• 40N, Right

• 0N

• 60N, Right

Multiple Choice

These forces are...

• Balanced

• Unbalanced

Multiple Choice

These forces are...

• Balanced

• Unbalanced

Multiple Choice

Based on the information in the diagram, what is the net force acting on this object?

• 12 N downward

• 4 N upward

• 4 N downward

• 8 N to the left

Multiple Choice

If an object's mass is 30 Kg on the Earth, what would be it's weight on Earth?

• 30 N

• 300 kg

• 3000N

• 300 N

Multiple Choice

If the weight of a body on the Earth's surface is 600 N, what would be it's weight on moon? (Hint: The earth is six times bigger than the moon)

• 3600 N

• 100 N

• 6000 N

• 200 N

Multiple Choice

Calculate the Net force experienced by the object shown below

• 20 N

• 14.4 N

• 4 N

• 10 N

Multiple Choice

Which force diagram best represents the forces acting on the box?

Multiple Choice

Which force diagram best represents the forces acting on the box?

Multiple Choice

If there are two forces going in the SAME direction, you would ...?

• (-) Subtract the forces
• (+) Add the forces
• Neither
• (x) Multiply

Multiple Choice

A 267.5 N force is applied to slide a chair across the room, causing an acceleration of 4.2 m/s². What is the mass of the chair?

• 26.75 kg

• 63.69 kg

• 263.3 kg

• 1123.5 kg