Module 05 Lesson 02 Friction

Multiple Choice

Why is understanding friction important in our daily lives?

• It helps us understand how objects move and stop.

• It explains why objects float in water.

• It tells us how electricity flows.

• It helps us know why the sky is blue.

Multiple Choice

Which of the following statements about force is correct?

• Force only has magnitude.

• Force is always a contact force.

• Force can be a push or a pull and has both direction and magnitude.

• Force does not affect objects.

Drag and Drop

In the diagram below, a girl sits on a sled and slides down a hill of snow. The arrow that represents the force of friction opposing the motion of the sled down the hill is

Placeholder

.

Drag these tiles and drop them in the correct blank above

Arrow A

Arrow B

Arrow C

Arrow D

Multiple Choice

Which type of friction acts on an object when there is no motion between two surfaces?

• Kinetic friction

• Static friction

• Rolling friction

• Fluid friction

Limiting Friction

Limiting friction is the maximum force of static friction that exists just before an object begins to slide over another surface; it's the peak resistance overcome to initiate motion, where the applied force equals this limiting value, causing impending motion.

Once exceeded, the object moves, and the friction becomes kinetic friction, which is generally less than limiting friction

Limiting Friction

Multiple Choice

What is the relationship between static friction and limiting friction?

• They are the same thing.

• Limiting friction is always greater than static friction.

• Static friction is the friction before an object starts moving, and limiting friction is the maximum value it reaches.

• Static friction increases after the object starts moving, becoming limiting friction.

​Types of Kinetic Friction

There aren't distinct "types" of kinetic friction in the way there are different categories; instead, kinetic friction (or sliding friction) describes the force opposing motion between two surfaces moving relative to each other, but it encompasses different manifestations like sliding friction, rolling friction, and even fluid friction (drag), all acting when objects are in motion, unlike static friction which prevents movement.

Sliding Friction

This is the classic definition, occurring when one solid surface slides across another (e.g., a book sliding on a table, a sled on snow).

Main Forms of Kinetic Friction

A type of kinetic friction involving a round object (like a wheel or ball) rolling over a surface, generally much weaker than sliding friction.

Main Forms of Kinetic Friction

Rolling Friction

Fluid Friction (Drag)

The resistance experienced when an object moves through a fluid (liquid or gas), like air resistance on a car or a swimmer in water, also a form of kinetic friction.

Main Forms of Kinetic Friction

Match

Match the types of friction

Rolling

Sliding

Static

Fluid

• Rolling

• Sliding

• Static

• Fluid

Match

Match the following Types of Friction to their descriptions

• Static Friction

• Slide Friction

• Fluid Friction

• Rolling Friction

Pushing a stationary car

Sledding down a hill

A plane flying through the air

A skateboarder going down a ramp

• Pushing a stationary car

• Sledding down a hill

• A plane flying through the air

• A skateboarder going down a ramp

Multiple Choice

An object that is not moving is an example of _____________ friction.

• static

• rolling

• sliding

• fluid

Multiple Select

Select all the correct statements about kinetic friction.

• Kinetic friction acts when two surfaces are moving relative to each other.

• Kinetic friction is always greater than static friction.

• Kinetic friction depends on the materials of the surfaces.

• Kinetic friction acts when there is no motion between surfaces.

Multiple Choice

What is the equation for the kinetic friction force between two surfaces?

• F_f, kinetic = μ_k F_N

• F_f, kinetic = μ_s F_N

• F_f, kinetic = m a

• F_f, kinetic = F_N / μ_k

Fill in the Blanks

Multiple Choice

Which pair of surfaces has the highest coefficient of static friction according to the table?

• Cast iron on cast iron

• Glass on glass

• Leather on oak

• Steel on steel

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

If the child in the previous example pulls with an extra 2.0 N in the horizontal direction, what will be the acceleration of the shoe?

• 1.3 m/s^2

• 2.0 m/s^2

• 0.65 m/s^2

• 9.8 m/s^2

Multiple Choice

What is static friction?

• The friction that acts on moving objects.

• The friction between two objects that are not moving relative to each other.

• The friction that only acts when an object is sliding.

• The friction that decreases as the speed of an object increases.

Multiple Choice

A block is dragged along the following surfaces. Which would result in the greatest kinetic friction force?

• highly polished table

• rough table

• sandpaper

• The kinetic force would be equal for all surfaces.

Multiple Choice

What type of friction is shown in this car that is at rest?

• rolling

• static

• fluid

• sliding

Multiple Choice

Which equation can be used to calculate the static friction force?

• F = μs/FN

• F = μs + FN

• F = μsFN

• F = μkFN

Multiple Choice

Which has the lowest coefficient of friction?

• cast iron on cast iron

• glass on glass

• nonstick coating on steel

• steel on steel with castor oil

Multiple Choice

What standard units do we use to communicate a coefficient of friction?

• kg

• N

• m

• It is unitless.

Multiple Choice

A box takes 40 N to start moving when the coefficient of static friction is .54. What is the weight of the box? 

• 81 N
• 18 N
• 98 N
• 74 N

Multiple Choice

What does the coefficient of friction describe?

• How heavy two objects rubbing together are

• How fast two objects are rubbing together

• How rough or smooth two surfaces are rubbing together

• How much inertia two objects rubbing together have