Lesson: What a Drag

Contributed by: Integrated Teaching and Learning Program, College of Engineering,

University of Colorado at Boulder

Keywords:  energy, friction, 

static friction, kinetic friction, drag, aerodynamic, forces, coefficient of friction, fluid

Power of airhttps://youtu.be/K3FqG7yXEUs

https://youtu.be/sXXzf6zpUZo

https://youtu.be/jYaIXWNOa_A

Learning Objectives 

Recognize the different types of friction: static friction, kinetic friction, and drag

Understand how friction and drag work

Learn how to calculate friction and drag

Give examples of friction and drag

Pre-Lesson AssessmentWhere would you slide further:

on a sidewalk, on the grass, or on a frozen lake?

Why would you slide further on a frozen lake? Explain.

Introduction

Why do bobsledders crouch down?

What are other examples of this?

What do they have in common?

The position allows air to flow over them and they will move faster.

What would happen if you sprinkled sand on a bobsled track?

What if the tracks were made out of concrete?

What if the tracks were made out of concrete?

Friction changes the energy of motion (kinetic energy) into heat. You can experience this by rubbing your hands together very quickly. Friction on ice is very low, which means that the sleds can move quickly. Many of us know that ice is very slippery, which you know if you have ever slipped and fallen on a patch of ice!

Post-Introduction Assessment

Brainstorming: 

Name sports in which friction and drag are factors.

Lesson Friction is a lot like a little brother or

sister that never cooperates with you. If you want to go somewhere, they want you to sit still. If you want to move forward, they are pulling you backward, and so on. There are two types of friction: static friction and kinetic friction.

Static friction resists an object to start moving or sliding, which is a good thing when you start walking. If static friction didn't exist, it would be like you were constantly walking on ice!

Static Friction

Kinetic FrictionKinetic friction resists an object

that is already moving and always acts in a direction opposite of motion. Kinetic friction is the reason that anything moving or sliding will eventually come to a stop. It is important to note that static friction is always stronger than kinetic friction.

Coefficient of FrictionThe friction between two

objects is primarily dependent on two things: how hard are the objects pressing against each other and the coefficient of friction (μ)(pronounced "mu") between the objects.

For example, the coefficient of friction of a dry water slide is much higher than one with water on it. A measurement of the coefficient of friction is usually determined experimentally.

FF = μ x WA common equation used to determine

the amount of friction an object experiences on a flat surface is:

FF = μ x W

where FF is the force of friction measured in Newtons (N) or pounds (lbs), μ is the coefficient of friction which is unit-less, and W is the weight of the object.

DragDrag is a special kind of friction

that affects objects moving though any type of fluid. Air and water are two example fluids that vehicles move through. The amount of drag depends both on the shape and speed of an object. Drag has a greater effect on objects that move quickly.

Aerodynamic shapes have smooth edges and small profiles to reduce the effects of drag. Drag and friction play an important roll in races. Professional cyclists position their bodies a certain way and wear tight clothing, racecars are made with aerodynamic shapes, and bobsledders tuck themselves into a very sleek sled, all in an effort to reduce drag.

Lesson Summary Assessment

Who would go faster? A girl on a bicycle who is not pedaling and is standing up or a girl on a bicycle who is not pedaling and is crouched over her handlebars?

How much frictional force is available from a bike's tires while sliding if μ = .6 and the bike weighs 150 lbs with the person riding it?

What problems are most likely to occur in an engine that runs out of oil and is not properly lubricated?

How much frictional force is available from a bike's tires when not sliding if μ = .8 and the bike weighs 150 lbs with a person riding it? What does this mean compared to the value when sliding? Do the calculation.