FRICTION

What is FRICTION?

• When you push your book on your desk, the 2 surfaces rub together.
• The force that one surface exerts on another when the two rub against each other is called FRICTION.
• 4 types (static, sliding, rolling, and fluid)

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Static Friction

• The force that acts on objects that are not moving is called static friction.
• Ex: Moving a desk across a floor. If you push the desk with a force less than the force of static friction between the desk and the floor, the desk will not move.

Sliding Friction

• Sliding friction occurs when two solid surfaces slide over each other.
• Ex: Ballet dancers apply a sticky powder to the soles of their ballet slippers so they won’t slip on the dance floor.

Rolling Friction

• When an object rolls across a surface, rolling friction occurs.
• Rolling Friction is easier to overcome than sliding friction.
• Ex: Engineers use ball bearings (made of smooth steel) to reduce the friction between the wheels and the moving parts of a bicycle.

Fluid Friction

• Fluid friction occurs when a solid object moves through a fluid.
• Fluids, such as water, oil, or air, are materials that flow easily.
• Ex: A surfer must overcome the fluid friction of the water.

• Friction acts in the direction OPPOSITE to the direction of the object’s motion

What affects friction?

• The strength of the force of friction depends on 2 factors: the types of surfaces involved and how hard the surfaces push together.
• Example: Rough surfaces produce more friction

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Is Friction useful or not?

• Go to the following website, list 3 situations in which friction is useful…and 3 in which it is not useful
• Dirtmeister: Friction -- Investigate the Facts

FRICTION….

• Now I want you to go to the following website. FearOfPhysics.com: What is Friction? Read the web page and explain in your own words what the drawing is showing you.
• After you have explained what it is showing you, click on the bottom of the screen where it says “Click here to do an experiment with friction” …. show Mrs. Allen when you have successfully completed the friction activity (try at least THREE different scenarios = 5 points).

Bar Graph/Frictional Surfaces

Which surface produces the most friction?

Gravity

• Gravity is the force that pulls all objects to Earth.

• Gravity accelerates all objects to Earth at 9.8 m/s²…in other words acceleration due to gravity is 9.8 m/s² .
• Two factors affect the gravitational attraction between objects: mass and distance.

Gravity keeps the moon in orbit with the Earth

WEIGHT

• Measure of the force of gravity on an object
• Formula

Weight = mass X acceleration due to gravity

Free Fall

In fact, all objects will fall at 9.8m/s² if the air does not act to slow them down.  An easy proof of this is to drop a grape and a grapefruit at the same time from the same height. 

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AIR RESISTANCE

• A type of fluid friction
• Greater the surface area the greater the air resistance
• The greatest velocity a falling object reaches is called its terminal velocity.

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AIR RESISTANCE

However, if you drop a book and a piece of paper, you will notice that the paper does not accelerate as much as the book because the small force of gravity on the paper is easily opposed by the force of air friction on the paper.  A similar force acts on the book from the air but it is small compared to the force of gravity on it. 

PROJECTILE MOTION

• Objects thrown in a horizontal direction will hit the ground at the same time as one dropped straight down.

Projectile Motion