FrictionFriction

What is Friction?What is Friction?

• Friction - the force that opposes motion, or makes it difficult for an object to move across a surface.

• The amount of friction depends on the surface type and the force pressing the two surfaces together. Rougher surfaces will usually result in greater friction and smooth surfaces will result in less friction

Increasing or Decreasing Friction

Increasing or Decreasing Friction

• People are always trying to increase of decrease friction to perform everyday tasks.

• New tires on a car will help increase friction between the car and the road.

• Wax on skies will allow them to slide over the snow better.

Friction and Net ForceFriction and Net Force

• Net Force - the combination of all forces acting on a object.

In the picture below, friction between the box and the surface is equal to 2 newtons. If you pull the box to the right with 8 newtons of force the net force will be 6 newtons to the left.

Friction in a Fluid (Air)Friction in a Fluid (Air)

• Air Resistance is also called Drag and refers to forces that oppose the relative motion of an object through a fluid (a liquid or gas).

• Here the cyclists wear tight fitting clothes to reduce the amount of drag. This allows them to move through the air with less resistance.

Friction in a Fluid (Liquid)Friction in a Fluid (Liquid)

• When an object moves through a liquid it encounters a force that opposes its motion. To understand how much the opposing force will be one needs to consider the viscosity of the liquid. Viscosity is basically how much a liquid resists flowing. To keep it simple, honey has a high viscosity and water has a low viscosity. If you dropped a marble in a cup of honey the marble would encounter more friction (drag) than if you dropped the same marble in water.

ViscosityViscosity

Honey has High Viscosity Water has Low Viscosity

Static FrictionStatic Friction

• Static friction is friction between two solid objects that are not moving relative to each other. For example, static friction can prevent an object from sliding down a sloped surface or in the picture on the left the box does not move when the woman pushes on it.

Sliding FrictionSliding Friction

• Sliding friction is when two objects slide across from one another. Examples include the skis sliding on the snow or sliding a book across a desk.

• It is also known as Kinetic friction

Kinetic FrictionKinetic Friction

• Kinetic friction occurs when objects move. In the pictures on the left you see an example of rolling friction (car) and sliding friction (the tube).

Free FallFree Fall

• A free falling object is an object that is falling under the sole influence of gravity. Any object that is being acted upon only by the force of gravity is said to be in a state of free fall. There are two important motion characteristics that are true of free-falling objects:

* Free-falling objects do not encounter air resistance. (this happens in a vacuum or in outer space)

* All free-falling objects (on Earth) accelerate downwards at a rate of 9.8 m/s/s

Free FallFree Fall

• In the photo to the right, gravity is the force that is pulling on the rock.

• t = time in seconds, v = velocity• This photo shows that the rock will

speed up by 9.8 meters per second each second it falls. The drag, or air resistance (friction) on the rock will eventually cause the rock to reach terminal velocity (a point where the rock will no longer gain speed).

http://www.physicsclassroom.com/class/1dkin/U1L5b3.gif

Free FallFree Fall

• Free-fall is a type of motion in which the only force acting upon an object is gravity. Objects which are said to be undergoing free-fall, are not encountering a significant force of air resistance; they are falling under the sole influence of gravity. Under such conditions, all objects will fall with the same rate of acceleration, regardless of their mass.

• On earth we have and atmosphere so all objects will encounter air resistance (friction) when the object falling collides with air molecules.

• If the objects that fall are of similar shape they will fall at the same rate. Newton’s 2nd law of motion helps us understand this concept.

• You can try this by dropping a golf ball and a basket ball at the same time. They should hit the ground at the same time.

Free FallFree Fall

• Newton determined that F=ma (force = mass times acceleration).

• Using this formula it is also possible to determine the acceleration of an object in free fall and explains that acceleration due to gravity (on earth) is 9.8 m/s/s. (in this example we will round it to 10m/s/s).

• Acceleration is determined by dividing force by mass.

• As you see the two objects if dropped at the same time, from the same height will accelerate to earth at the same speed and hit the ground at the same time.

• An objects that have large flat surface areas will encounter more air resistance and will fall at a different rate. A feather or a parachute attached to an object will slow the object down. This is where friction helps us.

What type of Friction?What type of Friction?

Kinetic or static?

What type of Friction?What type of Friction?

• What type of Friction?

Kinetic or Static?

• Kinetic, all objects are in motion. The car is experiencing rolling kinetic friction, the tuber and shark are experiencing sliding kinetic friction.

How is Free Fall on Earth different than Free Fall in outer space?

How is Free Fall on Earth different than Free Fall in outer space?

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How is Free Fall on Earth different than Free Fall in outer space?

How is Free Fall on Earth different than Free Fall in outer space?

• On earth objects in free fall encounter the force of gravity and some air resistance. In outer space objects only encounter the force of gravity and NO air resistance. This is demonstrated in the hammer/feather experiment that the Apollo 15 astronauts conducted to prove Galileo’s theory.

(click or copy the link below to watch)http://www.youtube.com/watch?v=5C5_dOEyAfk