Tuesday January 31st

Friction, Air Resistance, And Terminal Velocity!

What will we learn today?

• Content Objectives: Students will be able to compare amounts of friction in different scenarios

• Literacy Objective: Students will be able to define friction as the force that opposes the motion of two surfaces that are in contact

Yesterday…• …we talked about forces, free-body diagrams,

and balanced/unbalanced forces.• And one student (Gabe) brought up an

interesting idea…•…if we drag this block across the table… isn’t there friction that slows it down?? So don’t we need to add that to our free-body diagram?

Friction• Friction is the force that opposes the motion

of two surfaces that are in contact• The size of the frictional force depends on:– The materials the surfaces are made from (how

rough they are)– The force pushing the two surfaces together

(more force = more friction!)

A Closer Look at Friction

Three Types of Friction

• Static Friction: frictional force that prevents two surfaces in contact from sliding past each other – (there is no movement! The objects are static,

defined as “fixed or stationary”)

Man’s forceFrictional

force

Balanced Forces means NO MOVEMENT

Static Friction

• Static Friction increases as the downward force increases

• More downward force = more contact between the two rough surfaces = more friction.

• Let’s test it out!

Three Types of Friction• Sliding Friction: frictional force that acts in the

opposite direction to the motion of a surface sliding on another surface

• This time, there IS movement• Smaller than force due to Static Friction

Unbalanced Forces means A CHANGE in velocity = movement!Man’s

forceFrictional force

Three types of Friction• Rolling Friction: frictional force that acts in the

opposite direction to the motion of a surface ROLLING over another surface

• Smallest frictional force (of the three type)

Let’s take a look at how POWERFUL friction can be…

• http://www.youtube.com/watch?v=jSu0Tvlm6LY

• http://www.youtube.com/watch?v=hOt-D_ee-JE

Friction lets us…

• Walk on the ground without slipping!

• Rock Climb!• Stop a moving car!• Hold a Pencil• Wear Glasses or sunglasses

without them falling off

A Special Type of Friction

• Air Resistance is a type of friction that opposes the motion of objects that move through the air

• Without Air Resistance, all objects would fall at the same rate of speed

• Parachutes significantly increase air resistance, allowing for a safe landing for skydivers!

Skydivers

• Reach a terminal velocity of 120 mph when laying “flat” and 150-200 mph when diving head-first

• Terminal Velocity is when air resistanceequals the accelerating force of gravity and the object cannot go any faster.

• After the parachute is deployed, air resistance significantly increases and skydivers decelerate to a rate of 18mph

Lets see Air Resistance in Action:

• http://www.youtube.com/watch?v=1_fcX1YiWIU

• Balanced Forces: http://www.youtube.com/watch?v=Ays6luaS4r0