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”)
Frictional
force
Man’s
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
Frictional
force
Man’s
force
Unbalanced Forces
means A CHANGE in
velocity = movement!
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=jSu0Tvlm6
LY
• http://www.youtube.com/watch?v=hOt-D_eeJE
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 resistance
equals 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_fcX1Yi
WIU
• Balanced Forces:
http://www.youtube.com/watch?v=Ays6luaS4
r0