Download Friction and Gravity - elementaryscienceteachers

Friction, Gravity, and Elastic Forces
PH 380
Previously, in physical science…
• You learned about forces and combining
forces. Now, you continue to combine
forces and identify two or more forces
acting on an object.
Std 2d Students know how to…
• Identify separately the two or more forces
that are acting on a single static object,
including gravity, elastic forces due to
tension or compression in matter, and
friction.
1. What is friction?
The force that two surfaces exert on each
other when they rub against each other.
Byproduct of friction…Heat!
2. What direction does friction act?
• Acts in direction opposite to the
objects in motion.
3. How do you show friction as a
force?
• An arrow opposite of the object’s
motion
Friction
force
Motion
4. What determines the strength
of friction?
The types of surfaces (smooth or rough)
and how hard the surfaces push together
5. What are 4 types of friction?
a. Static friction: acts on object NOT
moving:
b. Sliding Friction
Two solid surfaces slide over each other
ex: book on desk
c. Rolling Friction
Object rolls across a surface
ex: ball bearings
d. Fluid Friction
• Solid object moves through a fluid
ex: oil, water, air
(what do you use in your car to reduce
friction?)
• What are two ways in which friction
can be useful?
•
allows objects to remain in place!
•
allows moving objects to stop to
avoid colliding with other objects!
6. What is gravity?
• The force that pulls objects toward
each other
7. How do you show gravity as a
force on Earth?
• An arrow toward the center of Earth
Gravity
http://youtube.com/watch?v=L55jImEhNZ0
8. What is weight?
• The gravitational force exerted on an object
9. What is the acceleration due to
Earth’s gravity?
• 9.8 m/s2
What is your weight (on Earth)?
Weight = Mass × 9.8 m/s2
Weight = ?
10. What is free fall?
• When the force of gravity
alone causes an object to
accelerate downward.
• Causes an object to
accelerate downward at 9.8
m/s2
11. What is air resistance?
• Type of fluid friction as an object
moves through air.
• Depends on speed and surface area
Air resistance
Air resistance
Free fall:
Free fall:
Gravity
Net Force
accelerate
downward
Air resistance
Terminal velocity:
Terminal velocity:
Gravity
0 net force
Move at
constant
speed
Air resistance
Gravity
Net Force
Air resistance
Let’s drop coffee filters (again)!
Leave one coffee filter open
Crumple the other coffee filter
Drop them at the same time, what happens?
Draw a diagram of your observations
Draw in your force arrows, including gravity
and air resistance.
Fluid friction
Gravity
Air
resistance
Rolling
friction
Gravity
12. When is matter elastic?
• When it returns to its
original shape after it is
squeezed or stretched.
…all matter has some degree of
elasticity…
13. What are 2 types of elastic
forces?
• Compression: when an elastic
force squeezes or pushes matter
together.
ex: couch cushion
• Tension: when an elastic force
stretches or pulls matter apart
ex: guitar string
Sometimes the
tension forces can
be great…like
when catching a
marlin…
…and
sometimes
not so great.
Compression
forces squeeze
together…like
the shocks on a
car
Think of
compression on
a sponge…or a
Twinkie