Transcript Gravity, Air resistance, Weight, Mass & Friction

Gravity, Friction and Simple
Machines
BY: BRIANNA SHIELDS
Do Now
• 1. How do you calculate total force if the
two forces are in opposite directions?
• 2. Which of Newton’s laws of motion
explains inertia?
• 3. What happens to an object if two forces
are equal but in opposite directions?
GOAL
• To relate gravity and weight
• To define friction and air resistance and
design ways to increase it or decrease it
• To define work
• To describe various simple machines
• Gravity
• 1. Force of attraction-pulls things
downward
• 2. Objects fall at a rate= 9.8 m/s2
– acceleration due to gravity
Example
1 sec
2 sec
3 sec
After falling for 7
seconds what is the
acceleration of the
ball?
4 sec
5 sec
6 sec
7 sec
Speed of a falling
object
V = 9.8 x t
(V= a(g) x t )
Free Fall
• Terminal
Velocity
• Object no longer accelerates- it
travels at a constant speed
• Law of
Universal
Gravitation
• 1. All objects have gravity
• 2. Massive objects = greater
gravity
• 3. Objects close together= more
gravitational attraction
Less Gravity
Less Gravity
More Gravity
More Gravity
• Weight
• 1. Amount of gravity that pulls on
an object
– Closer to core = more gravitational pull
• 2. Weight changes with location
• WEBSITE: Your weight on other
worlds
• Weight
• 3. W= m x 9.8
– Weight= mass x acceleration due to
gravity
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Weight Calculations
• What would a 10kg book weigh in
newtons?
• W = ma = 10kg x 9.8 m/s^2 = ?
• What would a 50kg person weigh in
newtons?
• W = ma= 50kg x 9.8 m/s^’2 = ?
If a feather and a hammer are
dropped at the same time,
which will hit the ground first?
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Why does the hammer hit the ground
before the feather?
• Air
resistance
Q uic k Tim e™and a
TIFF (U ncompres s ed) dec ompres s or
are needed to s ee this pic ture.
• Prevents objects from falling at 9.8
m/s
• Slows objects down
• Object rubs against air molecules
• Pushes upward on objects
• Due to object’s shape
If a feather and a hammer are dropped at
the same time on the moon, which will hit
the ground first?
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Find out by watching “Brain Bytes” NASA Vodcast
• Friction
• 1. Exists Where 2 surfaces touch
• 2. Acts in direction opposite to
object’s motion
Friction acting on an incline
• Often, picking up a heavy object is actually
easier than trying to slide it across the floor
Pushing
FRICTION
• Types of
friction
• 1. Sliding- produced when solid objects slide
over each other
•
2. Rolling Friction- produced by wheels
• 3. Fluid friction- created when an object
moves through/across a fluid (water, oil or air)
Examples of Friction
• Come up with atleast two examples of
each friction: sliding, rolling and fluid
• Come up with atleast two examples during
which friction would be helpful
Label with the type of friction:
Label with the type of friction:
Label with the type of friction:
• The movement of lubricated engine parts
Label with the type of friction:
Label with the type of friction:
Label with the type of friction:
Label with the type of friction:
When Friction is Helpful
Compare these two animations. How can the differences
be explained?
No Gravity: What will happen when
the banana is shot from the cannon?
With Gravity: What will happen when
the banana is shot from the cannon?
You will view a photograph of two balls
being released from an apparatus.
Which will hit the ground first? The
one dropped straight down or the one
shot out sideways?
Website:
http://www.fearofphysics.com/XYInd
ep/xyindep.html
Projectile Motion Tutorial Online:
http://www.mhhe.com/physsci/physic
al/giambattista/proj/projectile.html
• Work
• A force acting through a distance
to move an object
– Must be moved over a distance
– Object must move in direction of
force
• Work = force x distance
• Work
• Examples
– Pushing a desk
– Throwing a baseball
– Lifting a shovel
• Not Examples
– Carrying a grocery bag
– Pushing on a wall
– Carrying a shovelful of snow
Work or Not??? Explain
•
•
•
•
Picking up a grocery bag
Carrying a grocery bag to the door
Throwing a baseball 20 meters
Lifting a suitcase to put into the overhead
luggage compartment
• Pushing against a cement slab until exhausted
Work or Not??? Explain
• Carrying a bag of cat litter home from the
store
• Slamming a tennis ball across the net
• Studying all night for the science test
• Finger pushing down the RETURN key
on a computer
Work or Not??? Explain
• Carrying a shovel full of snow from the driveway
to the lawn
• You and a friend pushing a heavy piano across a
wooden floor
• Standing for half and hour in the freezing cold
waiting for the bus to come
Work Example
• If you lifted an object weighing 200 N
through a distance of 0.5 m, how much
work would you do?
• W = F x D = 200N x 0.5m = 100nm
Work Example
• If you picked up a rock weighing 350 N for
a distance of 1.0m, how much work would
you do?
• W = F x D = 350N x 1.0m = 210 nm
• What do
machines
do?
• Change small forces into large
forces
• Allow small forces to move
large objects
• Change the size and direction of
the force
• Doesn’t reduce work- divides it
into smaller jobs
• Inclined
plane
• Ramp, slanted surface
• Ex: truck ramp
• Wedge
• Moving inclined plane with
edges
• Ex: knife, scissors, key
• Screw
• Bar wrapped with threads for
fastening
• Ex:
– Lightbulb
– Jar Lid
• Lever
• Bar that pivots up and down
• Ex: hammer, teeter toter
• Pulley
• Rope wrapped around a wheel
• Ex: window shade pull
• Wheel and
axle
• 2 different sized spinning circles
• Ex: fan, sawblade, screwdriver
Machine Assessment
• Work with your group members to create your
own examples (atleast two) for each machine:
–
–
–
–
–
–
Pulley
Lever
Wedge
Screw
Inclined plane
Wheel and axle
Simple Machine Websites
• Site 1: Edheads - Simple Machines
Activities - Lever ...
• Site 2: Simple Machines Made Simpler - by
The Spice B...
Homework
• Write the following assignment into your
agenda:
– Physics Homework