Answer these questions…
1. Why doesn’t earth fly off into space?
2. Where is Gondwanaland?
3. How do airplanes stay in the sky?
4. How much do you weigh in a pool of water?
How do you know if an object is in motion?
• It’s distance from another object is changing• The other object is the reference point ( a place
or object used for comparison to see if an object is moving)
How do you describe motion?
• Distance – How far it goes (measured in meters)
• Speed – How fast it goes
• Time – How long it takes to get there
• Formula for Speed = Distance/Time
• Example: 50 miles per hour
Constant Speed
• Constant Speed – When an object’s speed is the same at all times during it’s motion– Example – an ship traveling across the ocean
may move at the same speed for several hours
– Long Distance Swimmers/Runners – keep the same pace for a certain part of the race
– Practice Problem: A train travels at a constant speed of 80 miles in 2 hours. What is it’s speed?
Average Speed
• Most objects do not move at the same speed for the entire time that it is in motion.– Example: A cyclist may glide over level
ground, climb slowly up a hill, speed down hills
– Practice Problem: A car is driven 60 miles in 2 hours and 100 miles during 4 hours. What is the car’s average speed?
Velocity
• Speed in a given direction
• Example – 4 meters per second North
• Who would use velocity in their careers?
• How would they use velocity?
Theory of Plate Tectonics
• Earth’s plates move in various directions very slowly
• Pangaea – super continent
• Laurasia – Northern mass of super continent
• Gondwanaland – Southern mass of super continent
http://pubs.usgs.gov/publications/text/historical.html
http://library.advanced.org/17701/high/pangaea/
Figure this one out…
• Suppose scientists discover that a plate will move 5 centimeters per year.
• Can you predict how far the plate will move in 1,000 years?
• How far will the plate move in a day?
Alfred Wegener (1880-1930) Developed Theory of Pangaea
Alexander du Toit (1878-1948) Believed that Pangaea broke apart into 2 super-continents – Laurasia and Gondwanaland
Acceleration• A change in
velocity -either increasing speed, decreasing speed or changing direction
• Acceleration =
• (Final Velocity – Initial Velocity)/Time
Figure this one out…
• A roller coaster picks up speed as it rolls down a slope.
• As it starts down the slope it’s speed is 4 meters/second, but 3 seconds later it’s speed is 22 meters/second.
• What is the average acceleration?
Explain This…
• When you travel in a car, where do the pushes and pulls you feel come from?
• When you experience pushes/pulls associated with acceleration, you continue the motion you had BEFORE the acceleration began.
• Check out the go-kart picture
To Make Motion Graphs Easier:
1. A straight line means constant speed.
2. A horizontal line means no motion, 0 speed.
3. A positively sloped line means motion away from the reference point, + velocity
4. A negatively sloped line means motion toward the reference point, - velocity
5. A steeper slope = faster speed.
To Make Motion Graphs Easier:
6. The value of the slope is defined by the graphed units of time and distance
7. A curved line means a change in speed.
8. The rate of curvature defines the amount of acceleration.
Demonstration
• What affects the acceleration of an object?
• Which student moves faster?
• Which student moves further?
• Which student stays in place?
Why did this happen?
• Objects with a larger mass have a greater resistance to acceleration.
• Inertia – the tendency of an object to resist changes in motion
• Force – a push or pull (sometimes results in motion)
Demonstration – Penny Activity
• What will happen to the penny if you YANK the card out from under it?
Yikes!!! Help Me!!
Why did this happen?
• What actually happened? You caused the card to accelerate horizontally.
• Why did this happen? The force was applied to the card only – Inertia kept the coin from moving.
• Do you think it would be different if you pulled it slowly? It should go with the card everytime.
Balanced vs. Unbalanced Force
• Balanced Forces – all the forces acting on an object cancel each other out
• Unbalanced Forces – when there is a greater force in some direction causes movement
Friction
• A force that resists motion between two objects in motion that are touching
• Example – a runner’s shoes and the running surface
• Ice – not enough friction
• Mud – too much friction
• Example – air hockey table
Rolling Friction
• The friction that exists when a wheel turns on a surface.
• Caused by the small indentations created as one surface rolls over another
• Resists slipping and spinning
Gravity
• A force that pulls free falling objects to earth’s center
• All objects accelerate downward at a rate of 9.8 m/s2 (meters/second squared)
Demonstration – Money, Money
• Do all objects fall at the same rate?
• Which would fall faster a book or a dollar bill?
• Why?
• How can we get them to fall at the same rate?
Why did this happen?
• As objects fall, more massive objects are more attracted to the earth but are also harder to accelerate (because of inertia)
• Their inertia reduces their acceleration by exactly half as much as their greater attraction increases it
What is the difference between mass and weight?
• Mass – the amount of matter in an object
• Greater mass = greater inertia
• So mass= the amount of inertia in an object
• Weight – the gravitational pull of an object toward the earth
Newton’s Second Law
• The force of an object is equal to the product (think math – what is a product) of its acceleration and its mass
• Force = Mass x Acceleration
• Force is Measured in Newtons
• Mass – usually in Kg
• Acceleration = m/s2
Think about it…
• Since weight is a force you can rewrite Newton’s second law:
• Weight = mass x acceleration
Action vs. Reaction Force
• Action Force – a push on something
• Reaction Force - the force that pushes back
• Example – balloon, fire hose, bullet fire, passing gas in the bath tub
Newton’s Third Law Cont.
• How many objects is Newton talking about in his third law?
• Can Action and Reaction forces be added together?
• No• Why or Why not? Because they are acting
on different objects
Examples
• Figure Skaters – one skater pushes against the other – both move
• Squid – pushes water out - he moves forward
BUT….
• Remember the roller chairs….
• Who moved further?
• Speed and amount of movement when 2 objects are in contact depends on mass.