Chapter 11, Section 1 - Measuring Motion

Motion - An object's change in position, relative to a reference point (another object that stays in place).

Frame of Reference - A system of describing the precise location of objects in space and time.
When an object changes position with respect to a frame of reference, the object is in motion.

Distance - How far an object moves.
Displacement - The change in an object's position.
• If you run around the school and end up ten feet from where you started, then...
• the distance you ran was all the way around the school.
• your displacement was 10 feet (how far from your starting point you ended up).
Speed - How fast an object is moving.
• Speed = Distance/time
Velocity - The speed and direction of an object.
• Describing velocity requires a reference point.
• If you are sitting in a bus that is going east at 15 m/s, then your velocity is...
• 15 m/s to the east, relative to the ground.
• 0 m/s relative to the bus.
• Velocities can be combined...
• As the bus is moving, you get up and start walking to the rear of the bus at a rate of 1 m/s.  Your new velocity is...
• 14 m/s to the east, relative to the ground. (15 m/s - 1 m/s = 14 m/s)
Measuring Speed
• Constant speed is when an object covers equal distances in equal amounts of time.
• If you move 2 m/s, then you are moving two meters every second.
• Average speed is the total distance traveled divided by the time it took to travel that distance.
• This is the same as the formula speed = distance/time.
• Instantaneous Speed is the speed at any given instant.
• (Think about your car's speedometer - it tells how fast you're driving at any given moment.)
Graphing Motion
• When you graph an object's motion (using a line graph)...
• Distance goes on the vertical (Y) axis.
• Time goes on the horizontal (X) axis.
• The slope of the graph is equal to the object's speed.
• If the object stops, time is still going on - so this is represented by a flat line.

Homework:
Pg. 371 #1-6

Chapter 11, Section 2 - Acceleration

Acceleration - The rate at which velocity changes over time.
• Change in speed.
• Speeding up = positive acceleration.
• Slowing down = negative acceleration.
• Change in direction.
• Centripetal Acceleration - Acceleration due to moving in a circle.
Acceleration is measured in meters per second per second (how many meters per second faster you go each second).  This is written as m/s/s or m/s2.

To calculate acceleration, divide the change in velocity by the time over which the change took place.
• Change is represented by the Greek letter delta (which looks like a small triangle).
• So change in velocity is written as delta-v.

Graphing Acceleration
• When you graph an object's motion (using a line graph)...
• Speed goes on the vertical (Y) axis.
• Time goes on the horizontal (X) axis.
• The slope of the graph is equal to the object's acceleration.
• In the graph to the right, the object slows down, stops, then speeds up and finally travels at a constant speed.
• Downward slope - slowing down.
• Upward slope - speeding up.
• Flat line - constant speed (including stopped).
• If you look at a speed graph (distance over time), a curving line indicates acceleration.

Homework:
Pg. 377 #1-2, 4-6

Chapter 11, Section 3 - Motion and Force

Force - An action exerted on an object that can change the object's state of rest or motion.  (Any push or pull on an object.)
• Force has a magnitude (size) and direction).
Types of Forces
• Fundamental Forces (4) in nature.
• Gravity - Attraction between two masses.  (Very weak, but occurs over huge distances.)
• Electromagnetic Force - Electric and magnetic effects.
• Strong Nuclear Force - Holds the protons and neutrons in an atom's nucleus together.  (Very strong, but over tiny distances.)
• Weak Nuclear Force - Cause radioactive decay (when an element breaks down into another element).
• Contact vs. Noncontact Forces
• Contact Forces - Objects must touch to put a force on each other.
• Example - Physical push or pull.
• Field Forces - Forces that do not require contact.
• Example - Magnetism, gravity, etc.
Balanced and Unbalanced Forces
• Net Force - The combination of all the forces acting on an object.
• If the net force = 0, the object does not change its motion.
• If the net force does not equal zero, the object accelerates in the direction of the net force.
Friction - A force that opposes motion.  It occurs between two surfaces that are in contact.
• Static Friction - Friction between stationary surfaces.  (Opposes the initiation of movement.)
• Kinetic Friction - Friction between moving surfaces.  (Opposes the continuation of movement.)
• Sliding friction occurs when objects rub against each other.
• Rolling friction occurs when a round object moves over a flat surface.
Friction is necessary for movement.
• You can decrease friction by using low-friction materials (like on a non-stick pan) or lubricants (like grease).
• You can increase friction by making the surfaces rougher or by increasing the force between the surfaces.

Homework:
Pg. 385 #1-7

Resources
Speed and Velocity Song by They Might Be Giants