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. 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.)
- 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/s^{2}.
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.
- 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
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