LINEAR MOTION UNIT ONE PHYSICS ONE

I CAN:  DEFINE MOTION  DEFINE SPEED  DEFINE VELOCITY  EXPLAIN HOW VELOCITY CHANGES  CALCULATE AVERAGE VELOCITY  DISTINGUISH BETWEEN SPEED AND VELOCITY

MOTION IS RELATIVE  Motion is everywhere. It is easy to recognize, but hard to describe. Historically, motion was not well understood because of the failure to understand rate.  Rate = quantity/time

 Familiar rates to us include speed, velocity, and acceleration.

SPEED  The simplest form of motion is along a straight path.

 Everything moves, even things you think are resting. We can say this because everything is moving relative to something else.  Ex. A book on a table compared to the stars.

 Racers at the Indy 500 are moving at 300km/hr relative to the track.

 Speed: a measure of how fast something is moving. This is the rate at which distance is covered.

 Units to describe distance and time can be used to describe speed. Examples are: mph, k/h, centimeters/day, light- years/century and meters per second (m/s).

 Speed at a given time is called instantaneous speed. A car doesn’t always go the same speed, sometimes it is faster and sometimes slower, but at any time the speedometer will give you instantaneous speed.

 If you would like to know how long it would take you to travel somewhere you will need to figure out the average speed.  Average speed = distance covered  Time interval

 So, if we travel 60km in one hour the average speed is 60km/h. Or, if we travel 240km in 4 hours, our average speed is 60km/h.

VELOCITY  Velocity: speed in a given direction.  In everyday language, velocity and speed would be interchangeable. In physics, they are not. Velocity includes direction. 

 Motion at a constant velocity is motion in a straight line at a constant speed.  Changing velocity includes a change in either the speed or the direction. This can be accomplished in a car three different ways.  Steering wheel  Brake pedal  Gas pedal

I CAN…  FIND THE AVERAGE ACCELATION IN A PROBLEM GIVEN VELOCITIES AND TIME  DISTINGUISH BETWEEN VELOCITY, SPEED AND ACCELERATION  GIVE 3 WAYS IN A CAR TO CHANGE ACCELERATION

ACCELERATION  The measure of how fast the velocity is changing is called acceleration.  This applies to both increases and decreases (deceleration) in velocity (including direction).

 Acceleration is also a rate  Acceleration (straight line) = change in velocity  Time interval

 We experience acceleration when someone slams on a gas pedal, or brakes, or when they round a corner at a constant speed.

 Notice that speed and velocity are measured in units of distance per time. Acceleration is measured in units of distance per time, per time.  For example you accelerate at 5mph/s, or at 10km/h/s.  You will have two units of time for acceleration.

I CAN…  CALCULATE THE VELOCITY OF AN OBJECT IN FREEFALL  CALCULATE THE DISTANCES OF AN OBJECT IN FREEFALL  EXPLAIN FREEFALL

Free fall: How Fast  Compare an apple dropped from one- story vs. one dropped from a jet. Could you catch the apple dropped by the jet? Probably not, so this proves that objects accelerate as they fall.

 Gravity causes this acceleration.  In real life, air resistance also plays a part in how the object behaves. But without air resistance, we would say they object is in free fall, or affected only by gravity.

 The speed of freefall is approx. 10m/s2 (9.8m/s2 exactly)  So, every second an object will fall 10m/s faster.  Customarily the letter (g) will be used to represent the fall because it is due to gravity.

 Now think about an object that we throw up in the air, and then it falls. When the object is changing from upward to downward motion, the speed is zero.  The drop downward begins as if it had been dropped from rest position.

 What is the rate of deceleration when the object is moving upwards? The same as when it falls downward: 10m/s/s

Free fall: How Far  Speed and distance do not mean the same thing. How fast something moves is not the same as how far something moves.

 Table 2.2 shows us that after 1 second a falling object has an instantaneous speed of 10m/s. It hasn’t fallen 10 meters, this is its instantaneous speed. If the object falls 10 meters in the first second, its average speed is 10m/s. However, our object took one full second to reach 10m/s. The average speed must be between 0 and 10m/s.

 For any object moving in a straight line at a constant acceleration, we find the average speed they way you find any average. Add and divide by two.  Adding 10m/s + 0 = 10.  Dividing by 2 = 5   So, during the first second, the average speed was 5m/s.  In the first second the object fell 5 meters.

 Question: During the span of the second time interval in table 2.2, the object begins at 10 m/s and ends at 20 m/s. What is the average speed of the object during this 1-second interval? What is its acceleration?

 Table 2.3 shows the total distance of a freely falling object dropped form rest. These distances show a mathematical pattern.

 We can use this equation to find the distances objects fall from rest:   d = ½ gt 2  d= distance  g= gravity (10m/s/s)  t = time

 Freely falling objects are used to describe the relationship between distance traveled, acceleration and velocity acquired. We used the acceleration of gravity, but object need not be freely falling. Acceleration can be a slowing down of an object or a speeding up of an object.

I CAN…  Create graphs of position/time and velocity/time.  Analyze graphs to describe what motions are shown, i.e.., acceleration, velocity, etc.

Graphs of Motion  Velocity and acceleration can also be represented not only by equations and tables, but by graphs. This gives a visual of the relationships.

Air Resistance and Falling Objects  On Earth, dropping two different items will result in one reaching the floor ahead of the other. Air resistance is responsible for this phenomenon. This can be shown at any time. However, when placing the same two items in a vacuum chamber, they fall at the same time and the same speed

 Air resistance is more noticeable with some objects, like paper, feathers, than with others, such as baseballs or rocks. In many situations, air resistance is negligible. Falling objects in this case can be considered freely falling.

How Fast, How Far, How Quickly: How Fast Changes  “How fast” and “How Far” can often be confused. How fast something falls related to its speed, or velocity. How far something falls is specified with a distance.

 Also confusing is acceleration, or the rate at which a rate changes. Acceleration is not velocity, or a change in velocity (which could just be a directional change). Acceleration is the rate at which velocity changes.