Motion Notes Physical Science

Physics of Motion We will look at: Position Distance Displacement Speed Velocity Acceleration First you need to realize that motion is relative…

Motion is Relative Motion is relative to the observer’s position and their reference point Sometimes called a “frame of reference” Consider the picture… If this man is driving at 15 mph, how fast is his coffee cup moving? Does the man feel like the cup is moving? Why?

Motion is relative An object is in motion if it changes position relative to a stationary reference point. Use this to establish a COORDINATE SYSTEM pick an origin or 0 point decide which direction is positive

Draw an Example:

Direction We use North, South, East, West or left & right to describe the direction of movement. We can also use POSITIVE and NEGATIVE to describe direction.

POSITION A location with respect to the origin or zero point.

Finding Position 4 miles 6 miles 4 10 -4 6 -10 -6 Position depends on where you put ‘0’.

Distance and Displacement Distance measures the actual path an object takes Displacement measures your overall distance from the initial position to final position in a STRAIGHT LINE. DISPLACEMENT values must include a DIRECTION! Which color line represents distance? Displacement?

Scalar vs Vector Quantities All measured quantities can be classified as being either a scalar or a vector. Scalar _________ only (size of the quantity ….a number) Vector _________ and _________ Magnitude Magnitude Direction

Distance vs. Displacement http://physics.info/displacement/

Use the diagram to determine the resulting displacement and the distance traveled by the skier during these three minutes.

Answer The skier covers a distance of (180 m + 140 m + 100 m) = 420 m and has a displacement of 140 m, right or east.

Write in your own words- what is the difference between distance and displacement?

Formula for Displacement ∆X = Xf-Xi Xf= final position Xi= initial position Remember- displacement can be positive or negative! Why?

What is the coach's resulting displacement and distance of travel?

Answer The coach covers a distance of (35 yds + 20 yds + 40 yds) = 95 yards and has a displacement of 55 yards, left or -55 yards.

Speed Speed is the distance traveled in an amount of time We know some things move faster than others…but how do we measure it?

What two quantities must you know to determine speed? Speed= distance/time units- miles/hr, km/hr m/s

There are three types of speed you must know… Constant speed Instantaneous speed Average speed

Constant Speed When an object covers equal distances in equal amounts of time Ex- if a race car travels at a CONSTANT SPEED of 96m/s, it will travel a DISTANCE of 96 meters EVERY SECOND.

But most objects do not travel at a constant speed. The speed of an object can change from one minute to another. So we can use AVERAGE SPEED to describe its motion-overall distance moved over the entire time Use this equation… Average Speed = total Distance / total Time

Instantaneous Speed Instantaneous speed- measures speed at an instant in time

Does a speedometer of a car read instantaneous or average speed?

What 2 controls on a car enable a change in speed?

What if I want to describe speed AND direction? For example…what if you wanted to find a plane. Knowing the speed would only tell you how far away to look but not in what direction. For that we need… VELOCITY

Let’s get back to the car example… Name another control that enables a change in velocity.

So…What is the difference between speed and velocity? SPEED- reports the magnitude of distance over time (just the number). VELOCITY- reports the speed AND direction of motion.

Are speed and velocity always going to be the same? NO! Only if the object is moving in the SAME direction the whole time. What is an example of a motion where avg speed is not equal to average velocity? (demo)

Constant Velocity Same displacement per unit of time

So how do you calculate speed vs velocity? Mathematically: Velocity = displacement / time (Includes a direction!)

Vavg= (xf-xi) / (tf-ti) Let’s look closer… Average velocity is calculated by the equation: Vavg= (xf-xi) / (tf-ti)

Use the diagram to determine the average speed and average velocity between the following points. A. Going from A and B B. Going from A to B and ending at C

Answers: A. Between A and B, speed and velocity will be the same because they are in the same direction the whole time. Speed = 180 m/min Velocity = 180 m/min east B. When the motion continues to point C, the speed and velocity are different because the distance traveled and displacement are different. Speed= 320 m/2 min= 160 m/min Velocity= 40 m/2 min= 20 m/min east

Acceleration Chapter 10.2-3

Acceleration is a change in velocity in a period of time. So…Is it a vector or a scalar quantity? VECTOR It must include a direction (like velocity) This also means that you can change acceleration by changing what? Speed or Direction

Does travel at a constant speed mean you are not accelerating? Remember that you can change velocity by changing direction, thus CHANGING ACCELERATION!

Causes of acceleration Increasing velocity Example: Car speeds up at green light Decreasing velocity Example: Car slows down at stop light Changing Direction Example: Car takes turn (can be at constant speed) screeeeech

What do the numbers mean? Small acceleration – speed is increasing slowly Large acceleration- speed is increasing rapidly

Positive vs. Negative Acceleration Acceleration and velocity are in the same direction when speeding up and opposite directions when slowing down

Calculating Acceleration If an object is moving in a straight line… ∆V= change in velocity (Final Velocity – Initial Velocity) (m/s) A= acceleration ∆ T= time (s) (final time- initial time) ∆V So what are the units of acceleration? If A= V/T… Acceleration is in m/s2

Calculating Acceleration Acceleration = (Vf-Vi) t Let’s look at the picture below…what is this car’s avg acceleration? 0 s 1 s 2 s 3 s 4 s 0 m/s 4 m/s 8 m/s 12 m/s 16 m/s

Question A skydiver accelerates from 20 m/s to 40 m/s in 2 seconds. What is the skydiver’s average acceleration?

Constant velocity

Now consider a car moving with a rightward (+), changing velocity - that is, a car that is moving rightward but speeding up or accelerating.

Acceleration

Distance time graphs Draw 2 graphs One showing a slow constant speed One showing a faster constant speed

Position-Time Graphs Slow, Rightward(+) Constant Velocity Fast, Rightward(+) Constant Velocity                                                                                                                                                                               

More Position-Time Graphs Slow, Leftward(-) Constant Velocity Fast, Leftward(-) Constant Velocity                                                                                                                                                                             

Consider a car moving with a constant, rightward (+) velocity - say of +10 m/s. A car moving with a constant velocity is a car with zero acceleration. Draw a graph!

Constant velocity = zero acceleration

Positive Acceleration Now consider a car moving with a rightward (+), changing velocity - that is, a car that is moving rightward but speeding up or accelerating. Since the car is moving in the positive direction and speeding up, the car is said to have a positive acceleration.                                                                                      

Positive acceleration

Describe this graph!

Does the velocity of the wind affect such things as a sprinter’s speed or an airplane’s flight time?

Resultant Velocity animation http://www.glenbrook.k12.il.us/GBSSCI/PHYS/mmedia/ vectors/plane.html

“Adding Vectors” Example: A small airplane heads east with a speed of 200 mph with respect to the air (the “air speed”). This would be the plane’s speed if the air was NOT moving – no wind) If the wind/jet stream is moving east at 50 mph, what is the plane’s resulting velocity with respect to the ground (the “ground speed”)? 200 50 250 mph, east with the wind

If, later, the airplane is flying west into the 50 mph wind with an “air speed” of 200 mph, now what is the plane’s resulting velocity with respect to the ground (the “ground speed”)? 200 50 150 mph, west against the wind

Find the velocity in m/s of a swimmer who swims 110 m toward the shore in 72 s.

1.5 m/s toward the shore

Imagine that you could ride a baseball that is hit high enough and far enough for a home run. Using the baseball as a reference frame, what does the Earth appear to do?

Calculate the displacement in meters a cyclist would travel in 5 Calculate the displacement in meters a cyclist would travel in 5.00 h at an average velocity of 12.0 km/h to the southwest.

But first let’s look at some graphs… If I wanted to graph speed, what should I label my axes??? So the slope of the line=SPEED Distance Speed Time

Constant Speed What would a position-time graph look like for a constant speed?