2 Physics of Motion We will look at: DistanceDisplacementSpeedFirst you need to realize that motion is relative…
3 Motion is relativeWhat is meant by saying that motion is relative? For everyday motion, what is motion usually relative to?
4 Motion is RelativeMotion is relative to the observer’s position and their reference pointSometimes called a “reference frame”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?
5 Motion is relativeAn object is in motion if it changes position relative to a stationary reference point.
6 DirectionWe use N, S, E, W to describe the direction of movement.
7 Distance and Displacement Distance measures the actual path an object takesDisplacement measures your overall distance from the starting point in a STRAIGHT LINE.DISPLACEMENT values must include a DIRECTION!Which color line represents distance?Displacement?
9 Write in your own words- what is the difference between distance and displacement?
10 Speed Describes how fast an object moves. We know some things move faster than others…but how do we measure it?What two quantities must you know to determine speed?
11 What two quantities must you know to determine speed? Choose from: displacement, distance, time, velocityHint…what is speed measured in???Speed= distance/timeEx- miles/hour, m/s, etc.
12 There are three types of speed you must know… Constant speedAverage speedInstantaneous speed
13 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=SPEEDDistanceSpeedTime
14 Constant SpeedWhen an object covers equal distances in equal amounts of timeEx- if a race car travels at a CONSTANT SPEED of 96m/s, it will travel a DISTANCE of 96 meters EVERY SECOND.
15 Constant SpeedWhat would a d-t graph look like for a constant speed?
16 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.Use this equation…Average Speed = total Distance / total Time
17 Let’s try itA runner finished a 3 mile race in 22 minutes. He may not have run at the same pace the whole time, but you can still calculate his AVERAGE SPEED3 miles/22 minutes=.14 miles/minPACE:22 minutes / 3 miles = 7.33 min/mile
18 Instantaneous vs. Average speed Average speed- overall distance over time the object traveledInstantaneous speed- measures speed over small time interval (at an instant)
19 Does a speedometer of a car read instantaneous or average speed?
20 What 2 controls on a car enable a change in speed?
22 Let’s use our math skills Page 323…Read through “MATH SKILLS”DO problems 1-3 “Practice Problems”
23 Answers to problems on 323 1. v=d/t 110m/72 sec= 1.5 m/s toward shore 38m/1.7 sec= 22m/s toward first based=vt=(12.0 km/hr)(5.00 hr)= 6.00 x 104 m
24 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- the speed and direction of motion.
25 Let’s get back to the car example… Name another control that enables a change in velocity.
27 In order to analyze such situations, we need to understand what are known as vectors… All measured quantities can be classified as being either a scalar or a vector.Scalar_________ only (size of the quantity ….a number)Vector_________ and _________MagnitudeMagnitudeDirection
28 Speed is Velocity…kind of… Velocity is the vector for the scalar of speed.The magnitude is the same!!!Therefore in calculations, you can use speed or velocity interchangeablySpeed = distance / timeVelocity = distance / time in a direction
43 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.
48 “Adding Vectors” Example: A small airplane heads east with a speed of 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”)?20050250 mph, eastwith the wind
49 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”)?20050150 mph, westagainst the wind
50 Find the velocity in m/s of a swimmer who swims 110 m toward the shore in 72 s.