SACE Stage 1 Conceptual Physics 2 Motion 2

SACE Stage 1 Conceptual Physics 2. Motion

2. 1 Motion is Relative Everything moves. Things that appear to be at rest move. l They move relative to the Earth, Sun and the stars. l A book at rest on the table is moving, it is moving at about 30 km/s relative to the sun. l

2. 1 Motion is Relative We describe the motion of an object as relative to something else. l When a car has reached 60 km/hr, it has reached 60 km/hr relative to the ground. l Unless stated otherwise, when we describe speed in nature, assume it is relative to the ground. l

2. 2 Speed Moving objects will cover a certain distance in a given time. l A car travels in km/hr. (Distance / Time) l Speed is the measure of how fast something is moving. l It is the rate at which distance is covered. l

2. 2 Speed l When we see the word Rate in a statement, we know that something is being divided by time.

2. 2 Speed l Table 2. 1 – Approximate Speeds in different units. 20 km/hr 40 km/hr 60 km/hr 80 km/hr 100 km/hr 120 km/hr = = = 12 mi/hr 25 mi/hr 37 mi/hr 50 mi/hr 62 mi/hr 75 mi/hr = = = 6 m/s 11 m/s 17 m/s 22 m/s 28 m/s 33 m/s

2. 2 Speed – Instantaneous Speed l The speed of an object at a particular point in time is called its instantaneous speed.

2. 2 Speed – Average Speed l Average speed is the total distance covered divided by the total time taken to travel that distance.

2. 2 Speed – Average Speed l Average Speed is an easy calculation. l If it takes 4 hours to cover 240 km. What is your Average Speed?

2. 2 Speed – Average Speed Calculation

2. 2 Speed – Question 1 l With the Speedometer in the dash of every car is an odometer, which records the distance travelled. If the initial reading is set to zero at the beginning of a trip and the reading is 35 km one-half hour later, what has been your average speed?

2. 2 Speed – Question 1 Answer

2. 2 Speed – Question 2 l Would it be possible to attain this average speed and never exceed the 70 km/hr?

2. 2 Speed – Question 2 Answer l No, not if the trip started from rest. For any period of time when the car was travelling less than 70 km/hr, there would have to be periods of time where the car travelled greater than 70 km/hr to be able to attain an average speed of 70 km/hr.

2. 2 Speed – Question 3 l If a cheetah can maintain a constant speed of 25 m/s, it will cover 25 m every second. At this rate, how far will it travel in 10 s? 1 minute?

2. 2 Speed – Question 3 Answer l When can re arrange our formula to allow us to work out the distance travelled.

2. 3 Velocity What is the difference between speed and velocity? l Velocity is a speed in a given direction. l A car that is travelling at 60 km/hr has speed but when we say 60 km/hr North, it now has velocity. l

2. 3 Velocity – Question l The speedometer in a car moving northward reads 60 km/hr. It passes another car that travels at 60 km/hr Southward. Do both cars have the same speed? Do they have the same velocity?

2. 3 Velocity – Question Answer l Both cars have the same speed but will have opposite velocities as they are travelling in opposite directions.

2. 3 Velocity – Constant Velocity To achieve constant velocity, it is reasonable to see that an object must have constant speed and constant direction. l Motion at constant velocity is motion in a straight line. l

2. 3 Velocity – Changing Velocity If either the speed or direction change, then velocity has changed. l In a car there are three things that can change velocity. The accelerator increases speed, the brake decreases speed and the steering wheel changes the direction. l

2. 4 Acceleration describes how velocity is changing. l Velocity can change by changing its speed or direction. l Although the moon that orbits the Earth isn’t changing it’s speed, it is accelerating as it is changing it’s direction. l

2. 4 Acceleration

2. 4 Acceleration When driving a car, there is 3 ways to accelerate. l Speed up, slow down or change direction. l

2. 4 Acceleration – Question 1 l Suppose a car moving in a straight line steadilty increases its speed each second, first from 35 – 40 km/hr, then from 40 - 45 km/hr, then from 45 – 50 km/hr. What is its acceleration?

2. 4 Acceleration – Question 1 Answer l We see that the speed increases by 5 km/hr during each 1 s interval. The acceleration is therefore 5 km/hr each second.

2. 4 Acceleration – Question 2 l In 5 seconds a car is moving in a straight line changes its speed from 50 km/hr to 65 km/hr while a truck goes from rest to 15 km/hr in a straight line. Which undergoes the greater acceleration? What is the acceleration of each vehicle.

2. 4 Acceleration – Question 2 Answer l The car and the truck both increase their speeds by 15 km/hr over the same time interval so therefore have achieved the same acceleration.

2. 5 Free Fall Drop something, it falls. Does it accelerate? l Gravitation causes things to accelerate when they are dropped. l If the object is dropped and air resistance has been removed, the object is said to be in free fall. l

2. 5 Free Fall l Free falling objects are said to be under the influence of gravity alone. Elapsed Time (s) Instantaneous speed (m/s) 0 0 1 10 2 20 3 30 4 40 . . . t 10 t

2. 5 Free Fall l l Note that in the previous table how the speed changed each second. During free fall, the instantaneous speed changed by 10 m/s. The speed gain each second is its acceleration.

2. 5 Free Fall - Question l What would the speedometer reading on the falling rock shown be 4. 5 s after it drops from rest? How about 8 s after it has dropped? 100?

2. 5 Free Fall – Question Answer l The speedometer readings would be 45 m/s, 80 m/s and 1000 m/s, respectively. You can reason this from the table on the previous slide or use the equation v=gt where g = 10 m/s (gravity) = 10 m/s 2.

2. 5 Free Fall What about its upward path? l During the upward motion its velocity is constantly changing so it must accelerating, even when it is momentarily still at the top. l Acceleration due to gravity (10 m/s 2) is always constant. l

2. 6 Free Fall: How far? l l How far something falls and how fast it falls are 2 different things! After 1 s, an object will fall 5 m. What is its average Speed?

2. 6 Free Fall: How far? Question 1 l Consider the table on the previous slide, during the span of the second time inerval, the object begins at 10 m/s and ends at 20 m/s. What is the average speed of the object during this 1 s interval? What is its acceleration?

2. 6 Free Fall: How far? Question 1

2. 6 Free Fall: How far? l Table 2 -3 Free-Fall distances of Object dropped from Rest. Elapsed Time (s) Distance Fallen (m) 0 0 1 5 2 20 3 45 4 80 5 125 . . t ½gt 2

2. 6 Free Fall: How far? Question 2 l An apple drops from a tree and hits the ground in 1 second. What is its speed upon striking the ground? What is its average speed during the 1 second? How high above the ground was it when it first dropped?

2. 6 Free Fall: How far? Question 2

2. 7 Air Resistance and Falling Objects Drop a heavy book and a piece of paper and it is obvious that the book hits the ground first. l As the paper has less mass, the air affects it more than the book. l Remove air resistance and both fall at the same rate. l All things fall at the same rate (9. 8 m/s 2) l

2. 8 Chapter Review Motion is described relative to something. l Speed is the rate at which distance is covered measured in units of distance divided by time. l l Instantaneous speed is the speed at any instant. l Average speed is the total distance covered divided by the time interval.

2. 8 Chapter Review l Velocity is the speed together with the direction of travel. l Velocity is constant only when the speed and direction are constant (a straight line motion)

2. 8 Chapter Review l Acceleration is the rate at which velocity is changing with respect to time. l An object accelerates when its speed is increasing, when its speed is decreasing and/or its direction is changing. l Acceleration is measured in units of speed divided by time.

2. 8 Chapter Review l An object in free fall is falling under the influence of gravity alone, where air resistance does not affect its motion. l An object in free fall has a constant acceleration of about 10 m/s 2.

2. 8 Chapter Review l Important Terms l Acceleration (2. 4) l Average speed (2. 2) l Elapsed time(2. 5) l Free Fall (2. 5) l Instantaneous Speed (2. 2) l Rate (2. 1) l Relative (2. 1) l Speed (2. 2) l Velocity (2. 3)