SACE Stage 1 Conceptual Physics 2. Motion

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

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

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

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

2.2 Speed Table 2.1 – Approximate Speeds in different units. Table 2.1 – Approximate Speeds in different units. 20 km/hr = 12 mi/hr = 6 m/s 40 km/hr = 25 mi/hr = 11 m/s 60 km/hr = 37 mi/hr = 17 m/s 80 km/hr = 50 mi/hr = 22 m/s 100 km/hr = 62 mi/hr = 28 m/s 120 km/hr = 75 mi/hr = 33 m/s

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

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

2.2 Speed – Average Speed Average Speed is an easy calculation. Average Speed is an easy calculation. If it takes 4 hours to cover 240 km. What is your Average Speed? 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 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? 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 Would it be possible to attain this average speed and never exceed the 70 km/hr? Would it be possible to attain this average speed and never exceed the 70 km/hr?

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

2.2 Speed – Question 3 Answer When can re arrange our formula to allow us to work out the distance travelled. 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? What is the difference between speed and velocity? Velocity is a speed in a given direction. Velocity is a speed in a given direction. A car that is travelling at 60 km/hr has speed but when we say 60 km/hr North, it now has velocity. A car that is travelling at 60 km/hr has speed but when we say 60 km/hr North, it now has velocity.

2.3 Velocity – Question 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? 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 Both cars have the same speed but will have opposite velocities as they are travelling in opposite directions. 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. To achieve constant velocity, it is reasonable to see that an object must have constant speed and constant direction. Motion at constant velocity is motion in a straight line. Motion at constant velocity is motion in a straight line.

2.3 Velocity – Changing Velocity If either the speed or direction change, then velocity has changed. If either the speed or direction change, then velocity has changed. 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. 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.

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

2.4 Acceleration

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

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

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

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

2.5 Free Fall Free falling objects are said to be under the influence of gravity alone. Free falling objects are said to be under the influence of gravity alone. Elapsed Time (s) Instantaneous speed (m/s) t10t

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

2.5 Free Fall - Question What would the speedometer reading on the falling rock shown be 4.5s after it drops from rest? How about 8s after it has dropped? 100? What would the speedometer reading on the falling rock shown be 4.5s after it drops from rest? How about 8s after it has dropped? 100?

2.5 Free Fall – Question Answer 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 = 10m/s (gravity) = 10m/s 2. 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 = 10m/s (gravity) = 10m/s 2.

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

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

2.6 Free Fall: How far? Question 1 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 1s interval? What is its acceleration? 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 1s interval? What is its acceleration?

2.6 Free Fall: How far? Question 1

2.6 Free Fall: How far? Table 2-3 Free-Fall distances of Object dropped from Rest. Table 2-3 Free-Fall distances of Object dropped from Rest. Elapsed Time (s) Distance Fallen (m) t ½gt 2

2.6 Free Fall: How far? Question 2 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? 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. Drop a heavy book and a piece of paper and it is obvious that the book hits the ground first. As the paper has less mass, the air affects it more than the book. As the paper has less mass, the air affects it more than the book. Remove air resistance and both fall at the same rate. Remove air resistance and both fall at the same rate. All things fall at the same rate (9.8m/s 2 ) All things fall at the same rate (9.8m/s 2 )

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

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

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

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

2.8 Chapter Review Important Terms Important Terms Acceleration (2.4) Acceleration (2.4) Average speed (2.2) Average speed (2.2) Elapsed time(2.5) Elapsed time(2.5) Free Fall (2.5) Free Fall (2.5) Instantaneous Speed (2.2) Instantaneous Speed (2.2) Rate (2.1) Rate (2.1) Relative (2.1) Relative (2.1) Speed (2.2) Speed (2.2) Velocity (2.3) Velocity (2.3)