Describing Motion Chapter 1 Physical Science Ch1 L.1 Position and Motion How does the description of an objects position depend on a reference point? How can you describe the position of an object in two dimensions? What is the difference between distance and displacement? Ch1 L.1 Bellwork Reference point: starting point you choose to describe the location, or position, of an object. Position: an objects distance and direction from a reference point. Motion: the process of changing position Displacement: the difference between the initial (first) position and the final position of an object Launch Lab p.9 I have labeled several locations in this classroom Work in pairs One person starts at START point (with N,E,S,W labeled) and walks to one of four locations Record number of steps and direction Give directions to partner and see if they end up in the same place THINK: How did your instructions compare to your partners directions? Did the step sizes match? What things were similar/different? How did the description of your movement depend at the point at which you started? Describing position Reference point (starting point you choose to describe the location, or position, of an object) determines directions Directions are based on reference point Position: distance and direction from a reference point COMPLETE DESCRIPTION OF POSITION: distance, direction and reference point Using a reference point to describe position Why is this important? How does the position of an object depend on a reference point? Use two different reference points to describe the position of Dr. A in this image Dr. A The reference direction To describe an objects position you compare the location to a reference direction The blacktop is towards the basketball court Use (+) or (-) to describe direction (+) is one direction and (-) is the opposite direction If you make East (+) then West would be (-) Mini Lab p. 10 Get a meter stick Place a sticky note at the 50cm location This is the reference point Fill out the following chart Top row= example Repeat with the reference point moved to the 40cm location How did the chart change when the reference point changed? Why is a reference point useful in describing positions of an object? Distance (cm) Reference Direction Position (cm) 10 cmnegative40 cm positive 15 cmpositive 75 cm 30 cm Hide and seek? Four groups (I pick.) Each group will pick a hiding spot for a treasure and write down directions (no drawings) to this spot Groups will switch instructions and try to find the treasure REMEMBER: directions for an objects position is based on a reference point If you need to use two references then you are describing in two dimensions North and West would be two dimensions Up and to the Left would be another way to say it Think of it like an X,Y coordinate Use one reference for one direction and then switch to another reference to finish up Describing positions in two dimensions Describing changes in position Motion: process of changing position position of the object changes relative to the reference point Motion relative to a reference point Need to choose a reference point carefully to see what is moving (if there is any movement at all) Describing changes in position contd Distance and displacement Distance= total amount traveled Displacement= difference between initial position and final position When are distance and displacement different? When are distance and displacement the same? Changes in position contd Distance and displacement ONLY equal if motion is in one direction ONLY Ch1 L.1 Homework p.14 #1-8 Outline Ch1 L.1 Study for Ch1 L.1 Quiz Ch1 L.2 Speed and Velocity What is speed? How can you use a distance-time graph to calculate average speed? What are ways velocity can change? Ch1 L.2 Bellwork Speed: measure of the distance an object travels per unit of time Constant speed: rate of change of position in which the same distance is traveled each second Instantaneous speed: speed at a specific instant in time Average speed: total distance traveled divided by total time taken to travel that distance Velocity: speed AND direction of a moving object A measure of the distance an object travels per unit of time Units of speed divide distance traveled.time to go that distance m/s mph km/h What is speed? Distance vs time graph Think cruise control Dont speed up or slow down Stay at the same speed The same distance is traveled each second The rte of change of position in which the same distance is traveled each second Constant speed Changing speed Speed is not constant Think about driving around normally Slow down, maybe stop, accelerate etc. Different distance traveled each second Want: Instantaneous speed This is the speed at any instant in time (this is what spedometer shows) Average speed Total distance traveled/total time taken to travel that distance Does NOT tell you if constant or not average speed in m/s= total distance in m/total time in s v=m/s The weird v with the line on top means average velocity Melissa shot a model rocket 360m into the air. It took the rocket 4s to fly that far. What was the average speed of the rocket? It takes Ahmed 50s on his bicycle to reach his friends house 250m away. What is his average speed? A truck driver makes a trip that covers 2,380km in 28 hours. What is the drivers average speed? Average speed practice Distance-time graphs Show how distance changes over time If the distance is the same each second, the speed is constant Comparing speeds on a distance-time graph Can look to see what speed something is moving and compare it to the speed of something else. Who is slower? Who is faster? Steeper line= greater slope= faster speed Choose two points on the graph (x1,y1), (x2, y2) Calculate time difference x2-x1 Calculate the distance difference y2-y1 Divide distance difference by time difference (y2-y1)/(x2-x1) ***SAME thing as just calculating slope! Using a distance-time graph to calculate speed Distance-time graph with changing speed Straight lines= constant speed Curved lines= when speed changes Can still calculate average speed!!! Pick beginning and end point and calculate slope Describe what is happening here Velocity Speed tells you how fast Velocity tells you speed AND direction Representing velocity Arrow represents displacement of an object from a reference point Difference between starting and ending position Arrow points in direction of motion Longer arrow= faster speed Changes in velocity When speed of object changes When direction of object changes When both speed and direction change Ch1 L.2 Homework p Outline Ch1 L.2 Study for quiz Ch1 L.2 Ch1 L.3 Acceleration What are three ways an object can accelerate? What does a speed-time graph indicate about an objects motion Ch1 L.3 Bellwork Acceleration: is a measure of the change in velocity during a period of time Launch Lab p.27 (Modified) Look at the graph, discuss with your partner what the motion was during each five second period What does the horizontal segment on a distance-time graph indicate? At what times do the following motions take place? Change in direction Increase in speed Decrease in speed Acceleration- Changes in velocity Moving faster/slower= velocity changes Change in direction= velocity changes Acceleration: Measure of the change in velocity during a period of time Representing acceleration Has a direction Can be represented with arrow Direction of arrow depends on if its an increase or decrease in velocity Green= velocity Blue= acceleration Changing speed (constant direction) Increasing speed= increasing velocity Acceleration is in same direction as velocity Decreasing speed= decreasing velocity Acceleration is in opposite direction as velocity Changing direction If velocity changes (EVEN IF SPEED DOES NOT CHANGE) the car is accelerating If the direction changes Calculating acceleration Change in velocity/time positive= speeding up in forward direction OR slowing down in reverse direction negative= slowing down in forward direction OR speeding up in reverse direction Acceleration practice p.29 A bicyclist started from rest along a straight path. After 2.0s, his speed was 2.0m/s. After 5.0s, his speed was 8.0m/s. What was his acceleration during the time 2.0s to 5.0s Adrian drops a rock from a cliff. After 4.0s, the rock is moving at 3.9m/s. WHat is the acceleration of the rock? Speed-time graphs Shows how speed changes over time Time on x-axis, speed on y-axis Object at rest Horizontal line at y=0 because distance never changes and speed never changed Speed is always zero Speed-time graphs Constant speed distance-time distance increases at a STEADY rate over time speed-time horizontal line at whatever the speed is Speed-time graphs Speeding up distance-time looks more like an exponential graph. As the distance increases, the rate of increase gets larger over time. speed-time speed increases at a steady rate over time Speed-time graphs Slowing down distance-time as distance increases, the rate of increase gets smaller speed-time speed of object decreases at a steady rate over time Limits of speed-time graphs ONLY show relationship between speed and time Does not show when change is because of direction- only discusses SPEED Ch1 L.3 Homework p Quiz Ch1 L.3 Outline Ch1 L.3 Study for test Ch1