Chapter 8: Forces & Motion Week 3 Chapter 8: Forces & Motion

Describing Position & Motion April 5

Today’s Objectives Students will be able to: Differentiate between distance and displacement. Calculate average speed in m/s. Describe an object’s motion using speed and/or velocity.

Bellringer #1 How can you describe an object’s position? Choose an object or person in the image and describe its position. Ask students to give their description without telling you what object they chose. See if you can find it.

Describing Position & Reference Points Position describes an object’s distance and direction from a reference point. A reference point is the starting point used to locate another place or thing. If Mrs. Smidt called me before school and asked me to make copies of the worksheet to the left of the tablet, would that be a good description? Why or why not? The position of an object depends on a reference point. Even if the actual position of an object does not change, the description of its position changes if the reference point is changed. Point out that most of their descriptions used a reference point.

Distance & Displacement If Mrs. Smidt asks me to find something that is directly 20 steps from my desk to the TV, why might I have some difficulty finding that item? Displacement is the difference between the initial position and the final position. An object’s displacement and the distance it travels are not always equal. (zig-zagging around tables would make me take more than 20 steps)

Think/Pair/Share Which car traveled the greatest distance? Which car had the greatest displacement? What is the difference between distance and displacement? Why don’t you need to know the total distance the car traveled in order to determine its displacement?

Motion What is motion? Why does the description of an object’s motion depend on a reference point? How can you use distance and displacement to describe an object’s motion? Motion is the process of changing position. How else can you describe motion?

Speed What does speed describe? What are some ways that speed can be expressed? Speed is the rate at which an object moves, or the distance an object moves in a certain amount of time (m/s). What can you infer about the bus’s speed as it moves from position 1 to 2 to 3 to 4? When an object moves the same distance over a given unit of time, it is said to have a constant speed. How is the motion of the bus between positions 4 and 7 different from its earlier motion? When the distance an object covers increases or decreases over a given unit of time, the object is said to be changing speed. How can you calculate the bus’s speed at a certain point? What was the bus’s speed as it moved from point 1 to point 2?

Average Speed How could you describe the bus’s speed during the entire trip? Average speed is equal to the total distance traveled divided by the total time. What was the bus’s average speed? When planning a trip, how is knowing an object’s average speed more useful than knowing its instantaneous speed (speed at a particular moment in time)? 80m/30s=2.7 m/s

Velocity What else do you need to know to understand the motion of the bus? Velocity is speed in a given direction. You can use arrows to show the velocity of an object. The longer the arrow, the faster the object is moving. Earlier you determined the bus’s speed. What was the bus’s velocity?

Constant & Changing Velocity Constant velocity means that an object moves with constant speed and its direction does not change. Velocity changes when either the speed or the direction of motion of an object changes.

Acceleration Acceleration is a measure of how quickly the velocity of an object changes. When the velocity of an object changes, it accelerates. The action of slowing down is called negative acceleration. When an object’s acceleration is in the same direction as its motion, this is called positive acceleration. When the roller-coaster car increases speed, decreases speed, or changes direction, it accelerates. Acceleration takes place whenever velocity changes.

Wrap Up What did we learn about describing position and motion? What does an object’s position depend on? How does this relate to describing an object’s motion?