Pearson Prentice Hall Physical Science: Concepts in Action Chapter 11 Motion

11.1 Distance and Displacement Objectives: 1. Identify frames of reference and describe how they used to measure motion 2. Identify appropriate SI units by measuring distances 3. Distinguish between distance and displacement 4. Calculate displacement using vector addition

Frames of Reference Definition: a frame of reference is a system of objects that are not moving with respect to one another Definition: relative motion is movement in relation to a frame of reference Ex: people standing on the side of the road see the car speeding by, but people in the car look at one another and don’t appear to be moving at all When describing how fast something is moving, choose a frame of reference that allows you to describe motion in a clear and relevant manner

Distance Definition: distance is the length of a path between two points When an object moves in a straight line, the distance is the line connecting the starting point to the ending point The SI unit for distance is the meter (m) For long distances, km is appropriate and for distances smaller than one meter, the cm is appropriate

Displacement Definition: displacement is the direction from the starting point and the length of a straight line from the starting point to the ending point (distance is the length of the total path) Definition: a vector is a quantity that has magnitude (size, length or amount) and direction (positive/negative, north/south) Definition: vector addition is the combining of vector magnitudes and directions Add displacements together using vector addition

Vector Addition When the displacement isn’t along a straight line, use vector addition Definition: a resultant vector is the vector sum of two or more vectors a resultant vector may show displacement Q: Should your directions to a friend traveling from one city to another include displacements or distances? Explain.

11.2 Speed and Velocity Objectives: 1. Identify appropriate SI units for measuring speed 2. Compare and contrast average speed and instantaneous speed 3. Interpret distance-time graphs 4. Calculate the speed of an object using slopes 5. Describe how velocities combine

SI Units for Speed + Average & Instantaneous speed Definition: speed is the ratio of distance moved divided by the amount of time it took Definition: Average speed is total distance divided by total time for the entire trip Total distance/total time or v= d/t Definition: instantaneous speed is speed measured at that moment (ex: speedometer) The SI unit of speed is meters per second (m/s)

Distance-Time Graphs Distance versus time graphs are a good way to describe speed The slope of a distance-time graph is speed

Using Slope

Velocity Velocity is the description of speed and direction of motion Velocity is a vector & can be + or – Two or more velocities add by vector addition Q: A plane’s average speed is 600 km/h. If the trip takes 2.5 h, how far does the plane fly? A: cross cancel the units: (600 km/h)(2.5 h) = 1500 km

11.3 Acceleration Objectives: 1. Identify changes in motion that produce acceleration 2.Describe examples of constant acceleration 3. Calculate the acceleration of an object 4. Interpret speed-time & distance-time graphs 5. Describe instantaneous acceleration

Changes in motion Definition: acceleration is a change in speed, direction or both Acceleration is a vector Definition: free fall is the movement of an object toward earth due to gravity alone Free fall is an example of acceleration and is a change in speed You can accelerate even if speed is constant by riding a bicycle around a curve (change of direction ) Riding a carousel is also acceleration

Constant Acceleration It is possible to change both speed and direction Ex: riding a roller coaster or driving along a winding road at the posted speed limit Definition: constant acceleration is a steady change in velocity This means that the velocity changes by the same amount each second An example is jet acceleration during a portion of takeoff

Calculate Acceleration To calculate acceleration for straight-line motion divide the change in velocity by the total time To find the change in velocity subtract the initial velocity from the final velocity (vf - vi) Change in velocity = (vf - vi) total time t To determine a change in velocity subtract one velocity vector from another If the motion is a straight line, the velocity can be treated as speed Then find acceleration by change in speed divided by time

Speed-time & Distance-time Graphs On a linear speed time graph, a positive slope shows positive acceleration

A negative slope shows negative acceleration

A distance-time graph of accelerated motion is a curve It is considered a nonlinear graph A steeper slope after several seconds on these graphs means that the speed is increasing

Instantaneous Acceleration Definition: instantaneous acceleration is how fast a velocity is changing at a specific instant or moment in time Acceleration is rarely constant Motion is rarely in a straight line A skateboarder is accelerating but the instantaneous acceleration is always changing