Pearson Prentice Hall Physical Science Concepts in Action

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