Ch. 11: Motion 11.1: Distance and Displacement 11.2: Speed and Velocity 11.3: Acceleration

Choosing a Frame of Reference To describe motion accurately and completely, a frame of reference is necessary. A frame of reference is a system of objects that are not moving with respect to one another. Choosing a meaningful frame of reference allows you to describe motion in a clear and relevant manner.

Measuring Distance Distance is the length of a path between two points. Start End Start End

Measuring Distance The SI (International System of Units) unit for distance is the meter.

Measuring Displacements Displacement is the direction from the starting point and the length of a straight line from the starting point to the ending point.

Combining Displacements Vector is a quantity that has magnitude and direction. The magnitude can be size, length, or amount.

Displacement Along a Straight Line The length of the arrow show the magnitude of the vector. Vector addition is the combining of vector magnitudes and directions.

Displacement That Isn’t Along a Straight Path The vector in red is called the resultant vector.

11.2: Speed & Velocity Speed is the ratio of the distance an object moves to the amount of time the object moves.

Speed Average Speed: is computed for the entire duration of a trip. Instantaneous Speed: is measured at a particular instant.

Average Speed vs. Instantaneous Speed

To determine a speed, you need to know two things. The distance traveled The time taken Speed is calculated by taking the distance traveled divided by the time taken.

Average Speed

SI Unit for Speed or Velocity meter second m s or

Three forms of the speed formula Equation Gives you… If you know… v = d/t Speed Time and distance d = vt Distance Speed and time t = d/v Time Distance and speed

What does “per” mean? The word “per” means “for every” or “for each” or “divided by”. If you want speed in meters per second, you have to divide meters by seconds.

Graphing Motion The slope of a line on a distance-time graph is speed.

Velocity Is a description of both speed and direction of motion. Velocity is a vector.

11.3: Acceleration A change in speed, change in direction, or change in both. Acceleration is a vector.

Changes in Speed Acceleration can be caused by positive (increasing) change in speed or by negative (decreasing) change in speed.

Free Fall Objects falling near Earth’s surface accelerate downward at a rate of 9.8 m/s2.

Constant Acceleration A steady change in velocity.

Instantaneous Acceleration How fast a velocity is changing at a specific instant.

Calculating Acceleration Calculate acceleration for straight-line motion be dividing the change in velocity by the total time.

Units for Acceleration If speed is in m/sec and time in seconds, then the units for acceleration are m/sec/sec. What this means is that the acceleration is the amount that the speed changes in each second.

SI Units for Acceleration The units of square seconds do not have a physical meaning in the same way that square inches mean surface area. meters seconds2 or m s2

Graphs of Accelerated Motion

Graphs of Accelerated Motion

A distance-time graph of accelerated motion is a curve.