Section 1 Measuring MotionChapter 12Bellringer

Describe the location of a favorite place, using a reference point and reference directions. For example, “The library is at the corner of Madison Street and Monroe Street. It is two blocks west of the school and four blocks north of the bus stop.”

Write your response in your Science Journal.

Section 1 Measuring MotionChapter 12What You Will Learn

• Properties used to describe the motion of an object include a reference point, direction, speed, velocity, and acceleration.

• Average speed can be calculated by dividing total distance by total time.

• A change in velocity is due to a change in speed, direction, or both.

• Speed and acceleration can be represented on graphs.

Section 1 Measuring MotionChapter 12Motion and Reference Points

• An object is in motion if it changes position over time in relation to a reference point.

• A reference point is any object that appears to stay in place.

• Features on Earth’s surface are often used as standard reference points.

Section 1 Measuring MotionChapter 12Motion and Reference Points, continued

• Reference directions—such as north, south, east, west, up, or down—can be used to describe the direction of an object’s motion.

• A grid can be used to describe motion in a two-dimensional system.

Section 1 Measuring MotionChapter 12Average Speed

• The speed of an object is the rate at which the object moves.

• The speed of an object is rarely constant. So, it is useful to find the average speed of an object.

Section 1 Measuring MotionChapter 12Average Speed, continued

• Average speed is the total distance traveled divided by the total time taken.

• A graph of position versus time can show an object’s speed. On a graph of position versus time, the slope of the line is equal to the speed of the object.

Section 1 Measuring MotionChapter 12Velocity: Direction Matters

• Velocity is the speed of an object in a particular direction.

• Velocity and speed do not have the same meaning.

• Velocity must include a direction.

Section 1 Measuring MotionChapter 12Velocity: Direction Matters, continued

• The velocity of an object is constant only if the speed and direction of the object do not change. So, constant velocity is always motion along a straight line.

• The velocity of an object changes if the object’s speed, direction, or both change.

Matter in MotionChapter 12Velocity

Section 1 Measuring MotionChapter 12Acceleration

• Acceleration is the rate at which velocity changes over time.

• An object can accelerate by changing speed, direction, or both.

• The units for acceleration are the units for velocity divided by a unit for time, usually meters per second per second, or (m/s)/s, which is also written as m/s2.

Matter in MotionChapter 12Acceleration

Section 1 Measuring MotionChapter 12Acceleration, continued

• Centripetal acceleration occurs when an object moves at a constant speed in circular motion.

• A graph of speed versus time can show positive acceleration, no acceleration, and negative acceleration.

Matter in MotionChapter 12