Motion

Describing and Measuring Motion An object is in motion if its distance

from another object is changing. Yes, you are blinking and writing, but you

are not moving from your chair. If you were to get up to sharpen your

pencil, you are now moving away from your chair.

To decide if you are moving, you use your chair as a reference point.

A reference point is a place or object used for comparison to determine if something is in motion.

An object is in motion if it changes position relative to a reference point.

What are some examples of reference points?

Measuring Distance You can use units of measurement to

describe motion precisely.

You use measurements all of the time! For recipes Repetitions at the gym Buying 1 lb. of gummy worms at the store

The system of measurement scientists use is called the International System of Units, or SI.

Why is it important for scientists to use the same measuring system?

Scientists use SI units to describe the distance an object moves.

When you measure distance, you measure length.

The SI unit of length is the meter (m).

You can change this measurement by adding prefixes: Milli Centi Deci

Speed and Velocity A measurement of distance can tell you

how far an object travels.

If you know the distance an object travels in a certain amount of time, you can calculate the speed of the object.

Speed is a type of rate.

Rate will tell you the amount of something that occurs or changes in one unit of time.

The speed of an object is the distance the object travels per unit of time.

Calculating Speed When calculating speed, use this

equation:

Speed= Distance/Time

The units that follow depend on what you measure distance and time by.

For example, if you measure distance in meters and time in seconds, you express speed in meters per second, or m/s.

The speed of most moving objects is not constant.

Can you think of an example?

Cyclists change their speeds many times during a race.

They might ride at a constant speed along flat ground, but move more slowly as they climb hills. They might move more quickly as they come down hills. And they may even stop to fix their bike.

To calculate average speed, you need to divide the total distance traveled by the total time.

For example: Suppose a cyclist travels 32 km during

the first two hours. Then the cyclist travels 13 km during the

next hour.

Take your total distance: 32 km+ 13 km Divide it by the total time: 2 h+ 1h

45/3=15 km/h

Instantaneous speed is the rate at which an object is moving at a given instant in time.

Describing Velocity Knowing the speed at which something

travels doesn’t tell you everything about its motion.

To describe an object’s motion completely, you need to know the direction of its motion.

When you know both the speed and direction of an object’s motion, you know the velocity of the object.

Velocity is the speed in a given direction.

At times, describing velocity is important.

Air traffic controllers must keep close track of the velocities of the aircraft under their control. Velocities continually change and

airplanes move overhead and on the runways. An error could lead to a collision.

Graphing Motion You can show the motion of an object on

a line graph in which you plot distance versus time.

X-axis: Time Y-axis: Distance

The slope of the line will tell you how fast one variable changes in relation to the other variable in the graph.

In other words, the rate of change.

To calculate the slope of a line: Divide the rise by the run.

The rise is the vertical difference between any two points on the line, and the run is the horizontal difference between the same two points.

Slope= Rise/Run

Acceleration A pitcher throws, the balls speeds up

towards the batter, it bounces off the bat, and flies over the fence. This ball went through several changes in

motion. Sped up- leaving pitcher’s hand Lost speed- traveling towards batter Stopped- hit the bat Changed direction, sped up, then slowed-

flying off the bat over the fence

Acceleration is the rate at which velocity changes. Remember… velocity is the speed and

direction of an object.

A change in velocity can involve a change in either speed or direction, or both.

In science, acceleration refers to increasing speed, decreasing speed, or changing direction.

Increasing: Object will accelerate.

Decreasing: Deceleration, or negative acceleration.

Changing Direction: An object that is traveling at a constant speed can be

accelerating. Recall that acceleration can be a change in direction as

well as a change in speed. Track runners accelerate as they round the curve in the track.

To determine the acceleration of an object moving in a straight line, you must calculate the change in speed per unit of time.

When calculating acceleration, use this formula:

Acceleration= Final speed-Initial speed/Time

If speed is measured in meters per second (m/s) and time is measured in seconds, the SI unit of acceleration is meters per second per second, or m/s2.

For example: Imagine a small airplane moving down the runway. 0.0 seconds= 0 m/s 1.0 seconds= 8 m/s 2.0 seconds= 16 m/s 3.0 seconds= 24 m/s 4.0 seconds= 32 m/s 5.0 seconds= 40 m/s

Final (40 m/s)- Initial (0 m/s) then divide by Time (5 seconds)= 8m/s2

The airplane accelerates at a rate of 8m/s2.

Graphing Acceleration You can use both a speed vs. time graph

or a distance vs. time graph to analyze the motion of an accelerating object.

Speed vs. Time A slanted, straight line on this graph

means that the object is accelerating at a constant rate.

Distance vs. Time A curved line tells you that the object is

accelerating, and that the speed is greater than the time before.