Physics UnitIntroduction to Motion

David W. Bombick, Ph.D.7th Grade Science

Definition of Motion

• An object is in motion if its distance from another object is changing.

• 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.

Measuring Distance

• Will use the International System of Units (SI)

• The SI unit of length will be the meter.• Prefixes will be used with the word ‘meter’

to describe fractions or multiples of a meter.

• For example, a centimeter is one hundredth of a meter; a millimeter is one thousandths of a meter; a kilometer is 1000 times a meter.

Calculating Speed

• The speed of an object can be calculated by knowing the distance travelled and the amount of time needed to travel this distance.

• Therefore, Speed = distance divided by time

• SI units are usually in meters and seconds

Average Speed• The average speed can be calculated by

adding up the total distance travelled and dividing by the total time the object has travelled

• An example would be someone who travels for 10 meters over 10 seconds; stops for 10 seconds; and travels 20 meters in 10 seconds has travelled at an average speed of 1 m/sec.

Speed versus Velocity

• Speed is distance travelled per unit of time.

• Velocity is speed with a known direction.

• Usually, when considering linear motion, speed is equivalent to the velocity.

Graphing Motion

• You can show the motion of an object on a line graph in which you plot distance versus time.

• Distance is shown on the y-axis.• Time is shown on the x-axis.• The speed (velocity) is the slope of the

line.• Slope is defined as the change in y divided

by the change in x.