Motion and Speed Describing Motion

Motion  Distance and time are important in describing motion.  Motion occurs when an object changes position.  You need a reference point to describe motion.  A reference point helps determine how far and what direction something has moved.

Relative Motion  Motion is relative.  Is the door to the classroom moving?  YES!!! The door is not moving relative to you or your desk or the building, but it is moving when compared to the sun and other planets.  Motion is relative

Distance  Distance is how far an object has moved TOTAL.  The SI unit of distance is the meter (kilometer, centimeter, etc.)  Distance is very important in describing motion.

Displacement  Displacement is the distance AND direction of an objects change in in position from the starting point.  A running back in football may run 26 yards but only gain 13 yrds from the line of scrimmage.  ALWAYS include DIRECTION when describing displacement (20m NORTH)

Speed  Speed is the distance an object travels per unit of time.  70 Miles per hour  You must always include distance and time when describing speed.

Rate  Rate is any change over time.  Rate of growth = how much you grow in X amount of time.

Calculating Speed  Speed = Distance / Time  S=D/T  So if you ran 2 km in 10 mins?????  S= 2/10  Speed =.2 km per hr  m/s….cm/s…mi/hr… etc

Motion w/ Constant Speed  If your in a car on an empty freeway with the cruise set at 70 miles/hr…. Your at a CONSTANT speed.  No change in speed means that you can calulate speed over any distance (mm to miles it is all the same)

Changing Speed  Most of the time we are not at a constant speed.  In a car the speed limits change going through towns.  You slow down to turn  Speed up to pass. Etc.  How do you express speed that is not constant????????

Average Speed  Average Speed is the total distance traveled divided by the total time of travel.  Average speed= total distance/total time  SOOOOOOOOOO  If you traveled 5km in.25 hours…. What would your average speed be?  5/.25=?????  20 km/h

Instantaneous Speed  Instantaneous Speed is the speed at a given point in time.  A speedometer shows instantaneous speed  If an object is speeding up or slowing down the I.S. is always different.  If an object is staying at the same speed, it’s I.S. is always the same

Graphing Motion  A distance-time graph makes it possible to display the motion of an object over time.  Page 43 in books

Velocity  Velocity describes the Speed AND direction of motion of an object.  There’s a tornado 6 miles west of WF and is traveling at 30 miles per hour… are we in danger???  We’re not sure… we need to know what direction the tornado is traveling…  If east… we take shelter… if west… we’re OK

Velocity (cont.)  If your driving 60 mph east and pass someone who is driving 60 mph west, your speeds are the same but your velocities are different because your moving in different directions.  Velocity can change even if the speed is constant.  It can also change if the direction is constant.

We’re done