Describing Motion

1
Describing Motion
2
Distance and Time

• Vocabulary
• Speed
• Velocity
• Constant Speed
• Average Speed
• Distance
• Displacement

3
Distance and Time

• How would you calculate your speed with only
  distance and time?
• How does your speed compare with the following
• Cheetah 120 km/h
• Horse 64 km/h
• Elephant 40 km/h
• Snake 3 km/h

4
Distance and Time

• What is motion and how do we describe it?
• Motion occurs when an object changes its
  position?
• So, how do we measure motion?

5
Distance and Time

• Not all motion is obvious.
• Motion is relative.
• The measurement of motion depends on your frame
  of reference.
• To measure motion, you must have a reference
  point.

6
Distance and Time

• An object is moving if its position changes
  against some background that stays the same.
• An important part of describing motion, is to
  describe how far it has moved, which is distance.
• The SI unit for distance is the meter (m).

7
Distance and Time

• Displacement is another term used to describe
  motion.
• Displacement is different from distance.
• Displacement includes the distance and direction
  an object moves.

8
Distance and Time

• Experience tells us that some objects move faster
  than other objects.
• Speed describes how fast an object moves.
• Speed is the distance an object travels in a
  certain amount of time.
• The SI units for speed are m/s.
• For faster speeds, km/h may be used, while for
  slower speeds units of cm/s may be used.

9
Distance and Time

• Speed Distance / Time
• S d/t
• Velocity is also used to describe how fast an
  object travels.
• Velocity includes speed and direction.
• Velocity Distance / Time
• V d/t

10
Distance and Time

• Some objects are moving, but their speed does not
  change.
• These objects are moving with constant speed.
• Constant speed means equal distances are covered
  in an equal amount of time.
• So, if a race car has a constant speed of 96 m/s,
  it means the race car travels 96 m every second.

11
Distance and Time

• Most objects however, do not move with constant
  speed.
• Imagine running a marathon. Because the course
  changes, at times you will run faster than at
  other times.
• In this case, average speed is used instead.
• Average speed is the total distance covered in
  the total time.

12
Distance and Time

• How have inventions, such as the automobile and
  airplane, increased the speed at which we can
  travel?
• Many inventions have increased the speed at which
  people can travel.
• If your average speed was 5 km/h, how long would
  it take you to walk to school? What time would
  you have to leave your house to be to school on
  time?

13
Distance and Time

• If you travel in a car or bus with an average
  speed of 70 km/h, how long would it take you to
  get to school? What time would you have to leave?
• How do your results compare with other students?

14
Graphing Distance and Time

• Sometimes, it is necessary to make a picture
  describing the motion of an object.
• Graphs are used to describe the distance covered
  by an object in a specific amount of time.
• What type of graph would best represent this
  information?

15
Graphing Distance and Time

• Line graphs are used to represent the distance
  covered by an object in a specific amount of
  time.
• The slope of the line provides the speed of an
  object.
• Objects moving with constant speed have a
  straight line.
• Objects moving with a faster speed have a steeper
  slope.

16
Graphing Distance and Time

• Time is plotted on the horizontal axis and
  distance is plotted on the vertical axis.
• If the object is not moving with constant speed,
  the graph is not a straight line.
• Average speed can be calculated by total distance
  covered divided by total time.
• The speed of an object at a specific point in
  time is known as the instantaneous speed.

17
Acceleration

• Vocabulary
• 1. Acceleration

18
Acceleration

• Imagine you are sitting in a car at a stoplight.
  When the light turns green, you step on the gas
  pedal and the car begins to move faster and
  faster.
• If you step on the pedal, and your speed changes
  very quickly, the passengers in the car can be
  thrown forward. If your speed changes gradually,
  your passengers have a smooth ride.

19
Acceleration

• Recall that speed is the rate of change of
  position, or how much distance you cover in a
  certain amount of time.
• Acceleration is the rate of change of velocity or
  speed.
• Acceleration occurs only if there is a change in
  velocity.
• If velocity is constant, then the acceleration is
  ZERO.

20
Acceleration

• There are three ways for acceleration to occur
  speed up, slow down, change directions.
• If an object is speeding up, then the
  acceleration is positive.
• If an object is slowing down, then the
  acceleration is negative.

21
Acceleration

• Acceleration is determined by the following
  equation
• Acceleration Change in Velocity / Time
• Change in velocity Final Velocity Initial
  Velocity
• A (Vf-Vi) / T
• (Vf-Vi) is often written as ?V
• Acceleration has SI units of m/s2

22
Graphing Acceleration

• Recall that velocity could be determined from a
  distance-time graph.
• What information would need to be used to create
  a graph to determine acceleration?

23
Graphing Acceleration

• Acceleration can be determined from a
  velocity-time graph.
• Velocity is plotted on the vertical axis and time
  is plotted on the horizontal axis.
• The slope of the line gives you the acceleration.

24
Graphing Acceleration

• Positive Acceleration Velocity is Increasing

25
Graphing Acceleration

• Negative Acceleration Velocity is Decreasing

26
Graphing Acceleration

• Zero Acceleration Velocity is Constant

27
Graphing Acceleration
28
Momentum

• Vocabulary
• Momentum
• Law of Conservation of Momentum

29
Momentum

• Velocity and acceleration are not the only
  important quantities when objects are in motion.
• For instance, a train is more difficult to stop
  than a car moving at the same speed.
• Why?

30
Momentum

• The train is more difficult to stop because it
  has a greater mass than the car and greater
  momentum.
• All moving objects have momentum.
• Momentum is the product of an objects velocity
  and its mass.
• momentum mass x velocity
• P m x v
• The letter p is used to denote momentum.

31
Momentum

• Momentum has SI units of kg x m/s
• If an object is not moving, it does not have
  momentum.
• The greater the mass or velocity the greater the
  momentum.

32
Momentum

• Imagine a large semi-truck traveling very fast,
  collides head-on with a small car traveling at
  the same speed?
• What happens to the truck? What happens to the
  car?
• Momentum can be used to predict the motion of the
  vehicles after the collision.

33
Momentum

• Momentum is conserved.
• The Law of Conservation of momentum states that
  the total of amount of momentum for a system is
  constant.
• The momentum of the truck plus the momentum of
  the car before the collision is equal to the
  momentum of the truck plus the momentum of the
  car after the collision.

34
References

• Glencoe, Integrated Physics and Chemistry. 2002
• Holt, Science Spectrum A Physical Approach. 2002
• Addison-Wesley, Science Insights Exploring
  Matter and Energy. 1997