Unit II Physical Science
Motion and Forces
Distance vs. Displacement
• What two changes
help you identify that
an object is in
motion?
– Change in Direction
– Change in Speed
_______________ is
necessary to
determine if
something is in
motion.
Frame of Reference:
background that appears to be
stationary to a moving object
Earth is the most common frame of
reference.
How can something appear to be
motionless but actually be moving?
• If an object has the
same frame of
reference.
• If the object is moving
at the same speed.
• Examples?
Dist. Vs. Disp.
• Distance
– Actual path that an
object takes
– Cannot be negative
• Displacement
– Straight line from start
to finish
– Can be negative or
zero
Think about your charts made to calculate
distance and displacement on the grid.
Explain how an object can have a zero or negative displacement.
Determine the displacement for A and B below.
12 km
6 km
Calculating Distance and
Displacement
• Pick a starting point on one side of the room,
and an ending point on the other side.
• Have your partner travel to that point.
• Draw a map on a separate sheet of paper.
• Using a meter stick, measure the distance
traveled and either calculate or measure the
displacement. (remember Pythagorean
theorem)
• Show both distance and displacement on your
map.
Speed and Velocity
• Speed: the rate at which an object
changes position
Represented by v in a formula.
• m/s : SI unit for speed
Instantaneous vs. Average
• Instantaneous
Speed:
speed at a
particular point in
time
• Average Speed:
speed taken over a
distance traveled
Ex: Calculating your
Ex: Speedometer of
speed over an
a car
entire trip
Speed = distance / time
v = d/t
What is a good way to represent and
describe motion?
A distance vs. time graph
What does the slope of the line in a distance
vs. time graph tell you?
slope = rise/run
speed = rise (d)/ run (t)
How is velocity different than
speed?
• Velocity is speed with a direction.
Ex: traveling 60 km/hr North
Two ways to change velocity:
1. Change speed (speed up/slow down)
2. Change direction (turn)
Example Problems
• A car travels 85 km from town A to town B,
then 45 km from town B to town C. The
total trip took 1.5 hrs. What was the
average speed of the car?
Ex. Continued
• A bicyclist travels for 1.5 hrs at an average
speed of 32 km/hr. How far does the
bicyclist travel in that time?
What can you
say about the
motion of the car
in the graph from
8-12 seconds?
– The car is
stopped
If you were paddling down river at
16 mph and the river moved at 10
mph, how fast would you actually
be moving? What if you were
paddling up river?
Acceleration
• Think of a situation where you were
accelerating. What was happening in that
situation?
Examples:
Acceleration
• Acceleration: is the rate of change in
velocity
Acceleration = change in velocity
time
a = vf – v i
time
• SI unit of acceleration = m/s/s or m/s2
Free Fall
What happens to an
object during free fall?
The object
accelerates at a
constant speed
Acceleration due to
gravity = 9.8 m/s/s
For every second an
object falls it will
move 9.8 m/s faster
Ways to Change Acceleration
• Change Speed
• Change Direction
Can you have negative
acceleration?
yes if the object is
slowing down
(deceleration)
v vs. t Graph
• What is the difference
between a linear
speed vs. time graph
and an non linear
speed vs. time graph?
Speed
Linear = constant
acceleration
Nonlinear = changing
acceleration
Time
Example Problems
1. A car traveling at 10 m/s starts to
decelerate steadily. It comes to a
complete stop in 20 s. What is its
acceleration?
Ex. Continued
2. An airplane travels down a runway for
4.0s with an acceleration of 9.0 m/s/s.
What is its change in velocity during this
time?
Ex. Continued
3. A boy throws a rock up into the air. It
reaches the highest point of its flight after
2.5 s. How fast was the rock going when it
left the boys hand?
4. What is the acceleration from 10 – 20 seconds in the graph
below?
0 - 5 = 0.5 m
10
s2
Speed
•
Graph the following table
representing a car trip. Make
sure to put the correct labels and
units making it a complete graph.
1. What was the average speed for
the entire trip
2. When was the car traveling at the
greatest speed?
3. What happened between the 3rd
and 4th hours
4. Give an example of a dependent
variable and explain why it is a
dependent variable.
Distance Time
(km)
(hours)
40
1
100
2
170
3
170
4
210
5
1. Which graph shows periods of constant speed?
Explain your answer.
• Graph A shows periods of constant speed (0–8 s,
8–12 s, 12–20 s).
2. Describe the motion of the object in Graph A.
• The object moves at constant speed for 8 seconds, is
at rest for the next 4 seconds, and then moves at
constant speed for the next 8 seconds.
3. Which graph in Figure 11-2 shows acceleration?
How do you know?
• Graph B shows acceleration. The upward slope of
the line indicates that an increasing distance is
covered each second.
4. Using Graph A in Figure 11-2, calculate the
average speed of the object in motion from 12 s to
20 s. Explain your calculation.
• The object moved a distance of 300 m in 8 s. The
object’s average speed is 37.5 m/s.
5. Compare Graphs A and B in Figure 11-2. At a
time of 2 seconds, which graph shows a
greater velocity? How do you know?
• Graph A; the slope is steeper.