Motion
Motion - Beware of moving
objects
In the study of motions objects will
be moving.
Terms
– A) Distance vs. Displacement
– B) Speed vs. Velocity
– C) Acceleration
– D) Acceleration due to Gravity
Measurements
All measurements must include a
unit!
Without a unit the answer is
incomplete.
For example:
A) Distance vs. Displacement
Distance - measures the total
amount of area covered.
If we start at point A and end at
point A the distance traveled is
found:
Add up all the legs of the journey:
4m + 8m + 4m + 8m = 24m
4m
A
8m
A) Distance vs. Displacement
Displacement is another word for
position.
For position the only important
value is how far apart are the
starting and ending point!
To calculate displacement:
1. Observe the starting and
ending points.
4m
2. Calculate the distance
between the two.
A
8m
A) Distance vs. Displacement
In this example the start and end
point are the same.
The total displacement is then 0m.
4m
A
8m
A) Distance vs. Displacement
Calculate the Distance and
Displacement of the following:
Start/
End
500m
Start
End
B) Speed vs. Velocity
Speed is a measure of how
quickly distance is covered.
Words “How quick” = time.
Speed = Distance/Time
–
–
–
–
Measured in m/s
Scalar Measure
Positive speed --> going faster
Negative speed --> slowing down
B) Speed vs. Velocity
Velocity measures how quickly the
position of an object changes.
– Velocity = Displacement/Time
– Velocity is a vector
– Unit is m/s
v = s/t
B) Speed vs. Velocity
A car travels
400m in 20sec.
What is its speed?
– v = s/t
– v = 400m/20sec
– v = 20m/s
A student has a speed
of 10m/s for 6 seconds.
How far does the
student travel?
v = s/t
10m/s = s/6sec
s = (10m/s)(6sec)
s = 60m
C) Acceleration
Acceleration measures the rate at
which an object changes velocity.
To calculate acceleration 2
measures of velocity must be
known: starting and ending.
The period of time must also be
measured.
C)Acceleration
An object accelerates (changes its
velocity) in a couple of ways.
– Increases velocity
– Decreases velocity
– Changes direction
When an object DOES NOT change
velocity/direction it is not
accelerating.
C) Acceleration
C) Acceleration
Acceleration is abbreviated with the
symbol a.
It is a vector measurement.
Compares changes in velocity over a
period of time.
The unit is m/s2
a = (vf-vi)/∆t
C) Acceleration
The bowling ball in my hands is at rest. When I throw it at
you it hits your head with a final velocity of 12m/s. The ball
takes 3 seconds to hit you. What is the ball’s acceleration?
a = (vf-vi)/∆t
vi = 0m/s
vf = 12m/s
t = 3s
a = (12m/s-0m/s)/3s
a = (12m/s)/3s
a = 4m/s2
C) Acceleration
A student is walking at 2m/s. Something
shiny flies by and the student takes off
reaching a final velocity of 6m/s. It takes
her 4s to reach this velocity. What is her
acceleration?
a
a
a
a
=
=
=
=
(vf-vi)/∆t
(6m/s-2m/s)/4s
(4m/s)/4s
1m/s2
D) Acceleration Due to Gravity
Gravity is a force felt on all
objects.
It pulls objects towards one
another.
The earth’s gravity pulls
everything towards the
center, we feel this as a
downward pull.
The rate at which objects are
pulled towards the earth is
the same.
D) Acceleration Due to Gravity
All objects are pulled at the same
rate.
Therefore, acceleration downwards
for all objects is the same.
Acceleration due to gravity is a set
value of 9.8m/s2
g = 9.8m/s2
D) Acceleration Due to Gravity
A student is thrown from the top of
Prep. The student is at rest on the
roof. The flight is 4s long. With what
velocity do they hit the ground?
a = (vf-vi)/∆t
9.8m/s2 = (vf-0m/s)/4s
9.8m/s2 = (vf )/4s
39.2m/s = vf