Student Reading Page: Motion Diagrams II Name______________
YOUR ASSIGNMENT: As you read this page (front & back), highlight new information
or main ideas you learn from it. Then get out a piece of notebook paper. Write down
the new information or main ideas you learned. You should have a minimum of 1 idea
per section. The five sections are marked with *.
1st*VECTORa quantity which tells you both the amount and the direction
Velocity is a vector because it tells you how fast and it tells you which direction with it’s
sign. (example: -23 m/s means going at 23 m/s backwards)
Position is a vector because it tells you the distance and the direction relative to zero.
(example: 14 km is a different position than -14 km)
Speed and distance are not vectors because they don’t tell you direction.
Speed & distance are called SCALARS because they only tell you how much.
Vectors of the same type can be added together when they are in the same direction or
subtracted when they are in opposite directions.
?????????????? Is acceleration a vector????????????
Lets find out.
2nd*Motion diagrams with constant acceleration.
In addition to the position and velocity of the object we can also represent the acceleration of
the object on the motion diagram. Since the acceleration is the change in velocity over a given
time interval, and since we draw motion diagrams at regular clock ticks, the acceleration is
proportional to the difference between the lengths of successive velocity arrows. Since the
acceleration involves two velocities (initial and final), we draw the arrows between those two
velocity vectors. The length of the acceleration vector is proportional to the difference
between the two velocity vectors because acceleration measures the change in velocity per
time interval.
Notice that v2 is longer than v1. The difference
in length between the two velocity arrows is the
length of the acceleration arrow. ( v1 + a = v2 )
3rd*Examples
of motion diagrams with constant acceleration:
1. A car starts from rest and speeds up. The direction of the acceleration is in the same direction
as the direction of its velocity.
Notice where the acceleration arrows are drawn on the diagram below. The acceleration arrows
are drawn above the position dot that is BETWEEN the two velocity arrows used to calculate it.
We need to know two velocities to find the acceleration. Technically, we don’t know the
acceleration at the first and last points. However, since we will only deal with motion with
constant acceleration, it is reasonable to expect the same acceleration at both first and last
points, therefore it is ok to draw acceleration arrows above those points.
4th*2.
A car slows down to a stop. The direction of the acceleration is opposite to the direction of
the car’s velocity.
3: An ostrich is speeds up, trying to catch up with his group. The speed of the ostrich is
increasing. The acceleration is in the same direction as the ostrich’s velocity (the ostrich is
speeding up).
group
4: After participating in a 20 mile bike race, Tom approaches his home. His speed decreases as
he prepares to stop. The acceleration is opposite to the direction of its velocity (Tom is slowing
down).
home
5th*5: A ball rolls down a ramp, increasing its speed as it moves down the ramp.
Note: motion diagrams are usually drawn parallel to the direction of motion.
Based on what you have read/written, is acceleration a vector????