Chapter 18
Section 2
Positive AccelerationIf the acceleration equation produces a positive
number as the result it means that you are
speeding up (the final speed is greater than the
initial speed)
 Negative AccelerationIf the acceleration equation produces a negative
number as the result it means that you are
slowing down (the final speed is less than the
initial speed)

Graphs acceleration (how the speed changes over time)
Remember, Distance vs. Time graphs show how much
distance is covered per unit time

14
Speed (m/s)
12
10
8
Rise = 4 m/s
6
4
Run = 2 s
2
0
0
1
2
3
4
5
Time (s)
1) In Speed vs. Time graphs:
Acceleration = Rise/Run
= 4 m/s ÷ 2 s = 2 m/s2
6
Section 3
Momentummeasure of how hard
it is to stop an object.
Depends on mass and
velocity
Inertia is the tendency of an object to resist a
change in its motion (the more mass something has
the harder it is to get it going, slow it down, speed
it up, or turn it)
What does that mean?
Like the law of conservation of mass (remember what
that is?) the law of conservation of momentum states
that the amount of momentum of objects is the same
before and after the objects collide
Think of playing pool…
What happens?
The momentum of the cue
ball decreases but the
momentum of the ball it hits
increases
What common external forces affect momentum?
Friction and gravity
In what ways can objects collide?
1. They can bounce off each other
(bowling ball and bowling pins)
2. They can stick to each other (2 pieces of candy)