Notes—Section 2.1: Describing Motion
I.
Motion
1. Motion has occurred when an object changes its position relative to a
reference point.
2. Af ter a ref erence point is chosen, a frame of reference can be
created.
a. measures the position of the objects
b. Ex: walls, ceiling, and floor provide a frame of reference to determine if an
object has moved within that room
3. Distance- important part of describi ng motion.
a. It describes how far it has moved.
b. SI unit is the meter (m)
4. Displacement- the distance and direction of an object's change in
position from the starting point.
II. Speed
1. Speed is the distance an object travels per unit of time.
2. To calculate speed, you use the f ormula s= d/t
3. The SI unit f or speed is measured i n m/s or km/ s.
4. If you are traveli ng at a constant speed, you can measure your speed
over any distance traveled.
5. Usually speed is not constant.
a. average speed is the total distance traveled divided by
the total time of travel.
b. Ex: s=d/t
s= 5km/0.25h
s= 20km/h
6. Instantaneous speed- the speed at a given point in time.
7. Changing instantaneous speed- instantaneous speed is changing when
something is speeding up or slowing down. The speed is different at
every point in time.
III.
Graphing Motion:
1. The motion of an object over a period of time can be shown on a
distance-time graph.
2. Time is plotted on the horizontal {x) axis, and distance is plotted
on the vertical (y) axis.
3. see graph on pg.43
4. A horizontal line on the graph represents a time when no distance
is covered.
IV. Velocity- measure of the speed of an object and the direction of
its motion.
1. Because the velocity depends on direction as well as the speed, the
velocity of an object can change even though the speed of the object
remains the same.
2. Cars moving around a race track at a constant speed have velocities
that change continuously because they are always changing directions.
V.
Motion of the Earth's crust
1. Changes in the landscape of the earth occur constantly as
continents drift slowly over the Earth’s surface.
2. The continents move because of the layers contained in the earth.
a. crust- outer layer
b. upper mantle
c. lower mantle- soft rock called the asthenosphere
3. Geologic activities such as earthquake and volcano eruptions occur
because of the motion of the earth's crust.
4. Plates of the earth's crust move so slowly that they are given
speeds in cm per year.
Notes—Section 2.2: Acceleration
I.
Acceleration: the rate of change of velocity
1. Remember that velocity includes the speed and direction of an
object.
2. Therefore, a change in velocity can include either a change in its
speed or d irect ion.
3. Acceleration occurs when an object changes its speed, its direction, or
both.
4. An object can accelerate even when it is slowing down. (if a car i s
sl ow i ng down, it has a negative acceleration)
5. Whether it is speeding up, or slowing down, because it is changing speed,
it is accelerating.
6. If acceleration is in the same direction as the velocity, the speed
increases and the acceleration is positive.
7. If the speed decreases, the acceleration is in the opposite direction
from the velocity, and the acceleration is negative.
8. Any time a moving object changes direction, its velocity changes and it is
accelerating.
II.
Calculating Acceleration:
1. Acceleration is the rate of change in velocity.
2. To calculate acceleration the change in velocity is
divided by the length of time interval over which the
Vf - Vi
t
3. The unit for acceleration is m/s2
4. See page 50 for calculating positive and negative acceleration.
Notes—Section 2.3: Motion and Forces
I. Force—a push or pull
1.
Every push, pull, stretch, or bend results in a force being applied to an
object.
2.
A force can cause the motion of an object to change.
3.
When two or more forces act on an object at the same time, the forces
combine to form the net force.
4.
Balanced forces occur when forces on an object are equal in size and
opposite in direction i.e. two people pushing the same box in opposite
directions
5.
When there is unequal force in opposite directions, the net force occurs
in the direction of the larger force. This is an example of unbalanced
forces and the net force will be the difference between the two forces.
6.
If the forces are exerted in the same direction, the forces are then
combined to give the net force.
II. Inertia and Mass
1.
Inertia is the tendency of an object to resist any change in its motion.
2.
An object that is moving will have uniform motion and continue to move
at the same speed and direction unless an unbalanced force acts on it.
3.
An object at rest tends to stay at rest. Its velocity is 0 unless a force
makes it move.
4.
Inertia is directly related to mass. The greater the mass of an object,
the greater its inertia.
III. Sir Isaac Newton
1.
Laws of Motion—rules that describe the effects of forces on the
motion of objects
2.
Apply to motion of all objects
3.
First Law of Motion—an object moving at a constant velocity keep
moving at that velocity unless an unbalanced net force acts on it
a.
Often called the Law of Inertia
b.
Ex—car crashing into building—driver keeps moving (wear seat
belts!!!)