This document discusses linear motion concepts including: - Motion is described relative to a reference point, such as the surface of the Earth. - Speed is the distance covered over a period of time and does not include direction. Velocity includes both speed and direction. - Instantaneous speed is the speed at a given moment, while average speed is the total distance divided by the total time. - Acceleration is the rate of change of velocity, and can be positive (speeding up) or negative (slowing down). The acceleration formula is the change in velocity over the change in time. - Graphs can show speed as the slope of a distance-time graph or acceleration as the slope of a velocity-

1. Ch 2 Linear Motion
Motion in a straight line

2. All Motion is Relative!
• …..even as we sit still, we are hurdling through space.
• When we describe motion we do it with regards
to something.
• The Train is leaving the station or is the station leaving the
• train?
• • A car in the Indy 500 travels 500 miles in the race but ends up
• at the same point.
• • On the school bus traveling at 30mph you throw a ball to a
• classmate, how fast is the ball moving?
• • We usually describe motion relative to the
surface of the Earth.

3. Speed
• Speed is a “scalar quantity” – this means it does
not include direction.
• Speed is my distance covered divided by the
time it takes me
• I am running at 10miles per hours, but I do not
tell you where – this is speed.
• Speed = distance/time
• – Miles per hour (mi/h), kilometers per hour
(km/h),
meters per second (m/s), light-years per century
• – The symbol / is read as “per”

4. Speed cont..
• Speed is the measure of how fast something
moves, it is a unit of distance divided by a
unit of time. We think of it in two ways.
• – Instantaneous speed
• – Average Speed
• Instantaneous Speed: speed at any given
instant. “speedometer”
– • Average Speed: total distance/ total time

5. Velocity
• Is a vector, meaning it includes
direction.
• Ex: a car travels at 60 km/hr heading
north.
• Going in a straight line at the same
speed is called constant velocity.
• But changing speed (hit the brakes) or
direction ( turn a corner), then we have
a change in velocity.
• Changing velocity (in either way) is
called acceleration.

6. Acceleration
• “how quickly velocity changes”. ( or how
fast you change speeds)
• Can be positive or negative.
• Positive acceleration – speeding up.
• Negative acceleration – slowing down,
and is sometimes called deceleration.
• Remember that changing directions –
even at a constant speed is an
acceleration.

7. Acceleration formula
• A= change in velocity/change in time.
• …some ways you might see this written:
• Vf = final velocity
• Vi = initial velocity
• ∆v = change in velocity ( subtract, final – initial)
a = vf – vi
∆t
Units can vary: m/s/s, mi/hr/s, km/hr/s or m/s2

8. Directions
• Directions can be positive or negative!
• Be careful with your signs.
• For example: someone moving backwards
or down would be moving in a negative
direction.

9. Graphing Motion
• The slope of a graph can tell you a lot
about the motion and save you some
equations!
• The slope of a distance time graph is
speed ( velocity).
• The slope of a velocity time graph is
acceleration!

10. Graphing Examples
• http://www.broadeducation.com/htmlDemos/AbsorbPhysicsAdvd/DistanceTime/page.htm