Ch 2 Linear Motion

Motion in a straight line

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.

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”

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

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.

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.

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

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.

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!

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