Speed vs. Velocity And Acceleration

 Distinguish between and calculate speed and velocity ◦ Velocity is a vector quantity (has direction) and speed is a scalar quantity (no direction) ◦ The speed and direction at a certain instant in time is called instantaneous velocity.  Describe and calculate acceleration.

A series of images of a moving object that shows its position after equal intervals of time. Each picture represent the location of the runner after 1 second. What can be said about the distance covered as the clock ticks? It increases so this guy is speeding up!

Use dots to represent object. Speeding Up Slowing Down Constant Speed

Speed is the same Direction is same Also referred to as “constant motion”

Scalar quantity Total distance divide by elapsed time Units: mph, km/hr, m/s

Vector quantity Could be negative Change in position divide by elapsed time Units: mph, km/hr, m/s

Speed and direction at a particular moment or “instant” of time. Speed changes! So you need a different description depending on what instant you see it.

G.U.E.S.S. method  G iven: “I know what values I have.”  U nknown: “I know where I am going.”  E quation: “ I know the physics relationship.”  S ubstitute: “I know how to apply the physics.”  S olve: “I know what the answer means.”

How long will it take to travel 50.0miles in a car traveling a constant speed of 65miles/hr? d= 50.0 miles s= 65miles/hr t=?s s=d/t so… t = d/s t=(50.0mi)/(65mi/hr ) t= 0.80 hr

Velocity is changing Vector quantity The change in velocity divided by the elapsed time Units: m/s/s or m/s 2

When studying one dimensional motion  Positive acceleration is toward the right.  Negative acceleration is toward the left. BUT… the direction of acceleration is independent of the direction of motion.

 When the objects velocity and acceleration are in the same direction the object will increase in velocity.  When the objects velocity and acceleration are in the opposite directions the object will decrease its velocity. And if this situation persists…  The object will eventually come to a stop. And it this situation persists…  The object will reverse directions.

G.U.E.S.S. method  G iven: “I know what values I have.”  U nknown: “I know where I am going.”  E quation: “ I know the physics relationship.”  S ubstitute: “I know how to apply the physics.”  S olve: “I know what the answer means.”

A dragster starting from rest accelerates at a rate of 49m/s 2. How fast is it going after an elapsed time of 15s? a=49m/s 2 Δ t=15s v i = 0 m/s v f = ? m/s (v f - v i ) = a Δ t (v f - 0)=(49m/s 2 )(15s) v f = 740 m/s