My, how you have changed!

Acceleration

Opening questions

1. Was the motion of your MPV constant throughout its run?

2. Suppose you walked at 3 miles/hour around a circular path at school. Is your rate of motion changing? Explain your response

What do the main subjects in the following pictures have in common

about their motion?

They are accelerating

DEFINITION AND UNITSAcceleration’s

Acceleration

• Anytime the velocity of an object changes, the object accelerates

– Get Faster– Get Slower– If the motion of object changes direction, but

stays at a constant rate

Velocity changes if…

Speeding up Slowing down

Velocity changes if…

• The direction of the motion changes, but the speed does not

• This type of acceleration is call centripetal acceleration

Units for acceleration

• m/s2

• (m/s) / s because the velocity (m/s) changes over time (s)

Derivation of units

The proof of why it is like it is

How we get m/s2 for units of acceleration

Units

• How do you simplify

(¾) / 9

answer

TYPES OF ACCELERATION

AVERAGE OR OVERALL ACCELERATION

Average or overall acceleration

• Acceleration of an object determined over a certain time period

Calculating average/overall acceleration

Acceleration = change in velocity / time

a = ∆v / t

∆v

• The ∆ means the change in…• So ∆v means change in velocity

• You find the change in a value by subtracting it initial value from its final value

• ∆v = final velocity – initial velocity

∆v = vf - vi

Final Equation for average/overall acceleration

a = (vf - vi) / t

vf = final velocity

vi = initial velocity

t = elapsed time

a = acceleration

Ex #1

• A flowerpot falls off a second story windowsill. The flowerpot start from rest and hits the sidewalk 1.5 s later with a velocity of 14.7m/s. Find the average acceleration.

What information is given?Identify the information.

• 1.5s • Is the time• 14.7 m/s• is the final velocity• Starts from rest• Means that the initial velocity is zero (0)

• “at rest” always means that vi = o

Answer #1

• a = (14.7 – 0) / 1.5 = 9.8

• What are the units?

• (14.7 m/s – 0 m/s) 1.5 s = (m/s) / s

• m / s2

UNIFORM OR CONSTANT ACCELERATION

Constant acceleration

• Occurs when rate of change in the velocity of an object is constant.

• This means the velocity values change at the same rate each second

• An object that accelerates at 3 m/s2, goes from rest (0 m/s) to:

• 3 m/s after 1 second, 6 m/s after 2 sec,• 9 m/s after 3 sec…

Constant acceleration and the graphs

• Constant acceleration is indicated by a smooth curve on a P-t graph, and a straight line on a V-t graph.

• Slope of the line on a V-t graph = value of constant acceleration

Ex

• What is the acceleration of the object at 5 min?

• What is the acceleration of the object at any time between 15 min and 40 min?

Constant Acceleration

• An object moving at a constant velocity has a constant acceleration of zero

• As evidenced in a graph by a straight horizontal line

Positive acceleration

• Positive slope of V-t graph, graph gets higher moving left to right

• Getting faster in the positive direction

• Getting slower in the negative direction

Negative acceleration

• Graph gets lower moving left to right

• Slowing down in the positive direction

• Speeding up in the negative direction

Question

• What is the difference between constant velocity and constant acceleration?

Answer

• Constant velocity means the object does not change its velocity over time

• Constant acceleration means that the object does not change its acceleration over time

REAL LIFE EXAMPLES OF CON A

Almost all movement in life can be divided up into segments with constant acceleration

Instantaneous acceleration

• Acceleration at a certain moment in time