Unit 4 – Lecture 2

AccelerationAcceleration – the rate of change of velocity

change in velocity over a change in time

aacceleration tf - ti

change in time

vf - vi

change in velocity

a

=

vf - vi tf - ii

Changerepresented by the Greek letter delta: Δequals the final condition

minus the initial conditionexample: ∆v = change in speedequation for change:

vfinal – vinitial

or… vf - vi ∆

Acceleration – cont’dAcceleration – the rate of change of velocity

change in velocity over a change in time

aacceleration ∆ t

change in time

∆vchange in velocity

a

=

∆v

∆ t

Acceleration – cont’dUnits for acceleration:

unit of distance / [unit of time]2

explanation: if unit for velocity: m/sif change in time measured in seconds = sunit = (m/s)/s = m/s2

for every second that passes, the object will accelerate this number of meters per second

Acceleration – cont’d

Acceleration – cont’dIn physics, acceleration is not only an

increase, but also a decrease in velocity.Two ways to accelerate:

change speedspeed up

positive accelerationslow down

negative acceleration

Acceleration – cont’dIn physics, acceleration is not only an increase,

but also a decrease in velocity.Two ways to accelerate:

change directionyou can feel this acceleration as you turn a

corner in a car – your body is being “pushed” toward the outside of the curve

Acceleration can also be negative [deceleration] depending on the change in frame of reference

Acceleration – cont’d

PracticeWhich has the greater acceleration, an airplane

that goes from 1000 km/h to 1005 km/h in 10 s, or a skateboard that goes from 0 to 5 km/h in 1 sec?

What is the acceleration of a race car that whizzes past you at a constant velocity 400 km/h?

If you’re running at 12 m/s and you accelerate at 0.33 m/s for 6 s, how fast are you going?

GravityGravity – the attraction of one

mass toward anotherdepends on the

masses of each andtheir distance fromone another

from our perspective…gravity is a constant acceleration toward the center of the earth

GravityGravity – the attraction of one mass toward

anotherdecreases over distance

explains why the moon has more of a gravitational effect [tides, etc] on the earth than the sun

Gravity – cont’dGalileo Galilei

experimented to find out the acceleration of free-falling objectsshowed that free-fall was constanthad difficulty showing this before using

inclined planesplane reduced force of gravity &

increased distance, but with same result

Galileo found greater accelerations for steeper inclines. The ball attains max acceleration when the incline is tipped vertically.

Air ResistanceAir resistance – the friction of air particles on

an object moving through itaccounts for varying speeds of falling

objects

Air Resistance – cont’dWithout air resistance, all objects fall with

the same constant acceleration due to gravity

RecapGalileo’s Observations:

a ball rolling down an inclined plane is moving with constant acceleration

greater accelerations for steeper planes, max acceleration when incline is tipped vertically

regardless of weight and size, when air resistance is small enough to be neglected, all objects fall with the same unchanging acceleration

Gravity – cont’dg = acceleration due to gravity

g = 9.8 m/s2 on earthg will change if

the object gets farther away from the center of the earth [distance change]

the object’s mass changes [but not in relation to the earth]

PracticeWhich of the following (if any) could not be

considered an “accelerator” in an automobile: gas pedal, brake pedal, steering wheel?

A sports car accelerates from 65 mph to 75 mph in 2 seconds while a minivan accelerates from 20 mph to 35 mph in 2 seconds.which has the greater acceleration?

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