Gravity and Free Fall (3)

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To-Do:

Grab a study guide from the side counter andcut it in ½ to share with a partner.

Do Not paste it in your journal

In your whirligig groups, discuss the “ScienceBehind It” column in your Deep Dive journalnotes.

Discuss topics such as Gravity, Air Resistance, Mass,Force, Surface Area, Etc.

Complete the right hand side of your notes foreach conclusion that you wrote.

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Gravity, Air Resistance,

Terminal Velocity, andProjectile Motion

 A Whole Bunch of Ideas in one Show

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By the end of this presentation,

you should be able to:  Describe and explain what is meant by

terminal velocity.

Describe and explain how forces changeon a falling object.

Recall and explain the meanings of the

terms „stopping distance‟, „thinkingdistance‟ and „braking distance‟ and thefactors which affect them.

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What is a force

Push or pull

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Some basics… 

Newton‟s second law states thatincreasing mass will ____________ 

the acceleration. Friction always acts in the _________ 

direction as motion.

When an object is thrown upward, thenet force will be the result of theinteractions between the upward forceand __________.

decrease

opposite

gravity

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What IS gravity? 

Gravity is NOT “the force that pullsyou down!”  

Gravity attracts all objects with massto all other objects with mass.

Why?... Who knows!

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the Force of Gravity

What is the force of gravity? 

The force of gravity is NOT the same

as the acceleration DUE to gravity!

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Free Fall

Objects whose only acceleration is dueto gravity are in free fall.

Theoretically, objects in free fallcontinue to accelerate as long as theyare falling.

 All free falling objects accelerateat the same rate regardless of their mass. 

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Galileo's Famous

Experiment

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 Acceleration of Gravity

 Any object which is beingacted upon only by theforce of gravity is said tobe in a state of free fall.There are two importantmotion characteristicswhich are true of free-falling objects: – Free-falling objects do

not encounter airresistance. –  All free-falling objects

(on Earth) acceleratedownwards at a rate of 9.8 m/s/s (often

approximated as 10m/s/s)

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If the velocity and time for a free-falling objectbeing dropped from a position of rest weretabulated, then one would note the following

pattern. Time (s) Velocity (m/s)

0 0

1 - 9.8

2 - 19.6 3 - 29.4

4 - 39.2

5 - 49.0

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Free Fall

Newton‟s 2nd Lawpredicts this!

a = Fm

The force of gravityfelt by a moremassive object is

greater

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 Air Resistance

 Air resistance is an upward forceexerted on an object as it falls by air

It is, in essence, a frictional force For simplicity, the amount of air

resistance is determined by two

factors – The cross-sectional area of the object

 – The speed of the object

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Terminal VelocityDetermine the acceleration of this 85 kg skydiver at points A-D.

9.8 m/s2 5.68 m/s2 1.56 m/s2 0 m/s2

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Terminal Velocity

The terminal velocity of a skydiver in afree-fall position with a semi-closed

parachute is about 195 km/h Higher speeds can be attained if the

skydiver pulls in his limbs. In this case,

the terminal velocity increases toabout 320 km/h!

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The more compact anddense the object, thehigher its terminal

velocity will be. Typicalexamples are the

following: raindrop,25 ft/s, a skydiver wasfound to be in a range

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By the end of this presentation, you should beable to:

Describe and explain what is meant byterminal velocity.

Describe and explain how forces change on afalling object.

Recall and explain the meanings of the terms

 „stopping distance‟, „thinking distance‟ and „braking distance‟ and the factors which affectthem.

W Ri h ds W thi Hi h S h l

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Terminal VelocityConsider a skydiver:

1) At the start of his jump the airresistance is _______ so he

 ____ downwards.

2) As his speed increases his airresistance will _______ 

3) Eventually the air resistance will be

big enough to _______ theskydiver’s weight. At this pointthe forces are balanced so hisspeed becomes ________ - this iscalled TERMINAL VELOCITY

zero

accelerates

increase

balance

constant

W Richards  Worthing High School

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How the forces change with time.

KEY

Weight (constant value and

always present)

Air resistance(proportional to thespeed)

Resultant force

(the bigger theunbalanced force the

greater theacceleration)

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Terminal

 VelocityConsider a skydiver:

4) When he opens his parachute theair resistance suddenly ________,causing him to start _____ ____.

5) Because he is slowing down his airresistance will _______ until it

balances his _________. Theskydiver has now reached a new,lower ________ _______.

increasesslowing down

decrease

weight

terminal velocity

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 Velocity-time graph for thesky diverVelocity 

Time 

Speedincreases… 

Terminal

velocityreached… 

Parachute opens – diver slows down

New, lower terminalvelocity reached

Diver hits the ground

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Projectile Motion

Projectile: When afalling object also

experienceshorizontal motion

Horizontal motiondoes not affect

vertical motion