Gravity, Air Resistance, Gravity, Air Resistance, Terminal Velocity, and Terminal Velocity, and Projectile MotionProjectile Motion

A Whole Bunch of Ideas in one A Whole Bunch of Ideas in one ShowShow

To-Do:To-Do:

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

Do NotDo Not paste it in your journal paste it in your journal

In your whirligig groups, discuss the In your whirligig groups, discuss the “Science Behind It” “Science Behind It” columncolumn in your Deep in your Deep Dive journal notes.Dive journal notes.

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

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

By the end of this presentation, By the end of this presentation,

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

by terminal velocity.by terminal velocity. Describe and explain how forces Describe and explain how forces

change on a falling object.change on a falling object.

What is a forceWhat is a force

Push or pullPush or pull

Some basics…Some basics…Draw a picture along with each fact to help you Draw a picture along with each fact to help you remember it!remember it!

An object’s mass _______ affect An object’s mass _______ affect the acceleration of the object due the acceleration of the object due to gravity on earth.to gravity on earth.

Friction always acts in the Friction always acts in the _________ direction as motion._________ direction as motion.

The ___________ of an object will The ___________ of an object will affect the velocity of a free falling affect the velocity of a free falling object dropping towards earth.object dropping towards earth.

Won’t

opposite

surface area

What IS gravity?What IS gravity?

Gravity is NOT “the force that Gravity is NOT “the force that pulls you down!”pulls you down!”

Gravity attracts all objects with Gravity attracts all objects with mass inward towards other mass inward towards other objects with mass.objects with mass.

the Force of Gravitythe Force of Gravity

What is the force of gravity?What is the force of gravity? The force of gravity is NOT the The force of gravity is NOT the

same as the acceleration DUE to same as the acceleration DUE to gravity!gravity!

Free FallFree Fall

Objects whose only acceleration Objects whose only acceleration is due to gravity are in is due to gravity are in free fallfree fall..

Theoretically, objects in free fall Theoretically, objects in free fall continue to accelerate as long as continue to accelerate as long as they are falling.they are falling.

All free falling objects accelerate All free falling objects accelerate at the same rate regardless of theat the same rate regardless of their mass.ir mass. (9.8 m/s (9.8 m/s22))

Galileo's Famous Galileo's Famous ExperimentExperiment

Acceleration of GravityAcceleration of Gravity

Any object which is being Any object which is being acted upon only by the acted upon only by the force of gravity is said to force of gravity is said to be in a state of be in a state of free fallfree fall. . There are two important There are two important motion characteristics motion characteristics which are true of free-which are true of free-falling objects:falling objects:– Free-falling objects do Free-falling objects do

not encounter air not encounter air resistance. resistance.

– All free-falling objects (on All free-falling objects (on Earth) accelerate Earth) accelerate downwards at a rate of downwards at a rate of 9.8 m/s/s (often 9.8 m/s/s (often approximated as 10 approximated as 10 m/s/s)m/s/s)

If the velocity and time for a free-falling If the velocity and time for a free-falling object being dropped from a position of rest object being dropped from a position of rest were tabulated, then one would note the were tabulated, then one would note the following pattern.following pattern.

Time (s)Time (s) Velocity (m/s)Velocity (m/s) 00 00 11 - 9.8- 9.8 22 - 19.6- 19.6 33 - 29.4- 29.4 44 - 39.2- 39.2 55 - 49.0- 49.0

Free FallFree Fall

Newton’s 2Newton’s 2ndnd Law Law predicts this!predicts this!

a = a = FFmm

The force of The force of gravity felt by a gravity felt by a more massive more massive object is greaterobject is greater

Air ResistanceAir Resistance

Air resistance is an upward force Air resistance is an upward force exerted on an object as it falls by airexerted on an object as it falls by air

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

resistance is determined by two resistance is determined by two factorsfactors– The cross-sectional area of the objectThe cross-sectional area of the object– The speed of the objectThe speed of the object

Terminal VelocityTerminal VelocityDetermine the acceleration of this 85 kg skydiver at points A-D. (Hint: F=ma)

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

Terminal VelocityTerminal Velocity

The terminal velocity of a The terminal velocity of a skydiver in a free-fall position skydiver in a free-fall position with a semi-closed parachute is with a semi-closed parachute is about 195 km/h about 195 km/h

Higher speeds can be attained if Higher speeds can be attained if the skydiver pulls in his limbs. In the skydiver pulls in his limbs. In this case, the terminal velocity this case, the terminal velocity increases to about 320 km/h!increases to about 320 km/h!

The more compact and dense the object, the The more compact and dense the object, the higher its terminal velocity will be. Typical higher its terminal velocity will be. Typical examples are the following: raindrop, 25 ft/s, a examples are the following: raindrop, 25 ft/s, a skydiver was found to be in a range from 53 m/s skydiver was found to be in a range from 53 m/s to 76 m/sto 76 m/s

Possible Test Questions: Possible Test Questions: What factor causes terminal velocity to What factor causes terminal velocity to

occur?occur? If an object is at terminal velocity, is it If an object is at terminal velocity, is it

speeding up, slowing down, or falling at speeding up, slowing down, or falling at a constant speed?a constant speed?

Describe and explain how forces change Describe and explain how forces change on a falling object.on a falling object.

Terminal VelocityTerminal VelocityConsider a skydiver:

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

____ downwards.

2) As his speed increases his air resistance will _______

3) Eventually the air resistance will be big enough to _______ the skydiver’s weight. At this point the forces are balanced so his speed becomes ________ - this is called TERMINAL VELOCITY

zero

accelerates

increase

balance

constant

W Richards Worthing High School

How the forces change with time.

KEY Gravity

(constant value & always present…weight)

Air resistance

(friction)

Net force(acceleration OR changing

velocity)

Terminal Terminal VelocityVelocity

Consider a skydiver:4) When he opens his parachute

the air resistance suddenly ________, causing him to start _____ ____.

5) Because he is slowing down his air resistance will _______ until it balances his _________. The skydiver has now reached a new, lower ________ _______.

increasesslowing down

decreaseweight

terminal velocity

Velocity-time graph for Velocity-time graph for the sky diverthe sky diverVelocit

y

Time

Speed increases…

Terminal velocity reached…

Parachute opens – diver slows down

New, lower terminal velocity reached

Diver hits the ground

On the Moon

Projectile MotionProjectile Motion

Projectile: When Projectile: When a falling object a falling object also experiences also experiences horizontal motionhorizontal motion

Horizontal Horizontal motion does not motion does not affect vertical affect vertical motionmotion