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

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.

Newton’s Universal Law of Newton’s Universal Law of GravitationGravitation

Each and every object in this universe there is the force of attraction between them which is called gravitation force that is always directly proportion to the masses and indirectly proportion to the square of the distance between them

http://www.youtube.com/watch?v=Jk5E-CrE1zghttp://www.youtube.com/watch?v=Jk5E-CrE1zg

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..

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

http://video.google.com/videoplay?docid=6926891572259784994#

Galileo's Famous Galileo's Famous ExperimentExperiment

Acceleration of GravityAcceleration of Gravity

Any object which is being acted Any object which is being acted upon only by the force of gravity upon only by the force of gravity is said to be in a state of is said to be in a state of free free fallfall..

There are two important motion There are two important motion characteristics which are true of characteristics which are true of free-falling objects:free-falling objects:– Free-falling objects do not Free-falling objects do not

encounter air resistance. encounter air resistance. – All free-falling objects (on All free-falling objects (on

Earth) accelerate downwards at Earth) accelerate downwards at a rate of 9.8 m/s/s (often a rate of 9.8 m/s/s (often approximated as 10 m/s/s)approximated as 10 m/s/s)

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

Free FallFree Fall

Air ResistanceAir Resistance

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

It is, a frictional forceIt is, a frictional force

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

http://www.youtube.com/watch?v=1ukf2vntU44http://www.youtube.com/watch?v=1ukf2vntU44

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

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