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

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Gravity, Air Resistance, Terminal Velocity, and Projectile Motion

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 change on a falling object.

What is a force Push or pull

Some basics… Draw a picture along with each fact to help you remember it!
An object’s mass _______ affect the acceleration of the object due to gravity on earth. Friction always acts in the _________ direction as motion. The ___________ of an object will affect the velocity of a free falling object dropping towards earth. Won’t opposite surface area

What IS gravity? Gravity is NOT “the force that pulls you down!”
Gravity attracts all objects with mass inward towards other objects with mass.

Newton’s Universal Law of Gravitation
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

the Force of Gravity What is the force of gravity?
The force of gravity is NOT the same as the acceleration DUE to gravity!

Free Fall Objects whose only acceleration is due to gravity are in free fall. All free falling objects accelerate at the same rate regardless of their mass. (9.8 m/s2)

Galileo's Famous Experiment

Acceleration of Gravity
Any object which is being acted upon only by the force of gravity is said to be in a state of free fall. There are two important motion characteristics which are true of free-falling objects: Free-falling objects do not encounter air resistance. All free-falling objects (on Earth) accelerate downwards at a rate of 9.8 m/s/s (often approximated as 10 m/s/s)

Free Fall Newton’s 2nd Law predicts this! a = F m
The force of gravity felt by a more massive object is greater

Free Fall

Air Resistance Air resistance is an upward force exerted on an object as it falls by air It is, a frictional force

Terminal Velocity Determine 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 Velocity The terminal velocity of a skydiver in a free-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 to about 320 km/h!

Possible Test Questions:
What factor causes terminal velocity to occur? If an object is at terminal velocity, is it speeding up, slowing down, or falling at a constant speed? Describe and explain how forces change on a falling object.

Terminal Velocity Consider a skydiver:
At the start of his jump the air resistance is _______ so he ____ downwards. zero accelerates 2) As his speed increases his air resistance will _______ increase 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 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 Velocity Consider a skydiver:
4) When he opens his parachute the air resistance suddenly ________, causing him to start _____ ____. increases slowing down 5) Because he is slowing down his air resistance will _______ until it balances his _________. The skydiver has now reached a new, lower ________ _______. decrease weight terminal velocity

Velocity-time graph for the sky diver
Parachute opens – diver slows down Velocity Speed increases… Terminal velocity reached… On the Moon Diver hits the ground New, lower terminal velocity reached Time

Projectile Motion Projectile: When a falling object also experiences horizontal motion Horizontal motion does not affect vertical motion