Net Forces and Gravity

-Free Fall and Air Resistance

Thought Experiment

Imagine you take an elephant and a mouse to the moon where there is no atmosphere. You drop the elephant and the mouse from the space ship. Which animal hits the ground first? Why?

Draw a free-body diagram of the forces acting on each body.

In a vacuum there is no air resistance and both will accelerate the same (acceleration due to gravity ---- g)

This is called Free-fall.

Free-fall is the gravitational acceleration of an object in the absence of air-resistance.

Thought Experiment # 2

Imagine you bring the elephant and the mouse back to Earth (unharmed by their Lunar adventure). You drop them both from a tall bridge. Which animal hits the water first? Why?

The net force is zero and the object no longer accelerates when the force due to air resistance is equal to the force due to gravity

Balanced forces = constant velocity (Terminal Velocity)

The mouse reaches terminal velocity first and loses!

The force of air-resistance Fair (usually just called "Air resistance") is a type of fluid friction and is the result of air molecules rubbing on the falling object.

It is always a resistance force (ie in the opposite direction to the motion of the object.)

maximum Fair = Fg At Terminal Velocity

1) The elephant encounters a smaller force of air resistance than the mouse and therefore falls faster. F

2) The elephant has a greater acceleration of gravity than the mouse and therefore falls faster. F

3)Both elephant and mouse have the same force of gravity, yet the acceleration of gravity is greatest for the elephant. F

4) Both elephant and mouse have the same force of gravity, yet the mouse experiences a greater air resistance. F

5) Each object experiences the same amount of air resistance, yet the elephant experiences the greatest force of gravity. F

6) Each object experiences the same amount of air resistance, yet the mouse experiences the greatest force of gravity. F

7) The mouse weighs more than the elephant, and therefore will not accelerate as rapidly as the elephant. F

8) Both elephant and mouse weigh the same amount, yet the greater mass of the mouse leads to a smaller acceleration. F

9) The elephant experiences less air resistance and than the mouse and thus reaches a larger terminal velocity. F

10) The mouse experiences more air resistance than the elephant and thus reaches a smaller terminal velocity. F

11) The elephant and the mouse encounter the same amount of air resistance, yet the elephant has a greater terminal velocity.F

True or False?

It's All About Unbalanced and Balanced Forces

Free body diagrams of the Elephant and Mouse show that the mouse quickly reaches terminal velocity, (Terminal Velocity occurs when Fg = Fair) while the elephant continues to accelerate for much longer. The elephant is therefore going faster when it hits the water. The elephant hits the water before the mouse.

http://www.sciencechannel.com/tv-shows/head-rush/videos/head-rush-terminal-velocity.htm

http://www.youtube.com/watch?v=_S5UxmW8FUc&safe=active

highest freefall

terminal velocity

The amount of air resistance experienced by an objects falling in a Gravitational Field in the presence of an atmosphere depends on:

- the velocity of the object Fair α v2

- the surface area of the object- the density of the air- turbulence caused by the shape of the object.

Air resistance is a complicated force...

Example 1: a) What is the net force on a free-falling -30 N object?

b) What is the net force on the object if it is experiencing an air resistance of 12 N?

c) What is the net force on the object when it has reached its terminal velocity of 23 m/s?

Read pages

1-5 Gravity and Weight (not apparent weightlessness on page 5)

16 and 17 Terminal Velocity

Complete page 34

Tomorrow we will measure g