Free Fall Chapter 4

Tiffany Rhodes

We all know the effects of gravity?

Right???

http://www.youtube.com/watch?v=cIwTYL1fwJk

http://www.youtube.com/watch?v=F-pX1X2cUv4

http://www.youtube.com/watch?v=lLmJ1Mm13IQ

What does it mean to be in Free Fall?

Occurs when an object is being acted upon by the force of gravity.

If gravity is the only force acting on an object, the object will accelerate at 9.8 m/s2. That is a constant on Earth!

All objects fall at the same rate!

Acceleration due to Gravity Constant Acceleration… 9.8 m/s/s

You can use the kinematic equations!!

Sometimes seen as ‘g’ rather than ‘a’ in equations

Usually given a negative symbol representing a downward direction.

g = 9.8 m/s/s, downward

Kinematic Review V = Vo + at

X = Xo + Vot + ½ at2

V2 = Vo2 + 2 a (X-Xo)

a = -g (gravitational constant. –9.8 m/s2)

You keep speeding up as you fall!

Time (s) Velocity (m/s) V = Vo + -at

0 0

1 - 9.8

2 - 19.6

3 - 29.4

4 - 39.2

5 - 49.0

Until You Hit ‘Terminal Velocity’

Happens around 10 seconds after YOU fall

Air resistance counters gravity causing no more acceleration to occur

Continue at a constant velocity to ground

Downward Gravity = Upward Air Resistance

Skydiver

Skydiver

The first 10 seconds, you are in constant acceleration. You can use your beloved kinematics!After 10 seconds, you have reached terminal velocity (no longer accelerate), Net force = 0 (Mechanical Equilibrium)

You can use your beloved constant Velocity equation!! V= displacement/time

Until you pull the parachute at about 30 seconds…

Sample Problem A Sky diver jumps out of a plane at an elevation of 5000 meters. He reaches terminal velocity at 10 seconds.

A. What is his velocity at 10 seconds?B. How far did he fall in the first 10 seconds? C. How far did he fall in the first 16 seconds?

Part A: What is his Velocity at 10s?

Vo = 0 T = 10 sec

A = -9.8 m/s2

Vf =?

Vf = Vo + atVf = 0 + (-9.8)(10)Vf = -98 m/s

In the first ten seconds, the skydiver accelerated to a terminal velocity of 98 m/s in the downward direction

Part B: How far did he fall in the first 10 s?

Vo = 0 T = 10 sec A = -9.8 m/s2

Vf =-98 m/s

Δx = ??

Δx = Vot + 1/2 at2 Δx = 0 + ½ (-9.8)(10)2

Δx = - 490 meters

In the first ten seconds, the skydiver fell a total of 490 meters towards the Earth

Part C: How far did he fall in the first 16s?

For the first 10s,

Δx = - 490 meters

For the next 6s, he travels at a constant speed

V = -98m/sT = 6 seconds

Δx = ???

Constant Velocity = Δx / time-98 m/s = Δx / 6 seconds-98 * 6 = Δx -588 m = Δx In the next 6

seconds, the skydiver fell a total of 588 meters towards the Earth

-490 + -588 = -1078 meters or 1078 meters down

Terminal Velocity

http://youtube.com/watch?v=1ukf2vntU44&feature=related

Rising Objects

Now consider an object thrown straight up:

• It moves upward for a while.

• At the highest point, when the object is changing its direction from upward to downward,

• its instantaneous speed is zero.

• It then falls downward as if it had been dropped from rest at that height.

4.5 Free Fall: How Fast

During the upward part of this motion, the object slows from its initial upward velocity to zero velocity.

The object is accelerating because its velocity is changing.

How much does its speed decrease each second?

4.5 Free Fall: How Fast

The speed decreases at the same rate it increases when moving downward—at 9.8 meters per second each second.

• The instantaneous speed at points of equal elevation in the path is the same whether the object is moving upward or downward.

• The velocities are different because they are in opposite directions.

• During each second, the speed or the velocity changes by 9.8 m/s downward.

4.5 Free Fall: How Fast

The change in speed each second is the same whether the ball is going upward or downward.

4.5 Free Fall: How Fast

Change in Displacement The distance which a free-falling

object has fallen from a position of rest is also dependent upon the time of fall.

∆X =Vot + 1/2at2

At t = 1 s

∆X = (0.5) * (-9.8 m/s2) * (1 s) 2 = -4.9 m

At t = 2 s∆X = (0.5) * (-9.8 m/s2) * (2 s) 2

= -19.6 m

Will an elephant and a mouse fall at the same rate?

Neglecting Air resistance…

ALL OBJECTS FALL AT THE SAME RATE!! 9.8m/s/s

An elephant and a mouse will hit the ground at

the same time!!

SAME ACCELERATION!

Why do we see otherwise in real life? AIR RESISTANCE

What if we remove AIR RESISTANCE??

Will a feather and a hammer fall at the same rate, if we negate air resistance?

http://youtube.com/watch?v=5C5_dOEyAfk&feature=relatedhttp://youtube.com/watch?v=eNn4yXEA_XQ&feature=related

QuickTime™ and aCinepak decompressor

are needed to see this picture.

Can we do this on Earth?

Since I am a superhero, I can ;)

Let me show you…

Do you think it is dangerous to drop a penny off the empire states building??

Penny Drop

Watch Mythbusters on Itunes, DVD or Youtube…

http://www.youtube.com/watch?v=QUS89-_fbtk

Homework: Free Fall Worksheet