Today’s Topic

Free Fall

What is Free Fall?

Free Fall is when an object moves downward (vertically) only as the result of gravity.

Physics’ Free Fall

When an object is in Free Fall we ignore air resistance. In other words, we treat it as if it were in a vacuum.

Free Fall

Galileo’s experiments showed that all objects fall at the same

speed no matter what their weight

Regardless of their mass, all objects fall with the same acceleration (ignoring air

resistance) Moon DropMyth Busters

Free Fall

When gravity acts on an object we call this gravitational acceleration

or g.Scientists have found that the

average gravitational acceleration on Earth is

Would the value of g be the same on the moon or on Mars?

-9.81 m/s2

Free Fall

For free fall problems we will use the kinematic equations with g plugged in for a:

g is just a special kind of acceleration.

d =vOt+ ½ gt2vf =vO+ gt

vf2 = v0

2 + 2gd

Free Fall

How Fast? How Far?

Free Fall

Remember, velocity and acceleration are not the same

thing.

Free Fall3 types of objects in freefall1.an object dropped

v0 = 0 m/s and d is negative

2.an object thrown down d is negative

3.an object thrown up at the top vf = 0 m/s, for the whole path d = 0 m

Free Fall ProblemsIf a ball is dropped off a

high building, calculate the distance it has traveled and its velocity after 3 seconds.

Distance:

d = ½ gt2

= ½ (-9.81) (3)2

=-44.1 mVelocit

y:

vf = gt= (-9.81) (3)=-29.4 m/s

vi= t=

a=

3s0m/s

-9.81 m/s2

How high does it go? An upward object’s velocity slows

down as it goes up until it is zero at the highest point.

To calculate the time to maximum height:1. Calculate the time it takes to reach the

highest point using: vf=vi + at

vi = vf = a=

t= (vf-vi)/a =

t=0.61 s Use this time to calculate the distance

of the highest point using:d=vit +1/2 at2

d=1.83 m

Example: A referee tosses a coin up at 6m/s. What is the time it takes to reach the maximum height and what is the max height of the coin?

6.00m/s 0 m/s -9.81m/s2

(0-6)/-9.81

=6 (.61) +.5 (-9.81) (.61)2

To calculate just for the max height, solve vf

2=vi2+2ad for d

How long is it in the air? The time it takes an object to go up is equal

to the time it takes to go down. So, the total time is equal to 2 times the

time it takes to reach its max. height. Proof: If we assume the coin is caught in

the referee’s hand the total displacement is?

We can solve d= vit +1/2at2 for t where d=0. (Solve using quadratic formula.) t=?

Equal Speeds For a given displacement along

the motion path, the upward speed of the coin is equal to its downward speed, but the two velocities point in opposite directions.

Mathematically, vf

2=vi2 + 2ad

vi = 6.00m/s and d = 1.50 m

vf2=62 + 2(-9.8)(1.5)

vf=±2.57 m/s

Which ball hits the ground with the greatest speed?

Ball A is thrown upward with a velocity of 30m/s; ball B is thrown downward with a velocity of 30m/s

Both balls hit the ground with the same speed.