Chapter 2: Section 3

Source: http://www.physicsclassroom.com

Learning TargetsRelate the motion of a freely falling body to

motion with constant accelerationCalculate displacement, velocity, and time at

various points in the motion of a freely falling object

Compare the motions of different objects in free fall

P2.1D, P2.1g, P2.2C

The Big Misconception"Wouldn't an elephant free-fall faster than a

mouse?“If we are considering the specific type of

falling motion known as free-fall, the objects move under the sole influence of gravityMore massive objects will only fall faster if

there is an appreciable amount of air resistance present.

However, objects that are truly in free fall do not encounter air resistance.

Subsequently, all objects free fall at the same rate of acceleration, regardless of their mass.

Air ResistanceIn the absence of air resistance, all

objects dropped near the surface of a planet fall with the same constant accelerationThis is why a feather and an apple fall at

the same rate if dropped in a vacuum In the presence of air resistance, an object will eventually reach terminal velocity

What is Free Fall?A free falling object is one that is

falling under the sole influence of gravity. Any object that is being acted upon only

by the force of gravity is said to be in a state of free fall.

Characteristics of Free FallThere are two important

characteristics 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 (m/s2)The acceleration due to gravity is

denoted with the symbols ag or g (on the surface of the Earth)

Reviewing AccelerationRemember that acceleration is the

ratio of velocity change to time between any two points in an object's path. To accelerate at 9.8 m/s/s means to

change the velocity by 9.8 m/s each second.

An Example of Free FallIf the velocity and time for a free-falling

object being dropped from a position of rest were tabulated, then one would note the following pattern:

Time (s) Velocity (m/s)

0 0

1 -9.8

2 -19.6

3 -29.4

4 -39.2

5 -49.0

Graphing Free FallPosition vs. Time

GraphThe slope of any position

vs. time graph is the velocity of the object

A curved line on a position versus time graph signifies accelerated motion

The small initial slope indicates a small initial velocity and the large final slope indicates a large final velocity.

The negative slope of the line indicates a negative (i.e., downward) velocity.

Graphing Free FallVelocity vs. Time

Graph

The slope of any velocity vs. time graph is the acceleration of the object

A diagonal line on a velocity versus time graph signifies accelerated motion

Constant negative slope indicates a constant negative acceleration

Acceleration is constant during upward and downward motion

When we throw an object up in the air, it will continue to move upward for some time, stop momentarily at the peak, and then change direction and begin to fall.Because the object changes direction, it may

seem that the velocity and acceleration are both changing.

Actually objects thrown into the air have a downward acceleration as soon as they are released

Velocity and Acceleration During Free Fall

Free falling objects always have the same downward accelerationWhen going up, velocity is positive and

acceleration is negative (-9.8 m/s2) - the object is slowing down

When falling down, velocity is negative and acceleration is negative (-9.8 m/s2) - the object is speeding up Remember that when the signs of velocity and

acceleration are the same, an object speeds up. When they are opposite, an object is slowing down

Calculating Free Fall VelocityTo calculate velocity during free fall, use the

velocity with constant acceleration equations

vf2 = vi

2 + 2a∆y

vf = vi + a∆t

*a = -9.8 m/s2

You can use any of the kinematic equations to solve free fall problems