Chapter 2: Section 3 Source:

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 gravity More 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 Resistance Demo – Penny and a feather In the absence of air resistance, all objects dropped near the surface of a planet fall with the same constant acceleration In the presence of air resistance, an object will eventually reach terminal velocity Terminal velocity is the maximum speed reached when the force of air resistance equals the force of gravity

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. Free-falling objects do not encounter air resistance. All free-falling objects (on Earth) accelerate downwards at a rate of 9.8 m/s 2 (often approximated as 10 m/s/s

Reviewing Acceleration Remember 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 2 means to change the velocity by 9.8 m/s each second.

An Example of Free Fall If 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)

Graphing Free Fall Position vs. Time Graph The 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 Fall Velocity 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 Your acceleration at any point is -9.8 m/s 2

When we throw an object straight 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 at the top is often a given. V top = 0 m/s, a top = -9.8 m/s2 V top

Calculating Free Fall Velocity To calculate velocity during free fall, use the velocity with constant acceleration equations v f 2 = v i 2 + 2a∆y v f = v i + a∆t ***a = -9.8 m/s 2 You can use any of the kinematic equations to solve free fall problems

Example 1

Example 2 2. Karina kicks a soccer ball straight up into the air. It returns to her 2.2 s later. How high did the soccer ball go? Δy= 0  y top = ? t top = ½ t total = 1.1s v top = 0 m/s The way up is the same as the way down only direction changes so make v i = v top Δy = 0 + ½ (-9.8)(1.1) 2 y top = 5.93m