Free Fall:

Objectives:

1.    Describe the motion of an object in free fall.

2.    Describe the motion of an object thrown up and allowed to fall until it hits the ground.

3.    Determine the speed and distance of an object at any time during free fall assuming that air resistance is negligible.

4.    Explain why the acceleration of an object in free fall does not depend on the mass of the object.

5.    Describe the effect of air resistance on a falling object.

Key Terms:

elapsed time    free fall

Web Resources:

Notes:

Free Fall is the term that describes the acceleration of objects toward the Earth when dropped at rest.  The acceleration of the falling object is due to the force of gravity between the object and the earth. Here are some simple rules.

 All objects accelerate at the same rate regardless of their masses. Acceleration due to earth’s gravity is 9.8m/s/s or 9.8m/s2 The term elapsed time refers to the amount of time that has passed since the object was dropped. Air resistance is assumed to be zero.

This means that for every second the velocity increases by a very predictable amount.  The formula for instantaneous speed is v = at  or  v = gt (g is the acceleration due to gravity)

Example:  1sec = 9.8m/s, 2sec = 19.6m/s, 3sec = 29.4m/s, 4sec = 38.2m/s…

The distance of an object can be determined by taking the average velocity between two points multiplied by the time of the fall.  This averaging of velocity must be considered because the speed of an object in free fall is constantly increasing.  The formula distance is d = 1/2 at2 or d = 1/2 gt2

Example:  How far does an object in free fall travel after 8 seconds? (assuming it started at rest)

Relationship Between Free Fall and Weight

All objects Accelerate towards Earth at the speed of gravity.  This can be proved by the equation

F/m = F/m        or        wt/mass = wt/mass

The force is equal to the weight of the item.  This principle is based on Newton's second law which states that the acceleration of an object is directly related to the force acting on an object and inversely related to the mass of the object.

Therefore, as the mass is increased the increase in weight counteracts this increase keeping the acceleration constant.  Look at these two masses.    100kg & 10kg (remember that weight = mass * gravity)

980N/100kg = 98N/10kg        or        9.8m/s2 = 9.8ms2

All falling objects meet air resistance.  This is reality.

 As an object falls, the air resistance gradually becomes equal to the pull of gravity. At this point the velocity becomes constant. (Terminal velocity) The heavier an object is the longer it takes to reach terminal velocity.