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Constant Acceleration
Free Fall
Forces and Newton's Law
Kinetic Energy
Potential Energy



Free Fall
Conservation of Energy
Conservation of Momentum


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Free Fall

The term free fall describes the motion of an object in a vertical direction (up or down). For example, if a pen falls from a desk to the floor or a person throws a pen into the air, we might measure the distance the object falls and the velocity with which it falls.

When an object moves in a free fall it is always moving with a constant acceleration, which is called acceleration due to gravity or gravitational acceleration. It is this gravitational force which prevents the freefalling object from flying. Gravitational acceleration always has a value of 9.8 m/s2. A special symbol "g" is assigned to gravitational acceleration and is used in formulas in order to distinguish acceleration due to gravity from other kinds of accelerations.

There are several formulas that you must be familiar with in order to solve problems that have to do with free fall.

This formula is used to calculate the distance that an object has traveled in air during its fall.

Example 1:

How to find the distance a dropped stone falls if you know how long it takes to hit the ground

A stone is dropped from the top of a six-floor building and reaches the ground 10 seconds later. How high is the building?


 Step 1:

In this problem we are given the following:

Vi = 0 m/s
t = 10 s
g = 9.8 m/s2

  Step 2:

Plug the collected data into the formula and solve the problem:

Answer: It took the stone 10 seconds to fall 4900m.

Example 2:

How to find the distance a ball travels if you know its initial velocity.

A ball is thrown with an initial velocity of 20 m/s and reaches its highest point in 4 seconds. Calculate the distance the ball traveled in the air until it reached its peak.

Man throwing a ball


Step 1:

In this problem we are given the following

Vi = 20 m/s
t = 4 s
g = 9.8 m/s2

Step 2:

Plug the data into this formula and solve the problem:

Answer: The distance the ball traveled is 158.4.

The formula below is used to calculate the velocity of an object when it's moving in a vertical direction. During free fall velocity is always changing because of acceleration due to gravity. Thus, physicists measure "final velocity," which means the velocity of the object right before it reaches the ground.

Example 3:

How to find the final velocity of a dropped object if you know its initial velocity and time.

A ball was dropped with initial velocity of 15 m/s, and reached the ground 3 seconds later. What was the final velocity of the ball right before it reached the ground?

Girl at the top of a building dropping a ball



In this problem we are given the following:

Vi = 15 m/s
t = 3 s
g = 9.8 m/s2

Step 2:

Plug the data into the formula:

Answer: The final velocity of the object is 44 m/s.