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Gravity Velocity Equations for Objects Projected Downward

by Ron Kurtus (revised 7 January 2011)

When you throw or project an object downward, it is accelerated until it is released at some initial velocity. If you know this initial velocity, there are simple derived equations that allow you to calculate the velocity when the object reaches a given displacement from the starting point or when it reaches a given elapsed time.

Examples illustrate these equations.

Note: You normally do not need to memorize these equations, but you should know where to find them in order to solve equations.

Questions you may have include:

This lesson will answer those questions. Useful tool: Units Conversion



Velocity with respect to displacement

The general gravity equation for velocity with respect to displacement is:

v = ±√(2gy + vi2)

where

(See Derivation of Displacement-Velocity Gravity Equations for details of the derivation.)

Since v is a downward vector, it has a positive value. Likewise, y and vi are positive numbers. Thus, only the + version of the equation applies:

v = √(2gy + vi2)

Velocity of object projected downward as a function of displacement or time

Velocity of object projected downward as a function of displacement or time

Velocity with respect to time

The general gravity equation for velocity with respect to time is:

v = gt + vi

where t is the time the object has fallen in seconds (s).

(See the Derivation of Velocity-Time Gravity Equations lesson for details of the derivation.)

This same equation applies for an object projected downward.

Examples

The following examples illustrate applications of the equations.

For a given displacement

Find the velocity of a rock that is thrown down at 2 m/s after it has traveled 2 meters.

Solution

You are given that vi = 2 m/s and y = 2 m. Since vi is in m/s and y is in meters, then g = 9.8 m/s2. The equation to use is:

v = √(2gy + vi2)

Substitute values in the equation:

v = √[2*(9.8 m/s2)*(2 m) + (2 m/s)2]

v = √(39.2 m2/s2 + 4 m2/s2)

v = √(43.2 m2/s2)

v = 6.57 m/s

For a given time

Suppose you throw the object downward at 10 m/s. Find its velocity after 4 seconds.

Solution

You are given that vi = 10 m/s and t = 4 s. Since vi is in m/s, g = 9.8 m/s2. The equation to use is:

v = gt + vi

Substitute values in the equation:

v = (9.8 m/s2)*(4 s) + 10 m/s

v = 39.2 m/s + 10 m/s

v = 49.2 m/s

Summary

You can calculate the velocity when an object that is projected downward reaches a given displacement from the starting point or when it reaches a given elapsed time from the equations:

v = √(2gy + vi2)

v = gt + vi


Help other people learn


Resources and references

Ron Kurtus' Credentials

Websites

Gravity Resources

Equations for a falling body - Wikipedia

Gravity Calculations - Earth - Calculator

Kinematic Equations and Free Fall - Physics Classroom

Books

Top-rated books on Simple Gravity Science

Top-rated books on Advanced Gravity Physics


Questions and comments

Do you have any questions, comments, or opinions on this subject? If so, send an email with your feedback. I will try to get back to you as soon as possible.


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