How to Find the Average Speed If You Do Not Know the Distance Nor Time

The equation for average speed is S=D/T.

Finding average speed is a basic aspect of physics. It is defined as the mean speed that an object travels over a set period of time. More simply, the equation for average speed is distance/time = average speed (D/T=S). The equation gets more complicated when multiple speeds, times, or distances are used. Still, it is possible to solve for average speed as long as you have one of the three parts of the equation, distance, time, or speed. If you do not have two of these parts, you must derive them from existing data in the problem.

Things You'll Need

• Paper
• Pencil
• Calculator

Instructions

Finding Average Speed

1. Find the distance traveled. If the distance is not stated as a flat number, find it by using other variables in the problem,. Example: A car is driven at 30 mph for three hours, then driven at 60 mph for 15 hours. You change the original formula for finding average speed to find distance. D=SxT. You can find the distance by multiplying time traveled by speed for each set of drives, and add them together. (30x3)+(60x15)=990 miles.

2. 

   Time is a measurement that is commonly used as part of many other measurements such as speed (D/T).

   Find the time traveled. Like distance, time traveled may not be directly stated. Instead, it is sometimes necessary to derive it. Example: a car drives 90 miles at 30 mph then drives 900 miles at 60 mph. Again we change the original equation to solve for time instead of speed. T=D/S. (90/30)+(900/60)= 18 hours traveled.

3. Find the average speed. In either permutation of the problem above, distance and time were not directly stated. However, you were able to derive them from other information in the problem. You now know that T= 18 hours and D= 990 miles. Using the original equation S=D/T you can solve for average speed. 990 miles divided by 18 hours equals 55 mph.

Tips & Warnings

• Because you are trying to find average speed, it does not matter what direction the distance traveled was in, only the total distance traveled.

• Each of the three parts of the average speed equation can usually be derived from information available. If for some reason you cannot find at least two parts out of distance, time, or speed, then it will be impossible to solve for average speed.

References

• The Physics Classroom; Describing Motion With Words
• Mansfield Connecticut Schools; Average Speed
• Machine Head Software; Speed Distance Time Calculators