# Homework Help: Average Velocity

1. Jan 25, 2008

### StephenDoty

When do you use the equation v=total distance/total time
and when do you use the equation v = (v1+v2)/2

Thank you

Stephen Doty

2. Jan 25, 2008

### arildno

Look on the assumptions behind the formulas. Have you read them?

3. Jan 25, 2008

### StephenDoty

What asumptions????

The two formulas are both for finding the average velocity. One deals with distance and time the other deals with averaging the actual velocities.

I just want to know when to use each of the formulas.

Stephen Doty

4. Jan 25, 2008

### physixguru

average velocity= displacement/time

In the above equation d is the displacement from the object's starting position to its ending position, and t is the time over which the displacement occurred. Knowing d and t we can calculate an average velocity. However, we can not claim to know exactly what the velocity was at a certain instant in time, just the average over the whole time period.

EXAMPLE:
A cyclist travels a displacement of 300 m due North at a velocity of 10 m s-1. She travels the next 300 m in the same direction at a velocity of 15 m s-1. Calculate the average velocity of the cyclist.

Answer: 1st 300 m takes: 300/10 = 30 s

2nd 300 m takes: 300/15 = 20 s

average velocity = total displacement/ total time = 600 / 50 = 12 m s-1.

Many weaker pupils will assume the answer is 12.5 m s-1. You will have to explain why the equation:

AVERAGE VELOCITY = (v1 + v2) / 2 cannot be used in this example. The equation only applies to UNIFORMLY ACCELERATED MOTION. The cyclist spends longer travelling at 10 m s-1 than at 15 m s-1.