# Homework Help: Methods for calculating average velocity

1. Feb 15, 2015

### henry3369

1. The problem statement, all variables and given/known data
A child is pushing a merry-go-round. The angle through which the merry-go-round has turned varies with time according to θ(t)=γt+βt3, where γ= 0.383 rad/s and β= 1.00×10−2rad/s3.

Calculate the average angular velocity ωav−z for the time interval t=0 to t= 5.50s.

2. Relevant equations
Given

3. The attempt at a solution
Okay so I solved all the questions above this one which gave me the final and initial angular velocities and I got ωi = 0.383 rad/s and ωf = 1.29 rad/s.

When I calculate average angular velocity, why can't I use (ωi + ωf)/2
Instead, I have to use (Θf - Θi)/5.50 to get the correct answer. Shouldn't the first equation yield the same result because an average is the sum of the velocities divided by 2?

2. Feb 15, 2015

### Nathanael

Nooo this is not true at all!

This only applies to one very special case: constant acceleration.

The average speed (over an interval of time) is the constant speed which produces the same displacement (over the same interval). So if you imagine a graph like y(x) or v(t) then the average is the constant (a horizontal line) which produces the same area (over a certain interval of x or t) as the area under the actual curve.

In the special case where y(x) or v(t) has a constant slope (a.k.a. constant acceleration) the average happens to be the midpoint of the line and thus is (y(a)+y(b))/2
(You might like to convince yourself that the midpoint between to points on a graph with a constant slope is the only constant value which gives the same area between the two points.)

I hope I'm not making it confusing, but words tend to do that with visual ideas. It is important to remember that the (y(a)+y(b))/2 or (vi+vf)/2 is just a special case which only applies for constant slope (constant acceleration).

3. Feb 15, 2015

### henry3369

If the angular acceleration is constant, can (vi+vf)/2 be used?

4. Feb 15, 2015

### Nathanael

Yes. If and ONLY if the angular acceleration is constant.

5. Feb 15, 2015

### Brian T

What do you think is the more general case of avg velocity?

Think: if I went 100 miles in 2 hours, my avg was?

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