# Homework Help: Average Acceleration

1. Oct 5, 2006

### XPX1

A rocket blasts off and moves straight upward from the launch pad with constant acceleration. After 2.6 s the rocket is at a height of 80.0 m.
(a) What is the acceleration of the rocket?
______m/s2
(b) What is its speed at this time?
______m/s

Hello, I have been stumped on this question all day. I have no clue on how to do this problem, I have tried 80/2.6/2.6 to get the average acceleration squared, but that was not the correct answer. Can anybody please explain how to do this?

2. Oct 5, 2006

### Staff: Mentor

Last edited by a moderator: Apr 22, 2017
3. Oct 5, 2006

### XPX1

When it comes to physics, I am completely lost in general. According to the equations I followed.. The answer should be 11.834 m/s^2 because I plugged the velocity I found into the acceleration equation.

4. Oct 5, 2006

### Staff: Mentor

Show what you did. What velocity did you find and how did you find it?

5. Oct 5, 2006

### XPX1

What I did,

I took 80 m/s which is supposedly the final velocity, and divided it by 2.6, which gave me 30.76923077. I divided it again by 2.6 because the final answer is in m/s^2. which gave me 11.83431953. 11.83431953*2.6^2=80. I still dont see where I am going wrong here

6. Oct 5, 2006

### drpizza

maybe this will help?

Here's something a little different to try- make a table with
initial velocity, final velocity, average velocity, displacement, acceleration, and time.

For your problem, start filling in what's known; here's an example using different (but similar in magnitude) numbers: "After 3.0 seconds, the rocket is at an altitude of 90 meters"

initial velocity = 0 m/s
final velocity = ?
average velocity = ?
displacement = 90 m
acceleration = ?
time = 3.0 seconds

Looking at these, the average velocity must be 90m/3.0s = 30 m/s
Fill it in...
initial velocity = 0 m/s
final velocity = ?
average velocity = 30m/s
displacement = 90 m
acceleration = ?
time = 3.0 seconds

Now, since you have an average velocity and initial velocity, you can find the final velocity, since (vi + vf)/2 = v.avg
Final velocity must be 60m/s. Fill it in.

initial velocity = 0 m/s
final velocity = 60m/s
average velocity = 30m/s
displacement = 90 m
acceleration = ?
time = 3.0 seconds

Now, since the rocket accelerated from 0m/s to 60m/s in 3.0 seconds, the velocity changed by 60 m/s in 3.0 seconds, or it changed at a rate of 20m/s per s. Fill it in, done. :)

I recognize the usefulness of using x = x0 + vot + 1/2at^2, but until you understand what's going on, sometimes it might help you to learn the concepts by using the more basic equations. The only time the vvvdat table will fail you is if you need to use v^2=vo^2 + 2ad

7. Oct 5, 2006

### Staff: Mentor

Why do you think 80 m/s is the final velocity? Study drpizza's post and you'll see how to find the average and final velocity.

8. Oct 5, 2006

### XPX1

Wow, I got it correct! And thank you sooo much for the tip on using more basic equations like that, it is starting to make ALOT more sense!

9. Oct 5, 2006

### XPX1

One last question, it asks me to find the speed at this time. The speed equation Speed = Distance / Time, but isnt the distance 80m? I tried 80/2.6 = 30.769 but that did not work out, where am I going wrong?

10. Oct 5, 2006

### XPX1

****bump****

11. Oct 5, 2006

### Staff: Mentor

That equation tells you the average speed. You need the final speed. (Reread drpizza's post to see the difference.)

12. Oct 5, 2006

### Cerno

You might want to closely read the question again. See if that helps you at all.

13. Oct 5, 2006

### XPX1

Ah! So Velocity = speed, I guess you can tell by now how lost I am in physics. I got the answer correct, thank you.

14. Oct 5, 2006

### Mindscrape

Velocity does not equal speed. Velocity has direction, while speed does not. A ball thrown up in the air will return to your hand with the same speed, but the opposite velocity. Speed is a velocity's magninude.