# Homework Help: Laws of motion magnitude of average acceleration

1. Sep 30, 2008

### maxiJ

1. The problem statement, all variables and given/known data
A rock is rolled in the sand. It starts at 5.0 m/s, moves in a straight line for a distance of 3.0m and then stops. What is the magnitude of the average acceleration?

2. Relevant equations
aavg = change in V/ change in time
magnitude of acceleration = change of speed/time interval.

3. The attempt at a solution
found time = 1.67 s and tried to plug in for average acceleration. 5.0/1.67 = 2.99 m/s^2
the correct answer is 4.2 m/s^2.
?? can't figure out what i am doing wrong.

2. Sep 30, 2008

### Hootenanny

Staff Emeritus
Welcome to Physics Forums,

Tell me, how did you find the time?

3. Sep 30, 2008

### maxiJ

thanks =)

i took 5.0 m/s and divided by the distance of 3.0 m to get 1.67 seconds.
so the ball traveled 3.0 m in 1.67 s at 5.0 m/s.

4. Sep 30, 2008

### Hootenanny

Staff Emeritus
I'm afraid that's incorrect, it would be correct if 5.0m/s was the average velocity, but it isn't it's the initial velocity.

Do you know any kinematics equations?

5. Sep 30, 2008

### maxiJ

ah, yes.. i overlooked that.

well for average velocity its change in displacement over change in time, i think.

6. Sep 30, 2008

### Hootenanny

Staff Emeritus
Last edited by a moderator: Apr 23, 2017
7. Sep 30, 2008

### maxiJ

wow that was a lot easier than i made it out to be. thanks for the help.

8. Sep 30, 2008

### Hootenanny

Staff Emeritus
My pleasure.