# Is this problem really this simple?

1. Sep 11, 2006

### wakejosh

3. A 50-gram ball traveling at 25 m/s is bounced off a brick wall and
rebounds at 22 m/s. A high-speed camera records this event. If the ball is in contact with the wall for 3.5 ms, what is the average acceleration of the ball during this time interval?

so is this as simple as 25+22= 47 then, 47/2= 23.5 m/s/s ?

im confused with the wording of this question. Is it asking what is the average velocity of the ball during the 3.5 seconds while its in contact with the wall? if so, is it just an average of the entry velocity and the exit velocity? (sorry i dont know the real terms). Im also guessing that the answer would be in M/s/s but im not sure how to do the conversion, this is my first physics class.. any help is appreciated

2. Sep 11, 2006

### Staff: Mentor

No, it's asking for the average acceleration. (Note that velocity has units of m/s, but acceleration has units of m/s/s.)

To find the average acceleration, first find the change in velocity. Remember that direction (and thus sign) counts.

$$a_{ave} = \Delta v / \Delta t$$

3. Sep 11, 2006

### wakejosh

ok.. so

25 m/s -22 m/s = 3 m/s is the change in velocity

so my change in time is just 3.5 ms or 0.0035 s ?

so is it just 3 / 0.0035 = 857 m/s/s?

4. Sep 11, 2006

### Staff: Mentor

No. Realize that the initial and final velocities are in opposite directions and thus have opposite signs.

Pick a sign convention. Let's call "towards the wall" to be positive; "away from the wall", negative. Now state the initial and final velocities. (The change will be final minus initial.)

Right.

5. Sep 11, 2006

### wakejosh

yea.. i understand the negative part. in my work I actually figured it to be -3 m/s but my answers to choose from were all positive.

so is it going to be -857 m/s/s ? or something totally different?

6. Sep 11, 2006

### Staff: Mentor

How did you get that answer?

(In selecting your answer, I'm sure all they care about is the magnitude of the acceleration, which is always positive.)

7. Sep 11, 2006

### wakejosh

3 / 0.0035 = 857 m/s/s

8. Sep 11, 2006

### Staff: Mentor

Reread what I wrote in post #4. The change in velocity is not 3 (or -3).

9. Sep 11, 2006

### wakejosh

25 - (-22) = 47

so

47/0.0035 = 13,428 m/s/s

thats gotta be it. durrr.. sorry.. ty for the help

10. Sep 11, 2006

### Staff: Mentor

Now you're cooking. Using the sign convention I suggested, I get:

Vi = +25 m/s
Vf = -22 m/s

Change in V = Vf - Vi = -22 - 25 = -47
Thus the average acceleration is -13400 m/s/s.

The minus sign just means that the direction of the acceleration is away from the wall.