A ball bounces off a brick wall, Find the average acceleration

[Alexanddros81]

Homework Statement

Serway Physics Section 2.4 Acceleration

14. A 50.0-g Super Ball traveling at 25.0 m/s bounces off a brick wall and rebounds
at 22.0 m/s. A high-speed camera records this event. If the ball is in contact with the
wall for 3.50 ms, what is the magnitude of the average acceleration of the ball during
the time interval?

Homework Equations

The Attempt at a Solution

I am a bit confused with this one.

If the ball is moving to the right before it hits the wall with a speed of 25.0 m/s and after the rebound
is moving to the left with a speed of 22.0 m/s then should it be:

$a_{avg} =\frac {-22.0 m/s - 25.0 m/s)} {0.0035 s}$

or

$a_{avg} = \frac {22.0 m/s - 25.0 m/s} {0.0035 s}$

(taking the x positive direction to the right)

thanks

 

[gneill]

If the ball is moving to the right before it hits the wall with a speed of 25.0 m/s and after the rebound
is moving to the left with a speed of 22.0 m/s then should it be:

$a_{avg} =\frac {-22.0 m/s - 25.0 m/s)} {0.0035 s}$

or

$a_{avg} = \frac {22.0 m/s - 25.0 m/s} {0.0035 s}$

(taking the x positive direction to the right)

thanks

What thought experiment might you apply to choose between them? Suppose that the bounce was perfect instead of losing a bit of speed. Which formula would give a reasonable result?

 

[ZapperZ]

Homework Statement

Serway Physics Section 2.4 Acceleration

14. A 50.0-g Super Ball traveling at 25.0 m/s bounces off a brick wall and rebounds
at 22.0 m/s. A high-speed camera records this event. If the ball is in contact with the
wall for 3.50 ms, what is the magnitude of the average acceleration of the ball during
the time interval?

Homework Equations

The Attempt at a Solution

I am a bit confused with this one.

If the ball is moving to the right before it hits the wall with a speed of 25.0 m/s and after the rebound
is moving to the left with a speed of 22.0 m/s then should it be:

$a_{avg} =\frac {-22.0 m/s - 25.0 m/s)} {0.0035 s}$

or

$a_{avg} = \frac {22.0 m/s - 25.0 m/s} {0.0035 s}$

(taking the x positive direction to the right)

thanks

Can't you tell that your second a_avg is wrong? 22.0 m/s is to the left, so why is it positive?

Zz.

 

[Alexanddros81]

Here is my solution:

Is this ok?

 

[jbriggs444]

[...] 25.0 m/s [...] 22.0 m/s [...] 50.0 g Super Ball [...] 3.50 ms [...] -13428.57 m/s² [...]
Is this ok?

The calculation looks fine. Carrying seven significant figures in a result obtained from inputs with only three significant figures is excessive.

 

[Alexanddros81]

@jbriggs444 :
Ok!