# Tennisball bouncing on wall - momentum

Feodalherren

## Homework Statement

A tennis ball (m  0.2 kg) is thrown at a brick wall. It is
traveling horizontally at 12 m/s just before hitting the
wall and rebounds from the wall at 8 m/s, still traveling
horizontally. The ball is in contact with the wall for 0.04 s.
What is the magnitude of the average force of the wall on
the ball?

P=mV
FΔt=ΔmV

## The Attempt at a Solution

I didn't even know where to begin to I went to the solutions manual which did this:

[-(0.2kg)(8ms)-(0.2kg)(12ms)] / 0.04s = -100N; magnitude would be 100

Where the crap is the negative sign coming from?!

Sum
Hi Feodalherren, the -ve sign tells u that the 100N force exerted by the wall to the ball is in the opposite direction. Its a consequence of newton's 3rd law of motion.

Feodalherren
In that case, why are both of the terms negative?

Sum
Only the reverse velocity should be -ve. It seems the method used in ur manual is unclear. This might help:
f=ma=m(v-u)/t
with m=0.2kg, u=12m/s, v=-8m/s & t=0.04s
substitute in the equation:
f=0.2(-8-12)/0.04=-100N

Feodalherren
That actually didn't help at all :(. Can somebody clarify why there are two negative signs? I have test tomorrow and I need to get this down.

Feodalherren
If an apple were placed in orbit at the same distance from
Earth as the Moon, what acceleration would the apple have?

How can the acceleration be the same if F=ma gives us a = F/m
which says that acceleration IS dependent on mass....?!

Last edited:
Feodalherren
And another, related to the first one:
How would the Sun’s gravitational force on Earth change
if Earth had one-half its present mass? Would Earth’s
acceleration change?

I still don't understand why acceleration doesn't change if f=ma implies it. And I've read all about Galileo's experiment. Why doesn't a=F/m make any sense in this case?

Sum
That actually didn't help at all :(. Can somebody clarify why there are two negative signs? I have test tomorrow and I need to get this down.

if u cant get the obvious, then no one would help u. even angels cant make u understand! open ur eyes!

Feodalherren
You were just doing exactly the same thing as the book. You put two negatives in there without explaining why... Why for an example isn't it -8+12?

ps. There are no angels.

Sum
i didn't imply that angels exist!
That apart, i beleave you know that velocity is a vector (size & direction). In ur case the initial velocity (when the ball is moving toward the wall) is u=12m/s. The final velocity (ball moving away from the wall after being reflected/bounced, this velocity is -ve since it is in the opposite direction), v=-8m/s
u know that f=ma & a=(v-u)/t=(-8-12)/0.04=-500
now f=ma=-500*0.2
hope u get

Feodalherren
Now I did get it. Thank you!