Impulse momentum theory, a ball strikes a wall dunno if i got it right?

[girl52]

Homework Statement

a ball w/ mass of .15kg and a velocity of 5m/s strikes a wall and bounces right back with v=3m/s. ball is in contact with wall for 0.1 sec, how much force did the wall exert on the ball?

Homework Equations

Ft=mvf-mvi
F=?
t=0.1
m=.15
vf=3m/s
vi=5

The Attempt at a Solution

0.1F=(.15*3)-(.15*5)
0.1F= -.30
F= -3 N

How could the force be negative?? did i do something wrong?

 

[PhanthomJay]

Velocity and momentum are vectors, which means you have to take into account the direction of the velocities (plus or minus) before and after the collision. Your answer is not correct. If the force is negative, what does that imply?

 

[girl52]

Velocity and momentum are vectors, which means you have to take into account the direction of the velocities (plus or minus) before and after the collision. Your answer is not correct. If the force is negative, what does that imply?

OOOH okayy i don't know how i forgot that!
i just don't know which one is negative. i made the inital velocity negative, and i got 12N for the answer...is that right?

i got:
0.1F=(.15*3)+(.15*5)
F=12

when i made the final velocity negative i got 3N

im so confused

 

[PhanthomJay]

12 N for the average force is correct, but what is the direction of the 12 N force? You can choose any direction as negative, as long as you choose the opposite direction as positive. If you make the final velocity negative, then the initial velocity is positive. In which case,
0.1F = .15*(-3) - (.15*5)
F = - 12 N
How do you interpret the minus sign??

 

[girl52]

12 N for the average force is correct, but what is the direction of the 12 N force? You can choose any direction as negative, as long as you choose the opposite direction as positive. If you make the final velocity negative, then the initial velocity is positive. In which case,
0.1F = .15*(-3) - (.15*5)
F = - 12 N
How do you interpret the minus sign??

um...12N toward the wall?

 

[PhanthomJay]

The ball exerts an average 12 N force on and pointing toward the wall, so the wall must exert an average 12 N force on the ball in the direction, per Newton 3, pointing away from? or toward? the wall. The sign of the force corresponds to the signs you chose for the direction of the velocities.