Homework Help: Force exerted by a ball to

euphtone06 · Mar 18, 2007

1. The problem statement, all variables and given/known data
A barefoot field-goal kicker imparts a speed
of 37 m/s to a football initially at rest.
If the football has a mass of 0.41 kg and the
time of contact with the ball is 0.012 s, what
is the force exerted by the ball on the kicker’s
foot?
Answer in units of N.

2. Relevant equations
F=ma

3. The attempt at a solution
F= .41 * (37/.012) = 1264.166667 N
Answer was wrong, wouldn't this obey newtons third law?
Or am I overlooking something obvious??? Please help!

hage567 · Mar 18, 2007

You need to think of impulse and momentum for this problem.

euphtone06 · Mar 18, 2007

so would it be,
impulse = force x time
1264.16667 N / .012 s = ans?

abm4life · Mar 18, 2007

no... why are u trying to solve for impulse when you have to determine force? what are your known and unknown variables? define them... Delta t = 0.012s, massball = .... and so on

abm4life · Mar 18, 2007

after defining your variables see if you are famliar with the impulse momentum formula:

mvf - mvi = momentum (where vf = final velocity and vi = initial velocity)
impluse = Force*delta t

impulse = momentum... maybe you can go about solving from there

euphtone06 · Mar 18, 2007

Following your advice and retrying everything I still come up with the same answer that the Force = m ΔV/t

tim_lou · Mar 18, 2007

hmmm. the answer sounds correct. who told you that the answer was wrong? maybe the answer is supposed to have a negative sign???

hage567 · Mar 18, 2007

tim_lou said: ↑

hmmm. the answer sounds correct. who told you that the answer was wrong? maybe the answer is supposed to have a negative sign???

I get the same answer as the OP. I agree that a negative sign may be needed, since it is equal and opposite to the force of the foot on the ball for Newton's third law.

euphtone06 · Mar 18, 2007

It was just the negative sign! Thanks guys I was killing myself over this

phys99 · Apr 4, 2009

how did u solve for it?? anyone?? i have the same problem?

futureteacher · Oct 22, 2009

Hey i just got it finally, the 37 is added to the 51.5 not subtracted due to it being in the opposite direction. The correct answer is 3750.