What are some strategies for understanding and completing homework problems?

[TeaWay]

Momentum..initial & final & impulse.

a 70g tennis ball coming towards you at 25 m/s is hit with a tennis racquet and goes away from u at 30 m/s

A) what is the initial momentum of the tennis ball?
B) what is the final momentum of the ball ?
C) what is the change in momentum of the tennis ball ?
D) what was the impulse on a tennis ball
E) what was the average force applied by the raquet on the ball if it made contact with the ball for 0.020 seconds ?

a) initial momentum = 0.070kg x 25 m/s
=1.75 kg x m/s

b)final momentum = 0.070kg x 30 m/s
=2.1 kg x m/s

c) change in momentum = 2.1 kg x m/s - 1.75 kg x m/s
= 0.35 kg x m/s

d) impulse = F(change in time) = change in momentum

impulse = 0.35 kg x m/s

i don't know if i did it right but i got stuck on "E"
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[Delphi51]

If the initial momentum is positive, then the final momentum must be negative because it is in the opposite direction. At least for calculating the change.

For (e) use F*Δt = m*Δv = Δp

 

[TeaWay]

sorry if i might ask a lot of questions >< . I am a bit slow.. lol

so if i change the final momentum to a negative i get..

final momentum = -3.85 kg*m/s

but for question " E "

how do i find the average force.?

is it like

impulse/ Δt = f i don't know if I am doing this correctly.

 

[Delphi51]

impulse/ Δt =

yes!

 

[TeaWay]

yes!

YAY ! ty ty :D ! but then i have a lot more questions i have to ask about my other homework problems.. usually i stay after school to get help and understand the concepts and whatnot but i didn't have time to get to go through problems i need help with ; - ;.. sorry for the extra info that wasn't need . i just wanted to to feel like I'm not a complete idiot asking sooo many questions :\ .