Looking for equation for change in momentum

[jjg242]

One of the questions on my test review tells me to look for the equation in the book, but i have a different book because my backpack got stolen and I can only get my hands on a old physics book at the moment. can someone help me out and let me know what equation I am looking for here? thanks,

A pitched baseball, of mass 0.2 kg, arrives at home plate with a speed of 40 m/s and is batted straight back to the pitcher with a return speed of 60 m/s. What is the magnitude of change in the ball's momentum?

a. 40 kg-m/s b. 8.0 kg-m/s c. 18 kg-m/s d. 20 kg-m/s

 

[Doc Al]

You shouldn't need a special formula here. Do you understand that momentum is mass x velocity? The change in anything will be final value minus initial value. (Tricky part: Direction--and signs--matter.)

 

[jjg242]

so simply:

(0.2*40)-(0.2* -60) = 20 kg-m/s

is this correct?

 

[Doc Al]

No. Your main mistake is getting the signs wrong. But you also got initial and final reversed. Fill this out:

Final momentum =

Initial momentum =

Change (Final - Initial) =

 

[jjg242]

No. Your main mistake is getting the signs wrong. But you also got initial and final reversed. Fill this out:

Final momentum =

Initial momentum =

Change (Final - Initial) =

maybe I am finding the momentum wrong?

Final momentum = -12

Initial momentum = 8

Change (-12 - 8) = -20 since it's magnitude, just 20. I don't see how it could be anything else.

 

[Doc Al]

Exactly right! (I misread your answer before --sorry about that. You didn't make the sign error I thought you did! The only mistake was reversing initial and final, but that didn't really matter.)

 

[jjg242]

thanks, this forum is so helpful.