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> Questions About Impulse Etc.
Posted: Nov 22 2007, 06:06 AM


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K so I am working on a few problems that are driving me nuts and I need answers soon cuz this is allready over due. The first is this problem

Now assume that the pitcher in Part D throws a 0.145kg baseball parallel to the ground with a speed of 32 m/s in the +x direction. The batter then hits the ball so it goes directly back to the pitcher along the same straight line. What is the ball's velocity just after leaving the bat if the bat applies an impulse of -8.4Ns to the baseball?

So I used the equations
delta p = p final - p intial
p = mv

J = delta p = mv final - mv initial

and after manipulating the variable to figure out v final and I got

v final = - mv initial m final / J

and the answer i got was .08 that just seems wrong and according to mastering physics it is. I also tried
v final = m final / J and that gave me 3.08 which is also wrong.

Can some please let me know what I am doing wrong?

Posted: Nov 22 2007, 01:26 PM


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J = deltaP = Pfinal - Pinitial
=> J + Pintial = Pfinal
=> -8.4 + Pintial = Pfinal
=> -8.4 + m*Vintial = m*Vfinal
=> Vfinal = (-8.4/m) + Vintial
=> Vfinal = -57.93 + 32 = -25.93 m/s

The negative velocity of the ball makes sense because it means the ball is traveling in the opposite direction at 25.93 m/s

Hope this is the correct answer and you should make sure it makes sense...

Posted: Nov 22 2007, 03:23 PM

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Yes, sidkshatriya's answer is correct. Biologybabe679, you had the right idea, but did some bad algebra solving for vfinal. It looks like you understand the physics of the problem, it was just a math glitch.

Hope that helps!

--Stuart Anderson

A hallmark of intelligence is the ability to give precise answers to vague questions.

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