# Homework Help: Takehome quiz

1. Apr 24, 2007

### StellarZ

the ball has a mass of 300g and is traveling at 20m/s. what will be its new velocity?

section is on momentum and impulse

2. Apr 24, 2007

### Dick

It's velocity is unchanged, since nothing happened. Can you post a more complete statement of your problem? And try to work it out as well?

3. Apr 24, 2007

### StellarZ

2. The ball has a mass of 300g and is traveling at 20m/s. What will be its new velocity? Sketch a diagram that shows the velocity of the ball before and after contact with the bat.

That is all it says. i have the diagram finished and have an equation but i need the mass of the bat to solve it i thought.

4. Apr 24, 2007

### denverdoc

sure you need a force, otherwise Newton's first law as Dick suggested is operative. to get a numerical answer not only would you need mass of bat, but velocity, angle of impact, and the coefficient of restitution for the collision. Since its impulse-momentum, you likely need to make several assumptions here, such as an elastic collision, and without specifics as to the bat mass and speed all you can do is solve in terms of these unknowns.

5. Apr 24, 2007

### Dick

???? You may very well need a velocity and mass of the bat. So what are they? Your first post didn't even mention a bat.

6. Apr 24, 2007

### StellarZ

ok and now i found out the impulse delivered to the ball is 15J...now idk what to do

7. Apr 24, 2007

### StellarZ

the first problem had me solve for the impulse delivered to the ball and that came out to 15J....i did this by using the given force vs time graph for the bat on the ball....

8. Apr 24, 2007

### denverdoc

Well that info is helpful. Do you know of an eqn that relates impulse to change of momentum?

9. Apr 24, 2007

### Dick

StellarZ, the units of impulse are force*time which is momentum. They are not measured in joules. Listen to doc, I'm sure he can help you, but he probably needs more information.

10. Apr 24, 2007

### denverdoc

Indeed he does. This is a problem that arises periodically which is most frustrating to all parties, an incomplete post of the problem. You're new to PF, no big deal. We need all info from start to finish.

11. Apr 25, 2007

### StellarZ

2. The ball has a mass of 300g and is traveling at 20m/s. What will be its new velocity? Sketch a diagram that shows the velocity of the ball before and after contact with the bat.

then number one said

1. what is the impulse delivered to the ball?
there was a force vs time graph and it was graphed in a triangle shape so i took the area of the triangle.... 0.3seconds x 100Newtons / 2= 15 and thats all i know how to do.... thanks for all the help so far by the way

12. Apr 25, 2007

### denverdoc

You did well, to realize that the impulse was the integral of force over time. This is also equal to the change in momentum for the ball.

So lets compute the initial momentum,=mv=.3*20=6. Can you finish?

13. Apr 25, 2007

### StellarZ

no...what do i do with the momentum?

14. Apr 25, 2007

### denverdoc

what is the definition of momentum?