Projecticle motion

1. Dec 19, 2006

8parks11

You want to close an open door by throwing either a 400-g lump of clay or a 400-g rubber ball toward it. You can throw either object with the same speed, but they are different in that the rubber ball bounces off the door while the clay just sticks to the door. Which projectile will apply the larger impulse to the door and be more likely to close it?

2. Dec 19, 2006

mbrmbrg

What concepts might be useful here? (You can ignore projectile motion completely.)

3. Dec 19, 2006

8parks11

impulse = F x T
and since they both have equal mass it doesnt matter except

the clay has a larger T value (since it sticks)
so the impulse is larger?

not sure

4. Dec 19, 2006

mbrmbrg

yes, impulse does equal F*t, but it also equals the change in momentum.

5. Dec 19, 2006

8parks11

so the answer is the clay because it has a higher T value = higher momentum?

6. Dec 19, 2006

mbrmbrg

whoa... back up.
First of all: what's the definition of momentum?
Secomdly: are we interested in the momentum of the thrown object or itschange of momentum?

I'd suggest writing down all of the momentum and impulse equations for handy reference.

7. Dec 19, 2006

8parks11

J = FΔt,
J = mΔv
J = P2 - P1
the ones I used/need to know i guess...

anyways im interested in only the first part.
we are interested in the change of momentum.
so p2-p1 is higher for the clay because it sticks -> higher t.

8. Dec 19, 2006

8parks11

meh i got it so the rubber ball will have a negative velocity while the clay will have equal velocity with the door.

9. Dec 19, 2006

8parks11

to make it more explicit
P2-P1 becomes P2-(-P1) for the rubber ball and P2+P1 so it will have be much higher.

10. Dec 19, 2006

mbrmbrg

I think you may have gotten it. State that momentum=mv and $$\Delta p=m_fv_f-m_iv_i$$, and I'll be a little more sure.

So in the end of the day, if you're trying to open a door and can't be bothered to get off the sofa, will you chuck the clay or the rubber ball?

Last edited: Dec 19, 2006