Jack D. Ripper flipped out after missing a Must-Do-It question for the third time on his Minds On Physics assignment. Outraged by the futility of his efforts, he flings a 4.0-gram pencil across the room. The pencil lodges into a 221.0-gram Sponge Bob doll which is at rest on a countertop. Once in motion, the pencil/doll combination slide a distance of 11.9 cm across the countertop before stopping. The coefficient of friction between the doll and the countertop is 0.325. Determine the speed at which the pencil is moving prior to striking Sponge Bob.

Question

theeric · Answer

What I think is important here is conservation of momentum.

The pencil is the only thing that has momentum to start.

Upon colliding with Spongebob, this system now has the momentum of the pencil except more mass.

If you can find the momentum that the Spongebob-pencil system has, you'll know the momentum of the pencil, and you know it's mass so you can find its velocity.

So can you find the momentum of the Spongebob-pencil system?

theeric · Answer

Well you know the mass of the Spongebob-pencil system so you just need to find its velocity.

It's velocity is an important factor in how far it can slide along the table. So we'll look at that.

You know the kinetic coefficient of friction, and the mass, so you can find the normal force and so you can find the friction force. Again, you know the mass, so you can find the acceleration due to the friction force.

Now you know the distance it slides to a stop, the acceleration, and you want to know the initial velocity. I almost feel like I'm saying too much, here, but $v_f^2=v_i^2+2ad$.

theeric · Answer

From there, you can backtrack to get the velocity of the pencil.

theeric · Answer

Feel free to respond with any questions! :) It is a lot.

anonymous · Answer

thank you

anonymous · Answer

I was not sure where to get started

theeric · Answer

Awesome :)

Feel free to post any troubles you run into, if any. Take care!